Q1. Why is Si preferred over germanium in the manufacture of semiconductor devices?

Ans:

The Silicon diodes have higher peak inverse voltage (PIV) and current ratings and wider temperature ranges than germanium diode. Silicon can be operated at temperatures up to about 75-80⁰C for germanium.

Q2. What are 2 mechanisms of breakdown in a p-n junction and what is difference between them?

Ans:

There are 2 mechanisms by which breakdown can occur at a reverse biased p-n junction:

a) Avalanche breakdown b) zener breakdown

Differences are: a) Diode junctions which breakdowns below 5V are caused by zener effect while those above 5V are caused by avalanche effect.

b) Zener breakdowns occur in heavily doped junctions while avalanche breakdown occurs in lightly doped junctions.

Q3. Why germanium is more temperature dependent than silicon?

Ans:

Germanium is more temperature dependent than silicon because it is observed that reverse saturation current Io doubles for every 10 degree Celsius rise for Ge and every 6 degree Celsius rise for Si.

Q4. What are 2 types of capacitances across p-n junction ? which of these is dominating?

Ans:

In reverse- bias region we have the transition or depletion –region capacitance while in the forward- bias region we have the diffusion or storage capacitance. Diffusion capacitance is much larger than transition capacitance.

Q5. What is recovery time?

Ans:

A junction diode may be used as a switch in electrical circuits. The circuit can be ON or OFF by forward biasing or reverse biasing the junction diode. The interval of time elapses before the diode recovers its steady state is called as recovery time.

Q6. What is a rectifier?

Ans:

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification. Rectification may serve in roles other than to generate direct current for use as a source of power.

Q7. What is PIV (Peak inverse Voltage) and its importance?

Ans:

It is defined as a maximum voltage across a diode in the reverse direction. For a half wave rectifier the maximum value of PIV is Vm and for full wave the maximum value is 2Vm. It maintains input Power Factor presented to the supply and also determines input supply window.

Q8. Define Ripple and Ripple Factor.

Ans:

The pulsating output of a rectifier can be considered to contain DC components and AC components called the Ripples. The ripple current is undesirable and its value should be the smallest possible in order to make the rectifier effective.

Ripple factor is defined as a ratio of effective value of AC components of voltage or current present in output from the rectifier to the direct or average value of the output voltage or current.

Q9.Why transistor is called as current controlled device?

Ans:

A BJT is a current controlled device because its output characteristics are determined by the input current. The current controlled model is used often because it is approximately linear.

Q10. Why collector is made larger than emitter and base .

Ans:

The collector is moderately doped but large in size as it has to collect most of the majority carriers supplied by the emitter to the collector.

Q11. Why width of base is kept very small as compared to another regions (Emitter and Collector)?

Ans:

Base is lightly doped and very thin in size so that it can pass on most of the majority carriers supplied by the emitter to the collector.

Q12. Why emitter is always forward biased.

Ans:

The emitter is always kept forward biased with respect to base so that it can supply large number of majority carriers to its junction with the base.

Q13. Why collector is always reversed biased with respect to base?

Ans:

Collector is always reverse biased so as to remove the charge carriers away from its junction with the base.

Q14. Can a transistor be obtained by connecting two semiconductor diodes back to back ?

Ans:

No, a transistor can’t be obtained by connecting two semiconductor diodes back to back, because diode has equally doped regions and requirement for transistor action is a lightly doped base b/w heavily doped emitter and intermediate doped collector.

Q15. How α and β are related to each other ?

Ans:

α =

Where α is current amplification factor and is the ratio of collector current to the emitter current

β is current gain factor of a common emitter circuit and is the ratio of collector current to the base current.

Q16. Why common emitter configuration is most popular in amplifier circuits?

Ans:

Common emitter is commonly used because its current, voltage and power gains are quite high and output to input impedance ratio is moderate.

Q17. Why common collector configuration is called as emitter follower configuration?

Ans:

Common collector arrangement gives very high input impedance and very low output impedance. Therefore its voltage gain is always less than unity and hence it is used for impedance matching i.e. for driving low impedance from a high impedance source. This configuration is also called emitter follower.

Q18. Explain why BJT’s are called as bipolar devices and FET’s are called unipolar devices?

Ans:

BJT is a bipolar device, that is, it uses both electrons and holes for conduction. Minority charge carriers are the bases of conduction in BJTs (electrons conducting in P channel and holes conducting in N channel)

FET's are unipolar devices, only one of the charge carriers causes conduction. and furthermore it is the majority charge carriers that conduct in this device( holes conducting in P channel and electrons conducting in N channel).

Q19. Define pinch off voltage for JFET.

Ans:

Pinch off voltage is defined as minimum ground to source voltage for which drain current will become zero. If we apply this voltage between drain and source then at this potential drain current starts saturating.

Q20. Why FET called as voltage controlled device?

Ans:

FET is a "voltage-controlled" current source, because the output current is modulated by a the GATE VOLTAGE which changes the electro-static FIELDs inside the channel, hence the name FIELD-EFFECT transistor.

Q21. What do you mean by saturated drain to source current?

Ans:

The drain current in the pinch off region with Vgs=0 is referred to the drain to source saturation current, IDSS.

Q22. How do FET have a very high input impedance ?

Ans:

FET have a very high input impedance because its input circuit (gate to source) is reverse biased.

Q23. Differentiate between BJT and FET.

Ans:

· BJT is a Bipolar Junction Transistor, while MOSFET is a Metal Oxide Semiconductor Field-Effect Transistor. A BJT has an emitter, collector and base, while a MOSFET has a gate, source and drain.

· BJTs are preferred for low current applications, while MOSFETs are for high power functions.

· In digital and analog circuits, MOSFETs are considered to be more commonly used than BJTs these days.

Q24. Differentiate between DE MOSFET and E MOSFET

Ans:

DE MOSFET can be operated in both modes i.e. depletion (-ve supply) as well as enhancement mode(+ve supply) in case of n-channel MOSFET. But e MOSFET only operated in enhancement mode i.e. positive supply at gate (in case of n channel). Also E MOSFET is normally in off state always but it is not with DE MOSFET.

Q25. Define cut in voltage of PN junction diode.

Ans:

The forward voltage at which the current through the junction starts increasing rapidly, is called the knee voltage or cut-in voltage. At some forward voltage (0.3V for Ge and 0.7 for Si). The potential barrier is eliminated and current starts flowing. This voltage is known as Threshold, Cut-in or knee voltage.

Q26. What is meant by 3db frequency?

Ans:

In electronics, cutoff frequency or corner frequency is the frequency either above which or below which the power output of a circuit, such as a line, amplifier, or electronic filter is 1/2 the power of the pass band. Because power is proportional to the square of voltage, the voltage signal is square root of 1/2 of the pass band voltage at the corner frequency. Hence, the corner frequency is also known as the 3 dB point because 1/2 is approximately 3 decibels. A band pass circuit has two corner frequencies; their geometric mean is called the center frequency.

Q27. What is 3db power gain and 3db voltage gain?

Ans:

Although no unit is assigned to the gain of the amplifier, since it is the ratio. Accordingly a logarithmic unit named decibel (dB) was decided.

Power gain = 10 log10 (Pout/Pin) dB

Voltage gain = 10 log10 (Vout/Vin) ^2 dB

Q28. What is MULTISTAGE amplifier circuit?

Ans:

A transistor circuit containing more than one stage of amplifier is multistage amplifier. A practical amplifier is always multistage amplifier.

· In multistage amplifier, output of first is combined to next stage through a coupling device, this process is known as cascading.

Q29. Why RC COUPLING is most widely used coupling b/w two stages of cascaded amplifier?

Ans:

RC COUPLING is most widely used coupling b/w two stages of cascaded amplifier because

· It is cheaper in cost.

· Circuit is very compact.

· Provide excellent frequency response.

· Constant gain over audio freq. response.

Q30. Why overall gain of multistage amp is less than product of gains of individual’s stage?

Ans:

In practice total gain A is less than AV1* AV2*________* AVn-1* AVn , due to loading effect of following stages .

Q31. Why coupling capacitor provided in a self biased common emitter RC coupled amplifier is also called LOCKING capacitor?

Ans:

The coupling capacitor ‘Cc’ transmits ac signals but blocks the dc voltage of first stage from reaching the base of second stage. Thus the dc biasing of next stage is not interfered with.

For this reason Cc is also called blocking capacitor.

Q32. Why does RC COUPLING gives constant gain over middle frequency range?

Ans:

The voltage gain of amplifier is constant with the increase in frequency in this range, the reactance of coupling capacitor Cc reduces thereby increase the gain but at the same time lower capacitive reactance cause higher loading resulting in lower voltage gain. Thus the two effects cancel each other and uniform gain is obtained in mid frequency range.

Q33. Why is RC coupled amplifier widely used as voltage amplifier?

Ans:

RC coupled amplifier widely used as voltage amplifier because of excellent audio fidelity over wide range of frequency.

Q34. What do you mean by operating point or Q Point?

Ans:

IN o/p characteristics of CE configuration, the point obtained by values of IC (collector current) and Vce (collector emitter voltage) when no signal is applied at input is known as operating point.

· This is also known as Q-point, because it is a point on Ic-Vce characteristic when transistor is silent i.e. No signal at input.

Q35. What do you meant by transistor biasing?

Ans:

The process by which the required conditions such as proper flow of zero signal collector current and maintenance of proper collector emitter voltage during the passage of signals are obtained is known as transistor biasing.

Q36. What is Faithfull amplification?

Ans:

While doing amplification, care should be taken that the shape of the signal must remain same. The amplification of signal without change in the shape is called a faithful amplification.

Q37. What is thermal runway transistor amplifier circuit?

Ans:

The destruction of transistor by cumulative effect of rise of temperature is known as thermal runway.

Ic = Beta Ib (1+beta) Icbo

Collector current depends upon the collector leakage current. The collector leakage current strongly depends on the temperature. At the load the flow of collector current produces heat within the transistor and it cause destruction of transistor.

Q38. What is an ordinary power supply? Why unregulated power supply is not good enough for practical application?

Ans:

DC power for electronic circuits is obtained from commercial a.c. lines by using transformer rectifier and filter. This combination of transformer _rectifier and filter is called ordinary power supply.

Unregulated power supply is not good enough for practical application, because

· DC o/p voltage changes with change in ac supply voltage.

· The o/p voltage varies if the load varies.

· Variation of temperature varies the d.c. output voltage.

Q39. What is regulated power supply?

Ans:

A power supply that maintains the output voltage constant irrespective of a.c. mains fluctuations or load variations is called as regulated power supply.

