Pulse Modulation Techniques Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.