Pulse Modulation Techniques Questions and Answers

Q: Match each impairment with where it arises. List I A. Aliasing B. Quantisation noise C. Crosstalk List II P. At the sampler, from too low a rate Q. In the coder, from finite levels R. In the channel, from limited bandwidth Select the correct match.

The correct pairing is A-P, B-Q, C-R. Locating each impairment matters because it decides the remedy: a higher sampling rate or a better filter for the first, more bits for the second, and more bandwidth or longer guard time for the third. An impairment attributed to the wrong stage leads to an expensive change that fixes nothing.

Q: Arrange the stages of a pulse code modulation transmitter in order. 1. Anti-alias filtering 2. Sampling 3. Quantising 4. Encoding into bits Select the correct sequence.

The order is 1, 2, 3, 4, and each position is forced: filtering must precede sampling because aliasing is irreversible, quantising needs a sample to act on, and encoding needs a level to represent. A signal chain whose order can be reconstructed from the reasons for it never has to be memorised as a list.

Q: Three schemes are compared for the same message bandwidth. SchemeParameter variedNoise immunity PAMAmplitudePoorPWMDurationGoodPPMPositionGood Which conclusion follows from the table?

Reading down the table, the two schemes holding amplitude constant are the two with good immunity, which identifies the mechanism rather than merely ranking the schemes. Duration is a varied parameter rather than a lever on immunity, the bandwidth is held equal across the comparison, and narrowing the pulses would not remove amplitude modulation's dependence on amplitude. A table read for the property the winners share is more informative than one read as a ranking.

Q: A component above half the sampling rate reappearing as a lower frequency is called ______.

This is aliasing, and its defining property is that it cannot be undone, since the folded component occupies a frequency the wanted signal also uses. Quantisation is the rounding of a sample to a discrete level. Companding is the compression and expansion that improves small-signal resolution. And jitter is uncertainty in the timing of the sampling instants. Four impairments of a sampled system, each with a distinct cause and its own remedy.

Q: The idling pattern a delta modulator produces on a steady input is called ______ noise.

The one-step hunting about a constant value is granular noise. Slope overload is the opposite failure, when the input moves faster than the steps can follow. Quantisation noise is the general rounding error of any digital coder. And thermal noise comes from the hardware. Because granularity favours a small step and overload a large one, the step size has an optimum, which is precisely what adaptive schemes exploit. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: Which of the following is not an analogue pulse modulation scheme? Pulse code modulation Pulse amplitude modulation Pulse width modulation Pulse position modulation

Pulse code modulation is digital: its samples are quantised to discrete levels and encoded as bits, which is what allows regeneration along a route. The other three carry a continuously variable parameter and so accumulate impairments with distance. The quantisation step is the dividing line between the analogue pulse family and the digital one, and it is what every advantage of digital transmission rests on.

Q: How is pulse width modulation generated most simply?

A comparator fed with the message and a sawtooth switches whenever the two cross, so the output's duty cycle follows the message amplitude. That is why the same circuit appears in switching power supplies and motor drives as well as in communication. Differentiating extracts edges rather than creating them, multiplication by a pulse train gives amplitude modulation, and integrating with clipping describes neither. A comparator against a periodic reference is the standard way to turn an amplitude into a time, and recognising the pattern makes many pulse circuits immediately readable.

Q: How is pulse amplitude modulation generated?

A sample-and-hold circuit captures the message at each clock instant and holds it, which is flat-top sampling and the form nearly every practical system uses. The comparator against a ramp gives width modulation. A monostable triggered from an edge gives a fixed pulse rather than a proportional one. And counting between peaks measures frequency. Each pulse scheme has one characteristic circuit, and knowing which circuit produces which output is enough to identify an unlabelled block diagram.

Q: What does a monostable contribute to a pulse position modulator?

The monostable turns a moving edge into a short pulse of standard width, which is exactly what position modulation needs - all the information in the timing, none in the shape. The ramp belongs to the width modulator ahead of it, the slot clock comes from the multiplexer, and the holding capacitor belongs to a sample-and-hold. A stage that standardises everything except the one parameter carrying the message is what gives a scheme its noise immunity.

51.

Match each impairment with where it arises.

List I

A. Aliasing

B. Quantisation noise

C. Crosstalk

List II

P. At the sampler, from too low a rate

Q. In the coder, from finite levels

R. In the channel, from limited bandwidth

Select the correct match.

A.A-R, B-Q, C-P
B.A-P, B-R, C-Q
C.A-P, B-Q, C-R
D.A-Q, B-P, C-R

Answer: C

The correct pairing is A-P, B-Q, C-R. Locating each impairment matters because it decides the remedy: a higher sampling rate or a better filter for the first, more bits for the second, and more bandwidth or longer guard time for the third. An impairment attributed to the wrong stage leads to an expensive change that fixes nothing.

Discuss in Forum

52.

Arrange the stages of a pulse code modulation transmitter in order.

1. Anti-alias filtering

2. Sampling

3. Quantising

4. Encoding into bits

Select the correct sequence.

A.2, 1, 3, 4
B.1, 3, 2, 4
C.1, 2, 4, 3
D.1, 2, 3, 4

Answer: D

The order is 1, 2, 3, 4, and each position is forced: filtering must precede sampling because aliasing is irreversible, quantising needs a sample to act on, and encoding needs a level to represent. A signal chain whose order can be reconstructed from the reasons for it never has to be memorised as a list.