Q40. What do you mean by load and line regulation?

Ans:

Load regulation or VOLTAGE regulation is the change in the regulated output voltage when the load current changes from minimum to maximum value.

Vnl : load voltage at no load(occurs when load resistance is infinite).

Vfl : load voltage at full load(occurs when load resistance is of minimum value).

For ideal power supply, load regulation value should be zero, rather be very small not exceeding 1%.

Source or line regulation is defined as change in the regulated output voltage for a specified range of line voltage, typically 230V +-10 percent.

Q41. What is meant by clipping and clamping circuit?

Ans:

Clipping circuits or limiting circuits: used to select for transmitting that part of an arbitrary waveform which lies above or below some reference level.

Clamping is the process of introducing a D.C. level into an a.c. signal. A clamping circuit is a device that places either the positive or negative peak of signal at a desired level.

Q42. What is meant by filter?

Ans:

An electronic device which blocks the a.c. component but allows the D.C. component of the rectifier to pass to the load is called a filter circuit.

COMPONENTS used in filter circuit: Inductor and capacitor

TYPES of filter circuit:

· Shunt capacitor filter

· Series inductor filter

· Choke i/p(L-C) filter

· Capacitor i/p-pi filter

Q43. What is effect of removal of emitter bypass capacitor in a CE amplifier circuit?

Ans:

The removal of the bypass emitter capacitor results in an increase in the output resistance.

Q44. What is varactor diode?

Ans:

In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals.
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Varactors are used as voltage-controlled capacitors. They are commonly used in parametric amplifiers, parametric oscillators and voltage-controlled oscillators as part of phase-locked loops and frequency synthesizer. For example, varactors are used in the tuners of television sets to electronically tune the receiver to different stations.

Q45. What is a photo diode?

Ans:

A photodiode is a type of photo detector capable of converting light into either current or voltage, depending upon the mode of operation.[1] The common, traditional solar cell used to generate electric solar power is a large area photodiode.:
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Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode use a PIN junction rather than a p-n junction, to increase the speed of response. A photodiode is designed to operate in reverse bias.

Q46. What is schottky diode? Why it is called as hot carrier diode?

Ans:

The Schottky diode (named after German physicist Walter H. Schottky; also known as hot carrier diode) is a semiconductor diode with a low forward voltage drop and a very fast switching action.

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When current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6–1.7 volts, while a Schottky diode voltage drop is between approximately 0.15–0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency.

Because the energy of electrons transfer from semiconductor to metal side have more energy than the Fermi energy of electrons in metal side. That's why these are called hot carrier diodes.

Q47. What is LED? How light emission takes place?

Ans:

A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting. Introduced as a practical electronic component in 1962,early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.

When a light-emitting diode is forward-biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.

Q48. Compare the CB,CE ,CC Configuration?

Ans:

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Q49. What is pin diode?

Ans:

A PIN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts.

The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (one typical function of a diode), but it makes the PIN diode suitable for attenuators, fast switches, photo detectors, and high voltage power electronics applications.

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Q50. What is tunnel diode?

Ans:

A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanical effect called tunneling.
These diodes have a heavily doped p–n junction only some 10 nm (100 Å) wide. The heavy doping results in a broken band gap, where conduction band electron states on the n-side are more or less aligned with valence band hole states on the p-side.
Tunnel diodes are usually made from germanium, but can also be made in gallium arsenide and silicon materials. They can be used as oscillators, amplifiers ,frequency converters and detectors.

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FAQ (COMMUNICATION SYSTEMS)

BY:-

SHAHNAZ KHAN

MILIND SINGHAL
Q1. Define modulation.

ANS:

Modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"). Any of these properties can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used.

Q2. What are the types of analog modulation.

ANS:

In analog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques are:

· Amplitude modulation (AM) (here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal)

· Angle modulation

1. Frequency modulation (FM) (here the frequency of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal)

2. Phase modulation (PM) (here the phase shift of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal)

Description: Description: http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Amfm3-en-de.gif/200px-Amfm3-en-de.gif

Q3. Define depth of modulation.

ANS :

The modulation index (or modulation depth) of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level. It is defined differently in each modulation scheme.

· The AM modulation index is the measure of the amplitude variation surrounding an unmodulated carrier. As with other modulation indices, in AM this quantity (also called "modulation depth") indicates how much the modulation varies around its "original" level. For AM, it relates to variations in carrier amplitude and is defined as:

$Description: Description: h = \frac{\mathrm{peak\ value\ of\ } m(t)}{A} = \frac{M}{A},$ where $Description: Description: M\,$ and $Description: Description: A\,$ are the message amplitude and carrier amplitude, respectively.

· The FM modulation index, this quantity indicates by how much the modulated variable varies around its unmodulated level. It relates to variations in the carrier frequency:

$Description: Description: h = \frac{\Delta{}f}{f_m} = \frac{f_\Delta |x_m(t)|}{f_m} \$

· The PM modulation index, this quantity indicates by how much the modulated variable varies around its unmodulated level. It relates to the variations in the phase of the carrier signal:

$Description: Description: h\, = \Delta \theta\,$ ,

where $Description: Description: \Delta \theta$ is the peak phase deviation.

Q4. What are Various cases of amplitude modulation (in terms of mod. Index)

ANS :

Typically the modulation index of a signal will vary as the modulating signal intensity varies. However some static values enable the various levels to visualized more easily.

Amplitude modulated index of 0.5

When the modulation index reaches 1.0, i.e. a modulation depth of 100%, the carrier level falls to zero and rise to twice its non-modulated level.

Amplitude modulated index of 1.0

Any increase of the modulation index above 1.0, i.e. 100% modulation depth causes over-modulation. The carrier experiences 180° phase reversals where the carrier level would try to go below the zero point. These phase reversals give rise to additional sidebands resulting from the phase reversals (phase modulation) that extend out, in theory to infinity. This can cause serious interference to other users if not filtered.

Description: Description: Amplitude modulated index of more than 1.0

Amplitude modulated index of more than 1.0
i.e. over-modulated

Q5. What is the need for modulation.

ANS :

The need for modulation can be summarized as follows:

(1) The antenna needed for transmitting signals should have size at least λ/4, where, λ is the wavelength. The information signal, also known as baseband signal is of low frequency (and therefore the wavelength is high). If we need to transmit such a signal directly, the size of the antenna will be very large and impossible to build. Hence direct transmission is not practical.

(2) The radiated power by an antenna is inversely proportional to the square of the wavelength. So, if we use high frequency signals, the power radiated will be increased.

(3) If we transmit the baseband signals directly, the signals from different transmitters will get mixed up and the information will be lost.

Because of these reasons, we use the technology of modulation, for transmitting message signals effectively for long distances.

Q6. Types of AM modulators.

ANS :

Different types of AM modulators can be classified as:

· Linear (e.g. switching modulator)

· Non-linear (e.g. square law modulator)

Q7. Classification of modulation.

ANS :

Modulation may be classified as:

· Continuous wave (CW)

a. Amplitude modulation (AM)

b. Angle modulation

i. Frequency modulation (FM)

ii. Phase modulation (PM)

· Pulse modulation

a. Pulse amplitude modulation (PAM)

b. Pulse time modulation (PTM)

i. Pulse width modulation (PWM)

ii. Pulse position modulation (PPM)

c. Pulse code modulation (PCM)

Q8. What is single tone and multi tone modulation.

ANS :

Single-Tone Modulation

A single-tone modulating signal may be represented as x(t) = Am cos wm t. It has a waveform shown in Figure 2.1.

Figure 2.1 A 1-kHz single-tone modulating signal

A carrier wave is a sinusoidal voltage or current generated in a trAns:mitter and is subsequently modulated by a baseband or modulating signal. The carrier signal can have the equation c (t) = Ac cos wc t. Its waveform is given in Figure 2.2.

Figure 2.2 A 10-kHz carrier wave

The waveform for the standard AM or double-sideband full-carrier AM is shown in Figure 2.3. It is described by the equation, xc (t) = Ac [ 1 + m cos wm t ] cos wc t, where m is the modulation index.

Figure 2.3 A DSBFC AM signal

Figure 2.4 An AM signal with modulating and carrier signals shown

From Figure 2.4, the amplitude of the waveform may be represented by the equation

A = Ac + Am cos wm t

A = Ac [ 1 + m cos wm t ]

where m =

.[1]

Hence, the AM equation for single-tone modulation is

xc (t) = Ac [ 1 + m cos wm t ] cos wc t

xc (t) = Ac [ 1 + m1 cos wm1 t + m2 cos wm2 t + m3 cos wm3 t + ... ] cos wc t

The total transmitted power is

where

Q9. The antenna current of an AM transmitter is 8A. When only carrier is sent it increases to 8.93A. When carrier is modulated by single sine wave, find % modulation.

ANS :

Substituting the values of

Q10. Compare AM with DSBSC and SSBSC.

ANS :

AM Vs. DSB-SC(double sideband suppress carrier)

· In AM, carrier wave is independent of the message signal, which means: that transmission of carrier wave represents waste of power.

· To overcome the above mentioned shortcoming, the carrier component is suppressed from the modulated wave, resulting in double sideband suppressed carrier modulation.

· Thus, by suppressing carrier we obtain a modulated wave that is proportional to the product of the carrier wave and the message signal.

· Unlike amplitude modulation, the envelope of a DSBSC modulated wave is different from the message signal since the modulated wave of DSBSC undergoes a phase reversal whenever the message signal crosses zero.

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AM Vs SSB-SC(single sideband suppress carrier)

· AM and DSBSC are wasteful of bandwidth because both require transmission bandwidth twice the message bandwidth.

· Half bandwidth occupied by upper sideband and other half by lower sideband. However, the upper and lower sidebands are uniquely related to each other by virtue of their symmetry about the carrier frequency.

· Hence, transfer of information requires only one sideband. This is achieved in SSBSC by suppressing both the carrier and one sideband.

· Therefore, SSBSC benefits through using less bandwidth and elimination of high power carrier.

· However, the cost and the complexity of its implementation are its disadvantage.

Q11. Advantages of VSB.

ANS :

VSB(vestigial sideband) is a form of amplitude modulation intended to save bandwidth over regular AM. Portions of one of the redundant sidebands are removed to form a vestigial sideband signal.

The actual information is transmitted in the sidebands, rather than the carrier; both sidebands carry the same information. Because LSB and USB are essentially mirror images of each other, one can be discarded or used for a second channel or for diagnostic purposes.

