Pulse Modulation Techniques Questions and Answers

Answer: C

The displacement in time carries the message, with amplitude and width both held constant, so the scheme shares width modulation's noise immunity and improves on its power efficiency. The other three parameters are all fixed in this scheme. Position modulation puts everything into timing, which is why it needs accurate synchronisation and why it tolerates amplitude noise so well.

Answer: D

Amplitude modulation is the vulnerable one, because noise adds directly to the quantity carrying the information and no amount of processing can separate them. Width and position modulation hold amplitude constant and so allow the receiver to limit or threshold the pulses. The three are certainly not equal. The rule generalises across all modulation, pulse or continuous: a scheme that carries information in amplitude inherits amplitude noise. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Answer: A

Because every pulse is short and identical, the average transmitted power is low and does not vary with the modulation, whereas a width-modulated pulse train's power rises with the average pulse width. The repetition rate is the same for both, being the sampling rate. Amplitude requirements are comparable. And position modulation needs more synchronisation, not less, which is its principal cost. Cheaper on power and dearer on timing is the trade between these two schemes.

Answer: B

Position modulation measures displacement against a reference, so the receiver must know precisely where each slot begins, which means a synchronising pulse or frame marker. Its amplitude is constant, so it does not admit amplitude noise. Its bandwidth is comparable with width modulation's. And it multiplexes perfectly well. A scheme that encodes into timing necessarily buys a dependence on a timing reference, and that dependence is its weakness.

Answer: C

Since a width-modulated pulse's trailing edge already moves with the modulation, a fixed narrow pulse triggered from that edge gives position modulation directly. Rectifying and smoothing recovers the original signal instead. Clipping normalises amplitude without changing the encoding. And resampling would introduce a fresh set of problems. Noticing that one scheme's waveform already contains another's information is what makes these conversions so simple in practice.

Answer: D

The average value of a width-modulated train is proportional to the modulation, so a low-pass filter recovers the signal directly - which is why the scheme is so popular in power control as well as communication. Differentiation extracts the edges, useful for conversion to position modulation. A ramp comparison is how the signal is generated rather than recovered. And counting pulses recovers nothing, since the rate is constant. The simplest demodulator is often just an average, if the encoding has put the information into one.

Answer: A

Sampling leaves long gaps between a channel's pulses, and multiplexing fills those gaps with the pulses of other channels. Using unused spectrum is frequency division multiplexing, the alternative approach. Amplitude differences and information redundancy are not what the technique relies on. Every multiplexing scheme finds a resource the single channel leaves idle, and here that resource is time.

Answer: B

A frame is one complete cycle through every channel, so it contains one sample from each and repeats at the sampling rate. The synchronising interval usually coincides with the frame but does not define it. A group belonging to one channel is not a frame. And the channel capacity is a specification rather than a time interval. Frame structure is what turns a pulse train into an addressable set of channels.

Answer: C

Guard time absorbs pulse spreading and timing jitter so that one channel's energy does not appear in its neighbour's slot, which would be crosstalk. Gain control settling, synchronisation and amplitude equalisation are separate matters, and synchronisation has its own slot rather than living in the guard time. Any system that divides a resource into slots must leave margin at the boundaries, whether the resource is time or frequency. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: D

A channel restricted in bandwidth spreads its pulses in time, so energy from one slot arrives during the next and appears as another channel's signal. Unequal amplitudes make no such transfer. Staggered sampling instants are inherent to the scheme and cause no crosstalk. And exceeding the frame's capacity is a design error rather than an impairment. In time division systems crosstalk is a bandwidth symptom, exactly as in frequency division systems it is a filtering symptom.