Discuss in Forum

53.

Consider the following pairs of scheme and its principal weakness.

Pulse amplitude modulation - Sensitivity to amplitude noise

Pulse position modulation - Dependence on accurate synchronisation

Pulse code modulation - Large bandwidth requirement

Delta modulation - Inability to be multiplexed

How many of the pairs given above are correctly matched?

A.Only three
B.Only one
C.Only two
D.All four

Answer: A

The first three are correctly paired and the fourth is not, so three match. Delta modulation multiplexes as readily as any other digital scheme; its real weaknesses are slope overload and granular noise, the two opposing consequences of a fixed step size. A proposed weakness that would rule a scheme out of practical use, when the scheme is in practical use, is worth doubting on that ground alone.

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54.

Three schemes are compared for the same message bandwidth.

Scheme	Parameter varied	Noise immunity
PAM	Amplitude	Poor
PWM	Duration	Good
PPM	Position	Good

Which conclusion follows from the table?

A.Amplitude modulation would improve if its pulses were made narrower
B.Constant amplitude is what confers the immunity
C.A longer pulse duration always improves the noise immunity obtained
D.Immunity depends on the message bandwidth rather than on the scheme

Answer: B

Reading down the table, the two schemes holding amplitude constant are the two with good immunity, which identifies the mechanism rather than merely ranking the schemes. Duration is a varied parameter rather than a lever on immunity, the bandwidth is held equal across the comparison, and narrowing the pulses would not remove amplitude modulation's dependence on amplitude. A table read for the property the winners share is more informative than one read as a ranking.

Discuss in Forum

55.

A component above half the sampling rate reappearing as a lower frequency is called ______.

A.companding
B.jitter
C.aliasing
D.quantisation

Answer: C

This is aliasing, and its defining property is that it cannot be undone, since the folded component occupies a frequency the wanted signal also uses. Quantisation is the rounding of a sample to a discrete level. Companding is the compression and expansion that improves small-signal resolution. And jitter is uncertainty in the timing of the sampling instants. Four impairments of a sampled system, each with a distinct cause and its own remedy.

Discuss in Forum

56.

The idling pattern a delta modulator produces on a steady input is called ______ noise.

A.slope overload
B.quantisation
C.thermal
D.granular

Answer: D

The one-step hunting about a constant value is granular noise. Slope overload is the opposite failure, when the input moves faster than the steps can follow. Quantisation noise is the general rounding error of any digital coder. And thermal noise comes from the hardware. Because granularity favours a small step and overload a large one, the step size has an optimum, which is precisely what adaptive schemes exploit. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

57.

Which of the following is not an analogue pulse modulation scheme?

Pulse code modulation

Pulse amplitude modulation

Pulse width modulation

Pulse position modulation

A.Pulse code modulation
B.Pulse amplitude modulation
C.Pulse width modulation
D.Pulse position modulation

Answer: A

Pulse code modulation is digital: its samples are quantised to discrete levels and encoded as bits, which is what allows regeneration along a route. The other three carry a continuously variable parameter and so accumulate impairments with distance. The quantisation step is the dividing line between the analogue pulse family and the digital one, and it is what every advantage of digital transmission rests on.

Discuss in Forum

58.

How is pulse width modulation generated most simply?

A.By integrating the signal and clipping the accumulated output
B.By comparing the signal against a ramp waveform
C.By differentiating the signal and rectifying the result
D.By multiplying the signal with a train of narrow pulses

Answer: B

A comparator fed with the message and a sawtooth switches whenever the two cross, so the output's duty cycle follows the message amplitude. That is why the same circuit appears in switching power supplies and motor drives as well as in communication. Differentiating extracts edges rather than creating them, multiplication by a pulse train gives amplitude modulation, and integrating with clipping describes neither. A comparator against a periodic reference is the standard way to turn an amplitude into a time, and recognising the pattern makes many pulse circuits immediately readable.

Discuss in Forum

59.

How is pulse amplitude modulation generated?

A.By triggering a monostable from each zero crossing of the message
B.By counting clock cycles between successive peaks of the message
C.By a sample-and-hold circuit clocked at the sampling rate
D.By comparing the message against a triangular reference waveform

Answer: C

A sample-and-hold circuit captures the message at each clock instant and holds it, which is flat-top sampling and the form nearly every practical system uses. The comparator against a ramp gives width modulation. A monostable triggered from an edge gives a fixed pulse rather than a proportional one. And counting between peaks measures frequency. Each pulse scheme has one characteristic circuit, and knowing which circuit produces which output is enough to identify an unlabelled block diagram.

Discuss in Forum

60.

What does a monostable contribute to a pulse position modulator?

A.A ramp against which the message can be compared
B.A clock defining the boundaries of each channel slot
C.A holding capacitor that retains the sampled value
D.A pulse of fixed width at a variable instant

Answer: D

The monostable turns a moving edge into a short pulse of standard width, which is exactly what position modulation needs - all the information in the timing, none in the shape. The ramp belongs to the width modulator ahead of it, the slot clock comes from the multiplexer, and the holding capacitor belongs to a sample-and-hold. A stage that standardises everything except the one parameter carrying the message is what gives a scheme its noise immunity.

Discuss in Forum

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