VSB transmission is similar to single-sideband (SSB) transmission, in which one of the sidebands is completely removed. In VSB transmission, however, the second sideband is not completely removed, but is filtered to remove all but the desired range of frequencies.

This stands for Vestigial Sideband. It is a form is signal where one sideband is completely present, and the other sideband that has been only partly cut off or suppressed. It is widely used for analogue television transmissions. It comes in useful because the baseband video signal is wide (typically 6 MHz). To transmit this using AM would require a bandwidth of 12 MHz To reduce the amount of spectrum used, one sideband is transmitted fully, whereas only the lower frequencies of the other are transmitted. The high frequencies can be later enhanced using filters.

Q12. Compare linear and non-linear modulation.

ANS :

The mathematical relation between the message signal (applied at the modulator input) and the modulated signal (obtained at the modulator output) decides whether a modulation technique can be classiﬁed as linear or non-linear. If this input-output relation satisﬁes the principle of homogeneity and superposition then the modulation technique is said to be linear. The principle of homogeneity states that if the input signal to a system (in our case the system is a modulator) is scaled by a factor then the output must be scaled by the same factor. The principle of superposition states that the output of a linear system due to many simultaneously applied input signals is equal to the summation of outputs obtained when each input is applied one at a time.

Linear modulations are characterized by a bandwidth amplitude B which is equal to the modulation signal. AM is a linear modulation the modulated and the modulating signal present the same bandwidth amplitude centered respectively on the carrier frequency and the zero frequency. Furthermore the modulation does not affect the spectral content of the modulating signal: the ratio between two harmonic components of the modulating signal is leaved unchanged by the modulation.

E.g. DSBSC, BPSK, QPSK, etc.

Non linear modulation act as nonlinear system: the bandwidth of the modulated signal is wider and can be altered in terms of ratio between two spectral components with respect to the modulating signal.

E.g. FM and PM, BFSK, etc.

Q13. How will you generate DSBSC-AM.

ANS :

A double sideband suppressed carrier modulated wave consists of the product of the message signal and the carrier wave. A device for achieving this requirement is called a product modulator. Two forms of product modulators are the balanced modulator and ring modulator.

Q14. What are advantages of ring modulator.

ANS :

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Advantage of ring modulator or double balanced modulator is that there is no output from the modulator at the carrier frequency; that is, the modulator output consists entirely of modulation products.

Q15. Define demodulation.

ANS :

Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.

Description: Description: http://images.tutorvista.com/content/communication-systems/amplitude-demodulation-process.gif

Q16. Types of AM detectors..

ANS :

TYPES OF AM DETECTORS ARE :

· Envelope detector

A simple envelope detector

One major technique is known as envelope detection. The simplest form of envelope detector is the diode detector that consists of a diode connected between the input and output of the circuit, with a resistor and capacitor in parallel from the output of the circuit to the ground. If the resistor and capacitor are correctly chosen, the output of this circuit will approximate a voltage-shifted version of the original signal.

An early form of envelope detector was the cat's whisker, which was used in the crystal set radio receiver.

· Product detector

A product detector is a type of demodulator used for AM and SSB signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator, hence the name. At least partially, it multiplies the signal by the output of the local oscillator. This can be accomplished by heterodyning. The received signal is mixed, in some type of nonlinear device, with a signal from the local oscillator, to produce an intermediate frequency, referred to as the beat frequency, from which the modulating signal is detected and recovered.

· Square law detector

Q17. Types of linear detectors.

ANS :

Types of linear detectors are:

· Product detector

· Envelope detector

Q18. Block diagram of coherent detector.

ANS :

Coherent detector(for DSBSC detection)-

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Q19. Define multiplexing.

ANS :

In telecommunications and computer networks, multiplexing (also known as muxing) is a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy, and is now widely applied in communications.

The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the high-level communication channel into several low-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side.

Q20. Define sensitivity.

ANS :

The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.

Q21. Define selectivity.

ANS :

Selectivity is a measure of the performance of a radio receiver to respond only to the radio signal it is tuned to (such as a radio station) and reject other signals nearby in frequency, such as another broadcast on an adjacent channel.

Q22. Define stability.

ANS :

The system is said to be stable if the output signal is bounded for all bounded input signals.

Q23. Define superhetrodyne principle.

ANS :

In electronics, a superheterodyne receiver (sometimes shortened to superhet) uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency. Virtually all modern radio and television receivers use the super heterodyne principle.

The principle of operation of the super heterodyne receiver depends on the use of heterodyning or frequency mixing. The signal from the antenna is filtered sufficiently at least to reject the image frequency (see below) and possibly amplified. A local oscillator in the receiver produces a sine wave which mixes with that signal, shifting it to a specific intermediate frequency (IF), usually a lower frequency. The IF signal is itself filtered and amplified and possibly processed in additional ways. The demodulator uses the IF signal rather than the original radio frequency to recreate a copy of the original modulation (such as audio).

In most modern radio receivers, reception is based on the super heterodyne principle. The incoming radio frequency is mixed (heterodyned) with the output of an oscillator the frequency of which is adjusted so that the difference between it and the incoming signal is constant; the result is the intermediate frequency. Amplification is thereafter carried out at this intermediate frequency. Both preliminary selection of the incoming frequency and adjustment of the local oscillator frequency are accomplished by variable tuned circuits consisting of inductance and capacitance.

Q24. Transmitter supplies 8KW to the antenna when modulated. Determine total power radiated when modulated to 30%.

ANS :

Therefore, Pc=5.33 KW

At 30% modulation

Q25. Define frequency modulation.

ANS :

In telecommunications and signal processing, frequency modulation (FM) conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant. In analog applications, the difference between the instantaneous and the base frequency of the carrier is directly proportional to the instantaneous value of the input-signal amplitude.

Q26. Define modulation index of frequency modulation.

ANS :

As in other modulation indices, modulation index (freq. modulation) quantity indicates by how much the modulated variable varies around its unmodulated level. It relates to variations in the carrier frequency:

$Description: Description: h = \frac{\Delta{}f}{f_m} = \frac{f_\Delta |x_m(t)|}{f_m} \$

where $Description: Description: f_m\,$ is the highest frequency component present in the modulating signal xm(t), and $Description: Description: \Delta{}f\,$ is the peak frequency-deviation—i.e. the maximum deviation of the instantaneous frequency from the carrier frequency. If $Description: Description: h \ll 1$ , the modulation is called narrowband FM, and its bandwidth is approximately $Description: Description: 2 f_m\,$ .

If $Description: Description: h \gg 1$ , the modulation is called wideband FM and its bandwidth is approximately $Description: Description: 2 f_\Delta\,$ .

Q27. How is FM wave converted to PM wave.

ANS :

We can define the FM wave, produced when we modulate a carrier frequency,

, with a modulating signal,

, to be

where

is the instantaneous frequency of the wave at the instant, t. The term

is a constant whose value depends upon the modulating system. It determines “how many Hz of frequency change we get for each volt of modulating signal.”

The FM wave can now be used to convey information about the modulating pattern in a manner similar to the AM variations we examined in an earlier section. Note that — unlike an AM wave — the FM wave doesn't have a single frequency value. This makes an FM wave obviously different to an AM one. Instead, we can define two distinct quantities; its ‘unmodulated’ (i.e. carrier) frequency, and its modulated frequency,

, which can change from instant to instant. The instantaneous phase of the modulated wave at any instant can be obtained by substituting 12.6 into 12.3 and integrating to get

Q28. How PM wave is converted to FM wave.

ANS :

we can define a Phase Modulated (PM) wave as having the form

where

and

determines how much “phase change per volt of modulation” the PM modulator being used produces. The instantaneous frequency of the PM wave will therefore be

Q29. What are the types of frequency modulation.

ANS :

Frequency modulation can be classified as narrowband if the change in the carrier frequency is about the same as the signal frequency, or as wideband if the change in the carrier frequency is much higher (modulation index >1) than the signal frequency. [5] For example, narrowband FM is used for two way radio systems such as Family Radio Service, in which the carrier is allowed to deviate only 2.5 kHz above and below the center frequency with speech signals of no more than 3.5 kHz bandwidth. Wideband FM is used for FM broadcasting, in which music and speech are transmitted with up to 75 kHz deviation from the center frequency and carry audio with up to a 20-kHz bandwidth.

Q30. Compare WBFM & NBFM.

ANS :

The primary difference between the two types of FM is the number of sidebands in the modulated signal. Wideband FM has a large number (theoretically infinite) number of sidebands. Narrowband FM has only a single pair of significant sidebands.

WIDEBAND FM:
Parameters :
1. modulation index : Greater than 1
2. maximum deviation: 75 kHz
3. range of modulating frequency: 30 Hz to 15 kHz
4. maximum modulation index: 5 to 2500
5. bandwidth : large, about 15 times higher than BW of narrowband FM
6. applications : entertainment broadcasting
7. pre-emphasis and de-emphasis : is needed.

NARROWBAND FM:
parameters :
1.modulation index: less than or slightly greater than 1
2. maximum deviation: 5kHz
3. range of modulating frequency : 30 Hz to 3 kHz
4. maximum modulation index : slightly greater than 1
5. bandwidth : small. approximately same as that of AM
6. applications: FM mobile communication like police wireless, ambulance etc.
7. pre-emphasis and de-emphasis : is needed.

It is possible to determine if a particular FM signal will be wide or narrow band by looking at a quantity called the Deviation Ratio (DR). It is defined as the ratio of the maximum deviation of the FM signal to the maximum modulating frequency:

The DR is also the modulation index of the highest modulating frequency. If the DR ≥ 1.0 the modulation is called wideband FM (WFM). If the DR < 1.0 the modulation is narrow band FM (NBFM).

One of the drawbacks of wideband FM is the large bandwidth required. Commercial FM broadcasting requires 150 KHz of bandwidth to transmit a15 KHz audio signal, 5 times the bandwidth required for an AM signal.

Q31. Define phase deviation

ANS:

Description: Description: http://www.ni.com/cms/images/devzone/tut/dhall_analog_modulation.JPG

In angle modulation (i.e. phase and frequency modulation) with reference to the equation,

The maximum value attained by

(t), that is the maximum phase deviation of the total angle

, is called the phase deviation.

Q32. Define frequency deviation

ANS :

As mentioned above, the maximum departure of the instantaneous frequency from the carrier frequency is called the frequency deviation.

Q33. State carson’s rule

ANS:

Carson's bandwidth rule defines the approximate bandwidth requirements of communications system components for a carrier signal that is frequency modulated by a continuous or broad spectrum of frequencies rather than a single frequency.

A rule of thumb, Carson's rule states that nearly all (~98 percent) of the power of a frequency-modulated signal lies within a bandwidth $Description: Description: B_T\,$ of:

$Description: Description: \ B_T = 2(\Delta f +f_m)\,$

where $Description: Description: \Delta f\,$ , as defined above, is the peak deviation of the instantaneous frequency $Description: Description: f(t)\,$ from the center carrier frequency $Description: Description: f_c\,$ .

Q34. Disadvantages of FM system

ANS:

Disadvantages of FM system are-

· A much wider channel is required by FM, up to 10 times as large as that needed by AM. This is the most significant disadvantage of FM.

· FM transmitting and receiving equipment tends to be more complex, particularly for modulation and demodulation.

· Since reception is limited to line of sight, the area of reception for FM is much smaller than AM. This may be advantage for co-channel allocations, but is a disadvantage for FM mobile communication over a wide area.

Q35. How will you generate message from FM signal

ANS:

Message from FM signal can be generated by using one of these demodulation techniques-

· Slope detection

· Phase locked loops

Q36. Types of FM detectors

ANS:

Below is a list of some of the main types of FM demodulator or FM detector-

Slope FM detector
Foster-Seeley FM detector
Ratio detector
PLL, Phase locked loop FM demodulator
Quadrature FM demodulator
Coincidence FM demodulator

Q37. Types of phase discriminators

ANS:

· Foster-seeley discriminator

The Foster-Seeley discriminator is a widely used FM detector. The detector consists of a special center-tapped transformer feeding two diodes in a full wave DC rectifier circuit. When the input transformer is tuned to the signal frequency, the output of the discriminator is zero. When there is no deviation of the carrier, both halves of the center tapped transformer are balanced. As the FM signal swings in frequency above and below the carrier frequency, the balance between the two halves of the center-tapped secondary is destroyed and there is an output voltage proportional to the frequency deviation.

· Ratio detector

The ratio detector is a variant of the Foster-Seeley discriminator, but one diode conducts in an opposite direction. The output in this case is taken between the sum of the diode voltages and the center tap. The output across the diodes is connected to a large value capacitor, which eliminates AM noise in the ratio detector output. While distinct from the Foster-Seeley discriminator, the ratio detector will similarly not respond to AM signals, however the output is only 50% of the output of a discriminator for the same input signal.

Q38. Disadvantages of balance detector

ANS:

The balanced SLOPE DETECTORS has few disadvantages as under:

1.The linear characteristics is limited to small frequency deviation .The frequency deviation for which the resultant characteristics is linear ,depends upon 3db bandwidth of each turned circuit which is equal to 2W.

2.The discriminators characteristics depends critically upon the amount of detuning of the resonant circuits.

3.The tuned circuit output is not purely band limited and thus the low pass RC filter of envelope detector introduces distortion.

Q39. Define probability

ANS:

Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we are not certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The certainty we adopt can be described in terms of a numerical measure and this number, between 0 and 1, we call probability. The higher the probability of an event, the more certain we are that the event will occur. Thus, probability in an applied sense is a measure of the confidence a person has that a (random) event will occur.

Q40. Define noise

ANS:

In common use, the word noise means: any unwanted sound.

In physics and analog electronics, noise is a mostly unwanted random addition to a signal; it is called noise as a generalization of the acoustic noise ("static") heard when listening to a weak radio transmission with significant electrical noise. Signal noise is heard as acoustic noise if the signal is converted into sound (e.g., played through a loudspeaker); it manifests as "snow" on a television or video image. High noise levels can block, distort, change or interfere with the meaning of a message in human, animal and electronic communication.

In signal processing or computing it can be considered random unwanted data without meaning; that is, data that is not being used to transmit a signal, but is simply produced as an unwanted by-product of other activities. "Signal-to-noise ratio" is sometimes used to refer to the ratio of useful to irrelevant information in an exchange.

Q41. Classification of noise

ANS:

Noise can be classified as-

· Internal The noise which originates inside due to internal factors is called internal noise.

· External The noise which comes from outside due to external factors is called external noise.

Q42. Types of external noise

ANS:

External noise can be further classified as-

· Atmospheric or static

· Extraterrestrial or space

· Industrial or manmade

Q43. Types of internal noise

ANS:

Internal noise can be further classified as-

· Shot noise

· Partition noise

· High frequency or transit time noise

· Low frequency or flicker noise

· Thermal noise

Q44. Types of extraterrestrial noise and their origin

ANS:

Depending on source, extraterrestrial noise is of two types-

1) Solar noise ---- electrical noise emitting from sun due to big body high temperature.

2) Cosmic noise --- generated due to high temperature of distant stars.

Q45. Define transit time of transistor

ANS:

Transit time of a transistor is defined as the time taken by charge carrier to reach from emitter to collector.

Q46. Define flicker noise

ANS:

Flicker noise is generated below few KHz. The power spectral density(PSD) of this type of noise decreases with frequency.

In vacuum tube, main cause of flicker noise are slow changes which take place in oxide structure of oxide coated cathode but in case of semiconductor devices, this type of noise is generated from fluctuation in carrier density and creates more problem in semiconductor amplifying devices.

Q47. Define S/N ratio

ANS:

Signal to noise ratio is the ratio of signal power to the noise power at same point in the system.

SNR =

= signal power/ noise power

Signal-to-noise ratio (often abbreviated SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise.

Q48. Define noise figure

ANS:

Noise figure (NF) is a measure of degradation of the signal-to-noise ratio (SNR), caused by components in a radio frequency (RF) signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard noise temperature

(usually 290 K). The noise figure is thus the ratio of actual output noise to that which would remain if the device itself did not introduce noise. It is a number by which the performance of a radio receiver can be specified.

The noise factor F of a system is defined as:

$Description: Description: F = \frac{\mathrm{SNR}_\mathrm{in}}{\mathrm{SNR}_\mathrm{out}}$

where $Description: Description: \mathrm{SNR}_\mathrm{in}$ and $Description: Description: \mathrm{SNR}_\mathrm{out}$ are the input and output signal-to-noise ratios, respectively.

Q49. Explain thermal noise

ANS:

Thermal noise is generated in resistance or resistive component, is random and referred to as thermal noise. The noise power (Pn) produced in resistor is proportional to its absolute temperature and bandwidth over which noise is measured.

Where, Bàbandwidth

Tàtemperature

kàBoltzmann constant =

J/kelvin

Q50. Give the expression for noise voltage in a resistor

ANS:

Noise voltage in a resistor-

Q51. Explain white noise

ANS:

White noise is a random signal (or process) with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency.

Q52. Give the expression for equivalent noise temperature in terms of hypothetical temperature

ANS:

Here,

is noise temperature

A is gain

R is resistance

T is hypothetical temperature

Also,

Where, To is room temp.

F is noise figure

Q53. Give the friss formulae in terms of noise temperature

ANS:

Friis's formula can be equivalently expressed in terms of noise temperature:

$Description: Description: T_{total} = T_1 + \frac{T_2}{G_1} + \frac{T_3}{G_1 G_2} + ...$

Tn is the noise temperature

Gn is the power gain available

Q54. What is image frequency

ANS:

In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Image frequencies can be eliminated by sufficient attenuation on the incoming signal by the RF amplifier filter of the superheterodyne receiver.

$Description: Description: f_{img} = \begin{cases} f + 2f_{IF} , & \mbox{if } f_{LO} > f \mbox{ (high side injection)}\\ f- 2f_{IF}, & \mbox{if } f_{LO} < f \mbox{ (low side injection)} \end{cases}$

Q55. Define intermediate frequency

ANS:

In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final detection is done.

Q56. Define tracking

ANS:

To recover the original data stream from FH spread spectrum signal, we required regeneration at the receiver, the chipping sequence similar to that used at the transmitter end. Locally generated chipping sequence should be fully synchronized with the received version of chipping sequence, otherwise the recovered signal will be highly distorted. This synchronization process is performed in two stages. In stage 1, we acquire synchronization with the received sequence and we call it coarse synchronization. In stage 2, we stay in close synchronization within permissible limits, we call it fine synchronization or TRACKING.

Q57. What is TRF receiver

ANS:

A tuned radio frequency receiver (TRF receiver) is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned to the station's frequency. It was replaced by the Superheterodyne receiver invented by Edwin Armstrong.

Description: Description: http://www.thebigger.com/wp-content/uploads/2010/02/14.8.png

Tuned radio frequency receiver

Q58. Advantages of superheterodyne receiver over TRF

ANS:

The TRF's disadvantages as "poor selectivity and low sensitivity in proportion to the number of tubes employed. They are accordingly practically obsolete." Selectivity requires narrow bandwidth, and narrow bandwidth at a high radio frequency implies high Q or many filter sections. In contrast a superheterodyne receiver can translate the incoming high radio frequency to a lower intermediate frequency where selectivity is easier to achieve.

An additional problem for the TRF receiver is tuning different frequencies. All the tuned circuits need to track to keep the narrow bandwidth tuning. Keeping several tuned circuits aligned is difficult. A superheterodyne receiver only needs to track the RF and LO stages; the onerous selectivity requirements are confined to the IF amplifier which is fixed-tuned.

Although a TRF receiver cannot be engineered for a high degree of selectivity relative to its carrier frequency, there is no reason it cannot reach the same level of sensitivity as other designs.

Description: Description: http://upload.wikimedia.org/wikipedia/commons/thumb/8/88/Superhet2.svg/497px-Superhet2.svg.png

Superheterodyne receiver

Q59. What is the figure of merit of DSB-SC

ANS:

A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application.

In modulation systems for communication, figure of merit of a device meAns: the ratio of output Signal to Noise Ratio to the input Signal to Noise Ratio.

Figure of merit for Amplitude modulation is given by

$Description: Description: \mathrm{\frac{(SNR)_{O,AM}}{(SNR)_{C,AM}}}=\frac{k_a^2P}{1+k_a^2P}$

Figure of merit for DSB-SC receiver or that of an SSB modulation is always unity. Therefore noise performance of AM receiver is inferior to that of a DSB-SC receiver or an SSB receiver.

Q60. What is capture effect

ANS:

In telecommunications, the capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency will be demodulated.

The capture effect is defined as the complete suppression of the weaker signal at the receiver limiter (if it has one) where the weaker signal is not amplified, but attenuated. When both signals are nearly equal in strength, or are fading independently, the receiver may switch from one to the other and exhibit picket fencing.

The capture effect can occur at the signal limiter, or in the demodulation stage, for circuits that do not require a signal limiter.

Q61. Define pre-emphasis and de-emphasis

ANS:

In processing electronic audio signals, pre-emphasis refers to a system process designed to increase (within a frequency band) the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.

In telecommunication, de-emphasis is the complement of pre-emphasis, in the anti noise system called emphasis.

Q62. Define sampling theorem

ANS:

Sampling Theorem:

A band limited signal can be reconstructed exactly if it is

sampled at a rate at least twice the maximum frequency

component in it."

· The maximum frequency component of g(t) is fm. To recover

the signal g(t) exactly from its samples it has to be sampled at

a rate fs ≥ 2fm.

· The minimum required sampling rate fs = 2fm is called

Nyquist rate.

FAQ (CONTROL SYSTEM)

BY:-

MILIND SINGHAL
Q. What is a control system

ANS:1:

A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other devices or system.
A Control System is a device, or a collection of devices that manage the behavior of other devices. Some devices are not controllable. A control system is an interconnection of components connected or related in such a manner as to command, direct, or regulate itself or another system.

Q. What are two major types of control system

ANS:2:

· Open loop control system (unmonitored control system)

· Closed loop control system (monitored control system)

Q. Define open loop control system

ANS:3:

Open loop control is by far the more simple of the two types of control theory. In open loop control, there is some sort of input signal (digital or analog), which then passes through amplifiers to produce the proper output, and is then passed out of the system. Open loop controls have no feedback and require the input to return to zero before the output will return to zero.

Q. Define closed loop control system

ANS:4:

In closed loop control, the system is self adjusting. Data does not flow one way, it may pass back from a specific amplifier (such as velocity or position) to the start of the control system, telling it to adjust itself accordingly. Many physical systems are closed loop control at the lowest level since the data about velocity and current position modify the output (also position) at consistent rate.

Q. Components of feedback control system

ANS:5:

A controller seeks to maintain the measured process variable (PV) at set point (SP) in spite of unmeasured disturbances (D). The major components of a control system include a sensor, a controller and a final control element.

Q. Why negative feedback is invariably preferred in closed loop system

ANS:7:

Negative feedback occurs when the feedback signal subtracts from the reference signal. If the feedback signal adds to the reference signal, the feedback is said to be positive.

Negative feedback tries to reduce the error whereas positive feedback makes the error large.

Q. Define transfer function

ANS:8:

A Transfer Function is the ratio of the output of a system to the input of a system, in the Laplace domain considering its initial conditions and equilibrium point to be zero.

Q. What is signal flow graph

ANS:9:

A signal-flow graph (SFG) is a special type of block diagram—and directed graph—consisting of nodes and branches. Its nodes are the variables of a set of linear algebraic relations. An SFG can only represent multiplications and additions. Multiplications are represented by the weights of the branches; additions are represented by multiple branches going into one node. A signal-flow graph has a one-to-one relationship with a system of linear equations. In addition to this, it can also be used to represent the signal flow in a physical system; i.e., it can represent relations of cause and effect.

Q. What is sink and source in control system

ANS:10:

With reference to the signal flow graph,

· A node which has only data output with no inputs; it corresponds to an independent variable, is known as source or input node.

· A node that has only input with no output; it corresponds to a dependent variable, is known as sink or output node.

Q. Define non touching loop

ANS:11:

Two loops are non touching if they do not have any node in common.

Q. What is stepper motor

ANS:13:

A stepper motor (or step motor) is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any feedback sensor (an open-loop controller), as long as the motor is carefully sized to the application.

Switched reluctance motors are very large stepping motors with a reduced pole count, and generally are closed-loop commutated.

Q. What is servo motor

ANS:14:

A servomotor is a motor which forms part of a servomechanism. The servomotor is paired with some type of encoder to provide position/speed feedback. This feedback loop is used to provide precise control of the mechanical degree of freedom driven by the motor. A servomechanism may or may not use a servomotor.
A servomechanism, sometimes shortened to servo, is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism.
The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position, speed or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback that controls position—the operator does this by observation. By contrast the car's cruise control uses closed loop feedback, which classifies it as a servomechanism.

Q. Name the test signal used in control system

ANS:15:

Step signal
Ramp or linear signal
Parabolic signal
Impulse signal

Q. What is an order and type number of a system

ANS:16:

The order of the system is defined by the highest degree of the linear differential equation that describes the system. In a transfer function representation, the order is the highest exponent in the transfer function. In a proper system, the system order is defined as the degree of the denominator polynomial. In a state-space equation, the system order is the number of state-variables used in the system.

Q. Define transient response

ANS:17:

In electrical engineering and mechanical engineering, a transient response or natural response is the response of a system to a change from equilibrium. The transient response is not necessarily tied to "on/off" events but to any event that affects the equilibrium of the system. The impulse response and step response are transient responses to a specific input (an impulse and a step, respectively).
The response can be classified as one of three types of damping that describes the output in relation to the steady-state response.

Q. Define damping ratio

ANS:18:

The damping ratio is a parameter, usually denoted by ζ (zeta), that characterizes the frequency response of a second order ordinary differential equation. It is particularly important in the study of control theory. It is also important in the harmonic oscillator.
The damping ratio provides a mathematical means: of expressing the level of damping in a system relative to critical damping. For a damped harmonic oscillator with mass m, damping coefficient c, and spring constant k, it can be defined as the ratio of the damping coefficient in the system's differential equation to the critical damping coefficient.

The damping ratio is dimensionless, being the ratio of two coefficients of identical units.

An under damped response is one that oscillates within a decaying envelope. The more under damped the system, the more oscillations and longer it takes to reach steady-state. Here damping ratio is always <1.

A critically damped response is the response that reaches the steady-state value the fastest without being under damped. It is related to critical points in the sense that it straddles the boundary of under damped and over damped responses. Here, damping ratio is always equal to one. There should be no oscillation about the steady state value in the ideal case.

An over damped response is the response that does not oscillate about the steady-state value but takes longer to reach than the critically damped case. Here damping ratio is >1

Q. List the time domain specification

ANS:19:

· Delay time

· Rise time

· Peak time

· Maximum overshoot

· Settling time

Q. Define delay time

ANS:20:

Delay time

The delay time is the time required for the response to reach half the final value the very first time.

Q. Define rise time

ANS:21:

Rise time

Rise time refers to the time required for a signal to change from a specified low value to a specified high value. Typically, these values are 10% and 90% of the step height.

Q. Define peak time

ANS:22:

Peak time

The peak time is the time required for the response to reach the first peak of the overshoot.

Q. Define overshoot

ANS:23:

Overshoot

Overshoot is when a signal or function exceeds its target. It is often associated with ringing.

Q. Define settling time

ANS:24:

Settling time

Settling time is the time elapsed from the application of an ideal instantaneous step input to the time at which the output has entered and remained within a specified error band.

Q. What is need for controller

ANS:25:

In control theory, a controller is a device, possibly in the form of a chip, analogue electronics, or computer, which monitors and physically alters the operating conditions of a given dynamical system.

a controller manipulates the inputs to a system to obtain the desired effect on the output of the system.

Q. Various types of controller

ANS:26:

There are a number of different standard types of control systems that have been studied extensively. These controllers, specifically the P, PD, PI, and PID controllers are very common in the production of physical systems, but they each carry several drawbacks.

Q. What are dominating poles

ANS:27:

Dominant (or Insignificant) Poles

o The slowest poles of a system (those closest to the imaginary axis in the s-plane) give rise to the longest lasting terms in the transient response of the system.

o If a pole or set of poles are very slow compared to others in the transfer function, then they may dominate the transient response.

o If we plot the transient response of the system without accounting for the transient response of the fastest poles, we may find little difference from the transient response of the original system.

Q. What is controlled variable

ANS:28:

Controlled variable is the quality or condition (temperature, level flow rate, etc.) characterizing a process whose value is held constant by controller or is changed according to certain law.

Q. Define command

ANS:29:

Command is an input that is established or varied by some means: which are external to and independent of the feedback control system.

Q. Define error detector

ANS:30:

Error detector is an element that detects the feedback ; essentially it is a summing point that gives the algebraic summation of two or more signals.

Q. What is disturbance in context to control systems

ANS:31:

Disturbance is an undesired variable applied to the system which tends to affect adversely the value of the variable being controlled. The process disturbance may be due to changes in set point, supply, demand, environmental and other associated variables.

Q. What is servomechanism

ANS:32:

A servomechanism is an automatic control system in which the controlled variable is mechanical position (displacement), or a time derivative of displacement such as velocity and acceleration. The output is designed to follow a continuously changing input or desired variable (command signal). The servomechanisms are inherently fast acting systems and usually employ electric or hydraulic actuation. These systems are essentially used to control the position and speed of a mechanism which is either too heavy or too remote to be controlled manually.

Q. What do you mean by process control

ANS:33:

A process control refers to the control of such parameters as level, flow, pressure, temperature and acidity of a process variable. A particular parameter has usually only one optimum desired value and the control system is required to ensure that the process output is maintained at this level in spite of changes in external conditions which affect the process.

Q. What is regulator

ANS:34:

A regulator is a feedback control system in which the output is maintained at a preset value irrespective of the external load on the system. The reference inputs although adjustable are kept constant for long periods of time. The primary task is then to maintain the output at the desired value in the presence of disturbances.

In general a control system that regulates a variable in response to a fixed command system is known as regulator system.

Q. What is sequence control. What are its features

ANS:35:

A sequence control is a special type of open loop system which has the following main features-

(i) The finish of one action initiates the start of the next

(ii) The acts take place in certain fixed sequence

(iii) There is no comparison of desired and actual value

Q. What are the applications of control system

ANS:36:

· On and off of electric supply to units such as washing machine, toaster, fans:, air conditioners and other electrical appliances.

· Speed and direction control of transport vehicles.

· Control of temperature, pressure, water level and humidity etc. in process industry.

· Regulation of voltage at electric power plants.

· Space technology, missile launching and guidance.

· Military operations such as automatic positioning of guns, radar antennas, steering control of ships.

Q. what do you mean by compensation

ANS:37:

Compensation is related to the act of incorporating devices or circuits to the existing system so as to

· Decrease the steady state error so that the system operates with greater accuracy

· Make the system stable

· Improve the performance of the system

Compensation becomes a necessity when accuracy and stability cannot be met at the same time.

Q. What are the types of compensation

ANS:38:

The compensation is classified or typed according to the location of compensating network in the arrangement of system components.

· Series (cascade) compensation

· Feedback compensation

· Load compensation (combination of the above two types)

Q. What are the factors influencing type of compensation used

ANS:39:

The following factors influence the decision about the type of compensation to be adopted-

(i) Nature of signals in the system

(ii) Energy levels at various points of the system

(iii) Availability of suitable components

(iv) Cost considerations

Q. What are synchros. Why are they used

ANS:40:

Synchros are electromagnetic transducers which are primarily used to perform the following two functions-

(i) To transmit angular data from one location to another through converted electrical signal

(ii) To generate error signals in positional control systems

Q. What are the advantages of synchros

ANS:41:

· No sliding contact and no wear; system can operate at much higher speeds and have higher life expectancy

· Infinite resolution and no noise problem associated with sliding contacts of potentiometers

· High reliability and accuracy

· Continuous rotation; useful operating range full 360 degree of shaft rotation

· No drift problems because a.c. amplifiers can be employed

Q. List the types of servomotors

ANS:42:

Three basic types of servomotors are-

· AC servomotors based on induction motor design

· DC servomotors based on DC motor design

· AC brushes servomotors based on synchronous motor design

Q. Classify DC servomotors

ANS:43:

DC servomotors can be classified as-

· Field controlled DC servomotor

· Armature controlled DC servomotor

Q. Distinguish between field control and armature control

ANS:44:

Field control	Armature control
(i) Control voltage is applied to the field	(i) Control voltage is applied to the armature
(ii) Armature current is kept constant	(ii) Field current is kept constant
(iii) System has low power requirement	(iii) System requires high power
(iv) Suitable for small rated motors	(iv) Suitable for large rated motors
(v) System is open loop type	(v) System is of closed loop type
(vi) Operation is with low efficiency	(vi) Operation is with better efficiency

Q. Differentiate between AC and DC servomotors based on performance parameters

ANS:45:

Parameters	AC motor	DC motor
(i) power output	Low	High
(ii) efficiency	Low	High
(iii) maintenance	Low due to absence of commutator	Commutator needs frequent maintenance
(iv) operation	Smooth and stable	Noisy
(v) ratio frequency noise	No such noise	Produced due to brushes
(vi) stability problems	Less	More
(vii) drift	AC amplifier has no drift	Amplifiers used have high drift

Q. What are the applications of servomotor

ANS:46:

The servomotors have extensive applications in diverse control systems used in computers, machine tools, process controllers, robots, radars, tracking and guidance units and electromechanical actuators.

Q. Where are stepper motors used

ANS:47:

Stepper motors are commonly used in position applications in-

· computer peripheral systems such as X-Y plotters, floppy disc head drives

· automatic latches and drilling machines

· robotic control systems, and watches

Q. Define observability

ANS:48:

A control system is stated to be observable if its every state X(to) can be determined by measurements of the output Y(t) carried over a finite time interval.

Q. What are magnetic amplifiers

ANS:49:

The magnetic amplifier is an electromagnetic device used for amplifying electrical signals.

The amplifier uses a changing inductance to control the power delivered to load.

Two basic types of magnetic amplifiers used in control systems are- balanced magnetic amplifier and unbalanced magnetic amplifier. Such amplifiers have been invented and used as an alternative to vacuum tube amplifiers where robustness and high current capacity is required.

Q. Give advantages and disadvantages of magnetic amplifiers

ANS:50:

Advantages-

· high efficiency up to 90%

· long life, less maintenance and few components needed

· shock and vibration resistant, high overload capability and freedom from effects of moisture

· no requirement for warm up time

· no moving parts; the input and output circuits can be isolated

Disadvantages-

· it cannot handle low level signals

· not useful at high frequencies

· there is time delay associated with magnetic effects

· poor fidelity

FAQ (DIGITAL ELECTRONICS)

BY:-

PRATEEK ARORA
Differentiate b/w analog signal and digital signal. What are its advantages and disadvantages?

Difference b/w analog and digital

Analog	Digital
1. Transfer less data.	Ability to transfer more data as compared to analog.
2. Analog signals have easy processing.	Systems and processing is more complex.
3. Because of their continuous nature they travel slow.	Because of their digital nature they can travel faster
4. If we transmit data at long distance then unwanted disturbance is there.	This does not happen in digital signal.

Analog signal

Advantages:

1. One of the major advantages of the analog signal is that they have power to define infinite amount of data.

2. Density of the analog signals is much higher as compared to digital ones.

3. Analog signals have easy processing.

Disadvantages:

1. Unwanted noise in recording.

2. If we transmit data at long distance then unwanted disturbance is there.

3. Generation loss is also a big con of analog signals.

Digital signal

Advantages:

1. Because of their digital nature they can travel faster in over digital lines.

2. Ability to transfer more data as compared to analog.

Disadvantages:

1. Greater bandwidth is essential.

2. Systems and processing is more complex.

1) Draw all logic gates using nand & nor gates?

NAND GATE

Not gate

And gate

Or gate

Nor gate

Xor gate

NOR gate

NOT gate

OR gate

And gate

Nand gate

Xor gate

2) Why do we use gray codes for solving k-map?

In gray code only one bit position changes between any two successive (nearby) entries. This allows us to group the terms producing the same (normally 1) and eliminate the variable corresponding to the common bit position. As the K-map is used for reduction of min terms, this approach is used. It is the basis of the K-map itself.

3) What is the difference b/w latch and flip flop?

· The difference between a latch and a flip-flop is that a latch does not have a clock signal, whereas a flip-flop always does.

· Latches are Level sensitive where as Flip-flops are Edge triggered sensitive.

· Latch is a device which is used as switch, f/f is a storing element

4) Differentiate b/w toggling and race around condition. How it can be avoided?

Toggling occurs when both the inputs are high and the o/p thus undergoes a transition state.

           For ex. Consider the input values in J-K F/F the o/p is 0 in normal case will change to 1 and vice versa.

           If toggling occurs more than once then it’s a race around condition.

THE REMEDY FOR RACE AROUND PROBLEM CAN BE ELIMINATED BY USING A MASTER SLAVE J-K FLIP FLOP'S.

5) Differentiate b/w +ve and –ve logic?

Positive Logic: With reference to positive logic, logical 1 state is the most positive logic or voltage level and logic 0 state is the most negative logic or voltage level. In other words, active high level is 1 and active low level is 0.

Negative Logic: With reference to negative logic, logic 0 state is the most positive logic or voltage level and logic 1 state is the most positive logic or voltage level. In other words, active high level is 0 and active low level is 1.

6) Why Nand & Nor gates are called universal gates?

The NAND gate and the NOR gate can be said to be universal gates since combinations of them (either NAND gate or NOR gate only) can be used to accomplish any of the basic operations and can thus produce an inverter, an OR gate, XOR gate or an AND gate. SEE Q/2 for reference.

7) Differentiate b/w combinational and sequential circuits. Explain with examples?

· In a combinational logic circuit, the output(s) depend only on the present values on the input, not on any previous values. In a sequential circuit, the output(s) also depend on the previous values. A sequential circuit must contain a memory element (at least one flip-flop) to hold the state of the circuit.

· EXAMPLES:

Sequential: Flip flops, Latches, Counters.

Combinational: Decoders, multiplexers, demultiplexers, Code converter, encoders, decoders, comparators.

8) Design all logic gates(2 i/p’s) using multiplexer?

Mux as inverter

Mux as And gate

Mux as or gate

Mux as nor gate

Mux as XOR gate

9) Differentiate b/w decoder and demux?

In the demultiplexer, we select one of the output line with help of select lines and drive it by input while in decoder, output line is selected on basis of the input data that is been decoded and then data is sent on that line.

10) What is fan-in and fan-out of logic gate?

Fan in is the number of input that a gate has.

Fan out is the number of gate inputs you can connect to a gate output.

11) Explain noise margin of a system?

Noise margin is the maximum external noise voltage added to an input signal that does not cause an undesirable change in the circuit output.

12) Differentiate b/w synchronous and asynchronous sequential circuits. Explain with an example?

· Synchronous Sequential Circuit: Output changes at discrete interval of time. It is a circuit based on an equal state time or a state time defined by external means: such as clock.

Examples of synchronous sequential circuit are Flip Flops, Synchronous Counter.

· Asynchronous Sequential Circuit: Output can be changed at any instant of time by changing the input. It is a circuit whose state time depends solely upon the internal logic circuit delays.

Example of asynchronous sequential circuit is Asynchronous Counter.

13) Compare bipolar logic family and MOS logic family?

· In case of MOS family, Power dissipation per gate is very low i.e. about 0.05 microwatt . In case of bipolar family the power dissipation is in milliwatts.

· MOS family has high noise immunity than bipolar family.

14) Design a circuit to divide i/p frequency by 2(using T ff)?

If we connect two T F/F together in series, the initial input frequency will be "divided-by-two" by the first flip-flop ( f÷2 ) and then "divided-by-two" again by the second flip-flop ( f÷2 )÷2, giving an output frequency which has effectively been divided four times, then its output frequency becomes one quarter value (25%) of the original clock frequency, ( f÷4 ). Each time we add another toggle or "T-type" flip-flop the output clock frequency is halved or divided-by-2 again and so on, giving an output frequency of 2ⁿwhere "n" is the number of flip-flops used in the sequence

15) Applications of octal number system.

The applications of octal number system are:

· It is used for entering the binary data and displaying certain information’s.

· It is very important for the efficient use of microprocessors and other digital

Circuits.

16) State De-morgan’s theorem.

De Morgan suggested two theorems that form important part of Boolean algebra. They are:

· The complement of a product is equal to the sum of the complements.

(A . B)' = A' + B’

· The complement of a sum term is equal to the product of the complements.

(A + B)' = A'B’

17) Advantages and disadvantages of K-map.

The advantages of the K-map method are:

· It is a fast method for simplifying expression up to four variables.

· It gives a visual method of logic simplification.

· Prime implicants and essential prime implicants are identified fast.

· Suitable for both SOP and POS forms of reduction.

· It is more suitable for class room teachings on logic simplification.

The disadvantages of the K-map method are:

· It is not suitable for computer reduction.

· K-maps are not suitable when the number of variables involved exceed four.

· Care must be taken to fill in every cell with the relevant entry, such as a 0, 1 or

don’t care terms.

18) Advantages and disadvantages of Q-M method.

The advantages are:

· This is suitable when the number of variables exceed four.

· Digital computers can be used to obtain the solution fast.

· Essential prime implicants, which are not evident in K-map, can be clearly seen in the final results.

The disadvantages are:

· Lengthy procedure than K-map.

· Requires several grouping and steps as compared to K-map.

· It is much slower.

· No visual identification of reduction process.

· The Quine Mc-Cluskey method is essentially a computer reduction method

19) Define figure of merit.

Figure of merit is defined as the product of speed and power. The speed is specified in terms of propagation delay time expressed in nano-seconds.

Figure of merit = Propagation delay time (ns) * Power (mW)

20) State Advantages and disadvantages of TTL.

Advantage:

1. Easily compatible with other ICs

2. Low output impedance

Disadvantage:

1. Wired output capability is possible only with tristate and open collector types.

2. Special circuits in Circuit layout and system design are required.

21) Define state diagram. What is its use?

A graphical representation of a state table is called a state diagram.

i)Behavior of a state machine can be analyzed rapidly.

ii) It can be used to design a machine from a set of specification.

22) Differentiate b/w ROM & PLD?

ROM	PLD
It is a device that includes both the decoder and the OR gates with in a signal IC package.	It is a device that includes both AND and OR gates within a single IC package.
ROM does not full decoding of the variables and does generate all the minterms.	PLD’s does not provide full decoding of the variable and goes not generate all the minterms.

23) What is PLA? Differentiate b/w PLA and ROM.

In some cases the number of don’t care conditions is excessive, it is more economical to use a second type of LSI component called a PLA. A PLA is similar to a ROM in concept; however it does not provide full decoding of the variables and does not generates all the minterms as in the ROM.

24) Define static and dynamic RAM.

Static RAM:

Use flip flops as storage elements and therefore store data in definitely as long as dc power is applied.

Dynamic RAM:

Use capacitors as storage elements and cannot retain data very long without capacitors being recharged by a process called refreshing.

25) What is CPLD?

CPLDs are Complex Programmable Logic Devices. They are larger versions of PLD with a centralized internal interconnect matrix used to connect the device macro cells together.

26) Differentiate b/w ROM, PLA , PAL?

ROM	PLA	PAL
array is fixed and OR array is programmable.	Both AND & OR arrays are programmable.	OR array is fixed and AND array is programmable.
Cheaper and simple to use.	Costliest & complex than PAL & PROMs.	Cheaper and simpler.
All minterms are decoded.	AND array can be programmed to get desired minterms.	AND array can be programmed to get desired minterms.
Only Boolean functions in standard SOP form can be implemented using PROM	Any Boolean function in standard SOP form can be implemented using PROM.	Any Boolean functions in standard SOP form.

27) Give an example of weighted codes.

These codes are positionally weighted and each position within the binary equivalent of the number is assigned a fixed value. Thus, they obey positional weighting principal.

Binary Coded Decimal (BCD) and Excess-3 Codes are examples of weighted codes. In these two codes, the binary equivalent of a number will always remain same.

28) Compare ASCII with EBCDIC code.

ASCII	EBCDIC
Uses 7 bits	Uses 8 bits
ASCII is a character encoding set common on a Number of different computer systems.	EBCDIC is a character set particular to IBM mainframes.
ASCII is well documented and widely available.	Documentation of EBCDIC is hard to obtain.

29) What are the various method of error detection?

· Parity method

· Hamming code

· Checksum method

30) Write applications of multiplexer.

I. They are used as a data selector to select one output of many data inputs.

II. They can be used to implement combinational logic circuits.

III. They are used in time multiplexing systems.

IV. They are used in frequency multiplexing systems.

V. They are used in A/D & D/A Converter.

VI. They are used in data acquisition system

31) What is standard SOP form?

When two or more product terms are added by Boolean addition, the resulting equation is sum-of product (sop) form.

For ex, Y=ABC + A(bar)BC + ABC(bar)

32) Design half subtractor /half adder using mux?

33) Draw truth table of 2 bit magnitude comparator?

Inputs					Outputs

0	0	0	0	0		1	0
0	0	0	1	1		0	0
0	0	1	0	1		0	0
0	0	1	1	1		0	0
0	1	0	0	0		0	1
0	1	0	1	0		1	0
0	1	1	0	1		0	0
0	1	1	1	1		0	0
1	0	0	0	0		0	1
1	0	0	1	0		0	1
1	0	1	0	0		1	0
1	0	1	1	1		0	0
1	1	0	0	0		0	1
1	1	0	1	0		0	1
1	1	1	0	0		0	1
1	1	1	1	0		1	0

34) What is priority encoder?

A priority encoder is an encoder circuit that includes the priority function. In priority encoder, if 2 or more inputs are equal to 1 at the same time, the input having the highest priority will take precedence.

35) Define modulus of a counter.

The number of unique states that a counter may have before the sequence repeats itself is the modulus of the counter. Example, Modulus 10 would have the counter with states 0-9 and then reset to zero.

36) What do you mean by excitation table?

An excitation table shows the minimum inputs that are necessary to generate a particular next state when the current state is known. They are similar to truth table and state table but rearrange the data so that the current state and next state are next to each other on the left-hand side of the table, and the inputs needed to make that state change happen are shown on the right side of the table.

37) Compare various bipolar logic families in terms of speed, power dissipation, fan-in and fan-out .

38) Convert S-R to J-K ff and vice versa.

39) What is classification of D/A converter?

i. Binary weighted DAC

ii. R-2R ladder DAC

40) Give various types of A/D converter.

i. Flash-type ADC

ii. Single-slope ADC

iii. Dual-slope ADC

iv. Continuous counter type ADC

v. Successive-approximation type ADC.

41) Define various specifications of A/D and D/A converter.

Resolution:
This is the smallest possible change in output voltage as a fraction or percentage of the full-sale output range. For example, for an 8 bit converter, there are 28 or 256 possible values of analog output voltage. Hence the smallest change in the output voltage is 1/255th of the full scale output range. Its resolution is one part in 255, or 0.4 percent.

Linearity:
In a converter, equal increments in the numerical significance of the digital input should result in equal increments in the analog output voltage. In an actual circuit, the input-output relationship is not linear. This is due to the errors in resistor values and voltage across the switches.

Accuracy:
The accuracy of a converter is a measure of the difference between the actual output voltage and the expected output voltage. It is specified as a percentage of full-scale or maximum output voltage.

Setting time:

It is the time required for the analog output to settle within (1/2) LSB of the final value after a change in the digital input.

Temperature sensitivity:
The analog output voltage for any fixed digital input varies with temperature. This is due to the temperature sensitivities of the reference voltage source, resistors, OP-AMP etc.

Speed:
The speed of converter is defined as the amount of time required to settle to a particular accuracy.

42) Differentiate b/w truth table, state table and characteristic table?

STATE TABLE

A table, which consists time sequence of inputs, outputs and flip-flop states, is called state table. Generally it consists of three section present state, next state and o/p.

TRUTH TABLE

A truth table is a tabular listing of all possible input value combinations for a logical function and their corresponding output values.

CHARACTERISTIC TABLE

An Characteristic table shows the minimum inputs that are necessary to generate a particular next state when the current state is known.

43) Postulate Boolean algebra.

Operations of 0and 1

· X + 0 = X

· X + 1 = 1

Idempotent laws

· X + X = X

Involution law:

· ( X' ) ' = X

Laws of complementarily:

· X + X' = 1

Commutative laws:

· X + Y = Y + X

Associative laws:

· (X + Y) + Z = X + (Y + Z)= X + Y + Z

Distributive laws:

· X( Y + Z ) = XY + XZ

44) Design EX-OR as inverter.

45) Design EX-or using minimum Nand gate.

46) What is the function of enable line in MUX?

The function of the enable input in a MUX is to control the operation of the unit.

47) Define parity method in error detection?

A parity bit is an extra 0 or 1 bit that is attached to the original signal and used to detect errors. There are two parity methods, even and odd. In the even parity method, the value of the bit is chosen so that the total number of 1s in the transmitted signal, including the parity bit, is even. Similarly, with odd parity, the value of the bit is chosen so that the total number of 1s is odd. For example, for the following byte 11010000, the even parity bit would be 1, making the total number of 1s in the signal even, and the odd parity bit would be 0, making the total number of 1s in the signal odd.

48) What are shift register in counter circuit? Give types.

A register capable of shifting its binary information either from right to left or left to right is known as shift register. It consists of flip-flops connected is cascade. All flip-flops receive a common clock pulse which causes the shift from one stage to the next stage.

It is of four basis types:

i. Serial in serial out register

ii. Serial in parallel out register

iii. Parallel in serial out register

iv. Parallel in parallel out register.

49) Compare ring counter with Johnson counter.

Ring counter:

A ring counter is a circular shift register with only one flip flop being set, at any particular time, all others are cleared.

Johnson counter:

The Johnson counter is a K-bit switch-tail ring counter with

2^k decoding gates to provides outputs for 2^k timing signal.

50) What are the applications of register, counter and flip flop?

COUNTERS

i. In oscillators

ii. In RF transmitter

iii. In audio devices

SHIFT REGISTERS

i. It is used in ring counters.

ii. It is used in sequence generators.

iii. It is used for data conversion in computers i.e. serial to parallel, parallel to serial etc.

iv. It is used for time delays i.e. serial in serial out shift register are used for this purpose.

v. It is used in Johnson counter / twisted ring counter.

FLIP FLOP

i. Used as a memory Element.

ii. Used as a Delay Element.

iii. Used as a basic building block in sequential circuits such as counters and registers.

iv. Data Transfer.

v. Frequency Division & Counting.

51) Applications of de-multiplexer?

· It finds its application in Data transmission system with error detection.

· One simple application is binary to Decimal decoder.

52) Explain how race around condition can be avoided in F/F?

A master-slave J-K flip-flop is constructed from two flip-flops. One flip-flop acts as a master and the other as a slave and the overall circuit is thus, called as master-slave flip-flop. It make use of J-K master flip-flop and S-A slave flip-flop. The master is positive edge triggered and slave is negative edge triggered. Therefore, master responds to J-K inputs before the slave. V

If J = 1 and K = 0, the master sets on the positive clock edge. The high output of the master drives the J input of the slave, so when the negative clock edge arrives, the slave sets, copying the action of the master.

If J = 0 and K = 1, the master resets on the positive clock edge. The high output of master i.e. Qm goes to R input of the slave. Therefore, slave resets on arrival of negative clock edge.

If J = K = 1 for master, it toggles on positive clock edge and the slave them toggles on the negative clock edge.

When J = K = 1

Let clock = 1 then master is active and slave is in active Therefore, output of master

toggle. So S and R also will be inverted

When clock 0 Master becomes in active and slave is active Therefore, output of the

save will toggle These changed outputs are again returned back to the master inputs as feedback is connected in fig

But here clock is ‘0’, the master is still in active So it does not respond to these changed Outputs.

FAQ (Microprocessor)

BY:-

SHASHIKANT RAJORIA

MILAN GOEL
1.What is Microprocessor?

It is a program controlled semiconductor device (IC}, which fetches,

decode and executes instructions.

2.What are the basic units of a microprocessor?

The basic units or blocks of a microprocessor are ALU, an array of

registers and control unit.

3.What is Software and Hardware?

The Software is a set of instructions or commands needed for

performing a specific task by a programmable device or a computing

machine.

The Hardware refers to the components or devices used to form

computing machine in which the software can be run and tested.

Without software the Hardware is an idle machine.

4. What is assembly language?

The language in which the mnemonics (short -hand form of

instructions) are used to write a program is called assembly language.

The manufacturers of microprocessor give the mnemonics.

5. What are machine language and assembly language programs?

The software developed using 1's and 0's

are called machine language,

programs. The software developed using mnemonics are called

assembly language programs.

6. What is the drawback in machine language and assembly language,

programs?

The machine language and assembly language programs are machine

dependent. The programs developed using these languages for a

particular machine cannot be directly run on another machine .

7.Define bit, byte and word.

A digit of the binary number or code is called bit. Also, the bit is the

fundamental storage unit of computer memory.

The 8-bit (8-digit) binary number or code is called byte and 16-bit

binary number or code is called word. (Some microprocessor

manufactures refer the basic data size operated by the processor as

word).

8. What is a bus?

Bus is a group of conducting lines that carries data, address and control

signals.

9.Why data bus is bi-directional?

The microprocessor has to fetch (read) the data from memory or input

device for processing and after processing, it has to store (write) the

data to memory or output device. Hence the data bus is bi-directional.

10.Why address bus is unidirectional?

The address is an identification number used by the microprocessor to

identify or access a memory location or I /O device. It is an output

signal from the processor. Hence the address bus is unidirectional.

11. What is the function of microprocessor in a system?

The microprocessor is the master in the system, which controls all the

activity of the system. It issues address and control signals and fetches

the instruction and data from memory. Then it executes the instruction

to take appropriate action.

12. What are the modes in which 8086 can operate?

The 8086 can operate in two modes and they are minimum (or

uniprocessor) mode and maximum ( or multiprocessor) mode.

13. What is the data and address size in 8086?

The 8086 can operate on either 8-bit or 16-bit data. The 8086 uses 20

bit address to access memory and 16-bit address to access 1/0 devices.

14. Explain the function of M/IO in 8086.

The signal

M/IO

is used to differentiate memory address and 1/0

address When the processor is accessing memory locations

MI is asserted high and when it is accessing 1/0 mapped devices it is asserted

low.

15. Write the flags of 8086.

The 8086 has nine flags and they are

1. Carry Flag (CF)

2. Parity Flag (PF)

3. Auxiliary carry Flag (AF)

4. Zero Flag (ZF) 9. Direction Flag (DF)

5. Sign Flag (SF)

6. Overflow Flag (OF)

7. Trace Flag (TF)

8. Interrupt Flag (IF)

16. What are the interrupts of 8086?

The interrupts of 8085 are INTR and NMI. The INTR is general

maskable interrupt and NMI is non-maskable interrupt.

17. Write the special functions carried by the general purpose registers of

8086.

The special functions carried by the registers of 8086 are the following.

1. AX 16-bit Accumulator

2. AL 8-bit Accumulator

3. BX Base Register

4. CX Count Register

5. DX .Data Register

18. What is pipelined architecture?

In pipelined architecture the processor will have number of functional

units and the execution time of functional units are overlapped. Each

functional unit works independently most of the time.

19. What are the functional units available in 8086 architecture?

The bus interface unit and execution unit are the two functional units

available in 8086 architecture.

20. List the segment registers of 8086.

The segment registers of 8086 are Code segment, Data segment, Stack

segment and Extra segment registers.

21.Define machine cycle.

Machine cycle is defined as the time required to complete one

operation of accessing memory, I/O, or acknowledging an external

request. This cycle may consist of three to six T-states.

22. Define T-State

T-State is defined as one subdivision of the operation performed in

one clock period. These subdivisions are internal states synchronized

with the system clock, and each T-State is precisely equal to one

clock period.

23.What does memory-mapping mean?

The memory mapping is the process of interfacing memories to

microprocessor and allocating addresses to each memory locations.

24.Why EPROM is mapped at the beginning of memory space in 8085

system?

In 8085 microprocessor, after a reset, the program counter will have

OOOOH address. If the monitor program is stored from this address

then after a reset, it will be executed automatically. The monitor

program is a permanent program and stored in EPROM memory. If

EPROM memory is mapped at the beginning of memory space, i.e., at

OOOOH, then the monitor program will be executed automatically

after a reset.

25.What is DMA?

The direct data transfer between I/O device and memory is called

DMA. Direct memory access.

26. What is the need for Port?

The I/O devices are generally slow devices and their timing

characteristics do not match with processor timings. Hence the I/O

devices are connected to system bus through the ports.

27.What is a port?

The port is a buffered I/O, which is used to hold the data transmitted

from the microprocessor to I/O device or vice-versa.

28.What is the drawback in memory mapped I/0?

When I/O devices are memory mapped, some of the addresses are

allotted to I/O devices and so the full address space cannot be used for

addressing memory (i.e., physical memory address space will be

reduced). Hence memory mapping is useful only for small systems,

where the memory requirement is less.

29. How DMA is initiated?

When the I/O device needs a DMA transfer, it will send a DMA

request signal to DMA controller. The DMA controller in turn sends a

HOLD request to the processor. When the processor receives a HOLD

request, it will drive its tri-stated pins to high impedance state at the

end of current instruction execution and send an acknowledge signal to

DMA controller. Now the DMA controller will perform DMA transfer.

30. What is processor cycle (Machine cycle)?

The processor cycle or machine cycle is the basic operation performed

by the processor. To execute an instruction, the processor will run one

or more machine cycles in a particular order.

31. What is Instruction cycle?

The sequence of operations that a processor has to carry out while

executing the instruction is called Instruction cycle. Each instruction

cycle of a processor indium consists of a number of machine cycles.

32. What is fetch and execute cycle?

In general, the instruction cycle of an instruction can be divided into

fetch and execute cycles. The fetch cycle is executed to fetch the

opcode from memory. The execute cycle is executed to decode the

instruction and to perform the work instructed by the instruction.

33.What is Block and Demand transfer mode DMA?

In Block transfer mode, the DMA controller will transfer a block of

data and relieve the bus for processor. After sometime another block

of data is transferred by DMA and so on.

In Demand transfer mode the DMA controller will complete the entire

data transfer at a stretch and then relieve the bus to processor.

34. What is the need for timing diagram?

The timing diagram provides information regarding the status of

various signals, when a machine cycle is executed. The knowledge of

timing diagram is essential for system designer to select matched

peripheral devices like memories, latches, ports, etc., to form a

microprocessor system.

35. How many machine cycles constitute one instruction cycle in 8085?

Each instruction of the 8085 processor consists of one to five machine

cycles.

36. Define opcode and operand.

Opcode (Operation code) is the part of an instruction / directive that

identifies a specific operation.

Operand is a part of an instruction / directive that represents a value

on which the instruction acts.

37. What is opcode fetch cycle?

The opcode fetch cycle is a machine cycle executed to fetch the opcode

of an instruction stored in memory. Every instruction starts with opcode

fetch machine cycle.

38. Why status signals are provided in microprocessor?

The status signals can be used by the system designer to track the

internal operations of the processor. Also, it can be used for memory

expansion (by providing separate memory banks for program & data

and selecting the bank using status signals).

39. How the 8085 processor differentiates a memory access (read/write)

and I/0 access (read/write)?

The memory access and I/0 access is differentiated using 10

M signal.

The 8085 processor asserts 10

M low for memory read/write operation

and 10

M is asserted high for 1/0 read/write operation.

40. What is interrupt acknowledge cycle?

The interrupt acknowledge cycle is a machine cycle executed by 8085

processor to get the address of the interrupt service routine in-order to

service the interrupt device.

41. What is Software interrupts?

The Software interrupts are program instructions. These instructions

are inserted at desired locations in a program. While running a

program, if software interrupt instruction is encountered then the

processor executes an interrupt service routine.

42. What is Hardware interrupt?

If an interrupt is initiated in a processor by an appropriate signal at the

interrupt pin, then the interrupt is called Hardware interrupt.

43. What is the difference between Hardware and Software interrupt?

The Software interrupt is initiated by the main program, but the

Hardware interrupt is initiated by an external device.

In 8085, the Software interrupt cannot be disabled or masked but the

Hardware interrupt except TRAP can be disabled or masked.

44.How clock signals are generated in 8085 and what is the frequency of

the internal clock?

The 8085 has the clock generation circuit on the chip but an external

quartz crystal or L C circuit or RC circuit should be connected at the

pins XI and X2. The maximum internal clock frequency of 8085A is

3.03 MHz.

45. What are the operations performed by ALU of 8085?

The operations performed by ALU of 8085 are Addition, Subtraction,

Logical AND, OR, Exclusive OR, Compare Complement, Increment,

Decrement and Left

Right shift

46. What is a flag?

Flag is a flip flop used to store the information about the status of the

processor and the status of the instruction executed most recently.

47. List the flags of 8085

There are five flags in 8085. They are sign flag, zero flag, Auxiliary

carry flag, parity flag and carry flag.

48. What is the Hardware interrupts of 8085?

The hardware interrupts in 8085 are TRAP, RST 7.5, RST 6.5 and

RST 5,5. 41.

49. What is an ALE?

The ALE (Address Latch Enable) is a signal used to demultiplex the

address and data lines, using an external latch. It is used to enable the

external latch.

50. Explain the working of a handshake output port ?

In handshake output operation, the processor will load a data to port.

When the port receives the data, it will inform the output device to

collect the data. Once the output device accepts the data, the port will

inform the processor that it is empty. Now the processor can load

another data to port and the above process is repeated.

[1]

Multi-Tone Modulation

Interview Questions

Monday 13 May 2013

ECE FAQ's

Table of contents

FAQ (COMMUNICATION SYSTEMS)

Figure 2.1 A 1-kHz single-tone modulating signal

Figure 2.2 A 10-kHz carrier wave

Figure 2.3 A DSBFC AM signal

Figure 2.4 An AM signal with modulating and carrier signals shown

FAQ (CONTROL SYSTEM)

FAQ (DIGITAL ELECTRONICS)

FAQ (Microprocessor)

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