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ELEC 202

SECOND SEMESTER EXAMINATIONS 2017/18

COMMUNICATION SYSTEMS

1.    a)     Give three characteristics of a good communication system, along with one trade-off (compromise) that may be necessary in the design of a communication system. 5

b)    Explain how the problem of aliasing arises in communication systems, and how anti-aliasing filters can be used to prevent this. Discuss whether such a filter should come before or after the sampling process. 5

c)    Discuss whether the quantisation process can ever be considered reversible, and  the  implications  this  has   for  the  communication  of  analogue information over a digital communications system. 5

d)    Distinguish between baseband and bandpass signals, commenting also on  the difference between baseband and bandpass communications. 5

e)    Explain the meaning of modulation, giving two compelling reasons why it   is necessary. 5

Total

25

2.

a)    The carrier signal used in the AM signal shown in Figure Q2 is of frequency 100 kHz. Find the bandwidth of the modulating message. 5

b)    Referring to Figure Q2 above, calculate the modulation index, expressing it as a percentage. 5

c)    What percentage of the total power of this AM signal is contained within the upper sideband? 5

d)    Design, using a circuit diagram, a suitable non-coherent demodulator for the above AM signal, suggesting suitable values for any components used. 5

e)    Design, using a block diagram and relevant calculations, a binary ASK

transmission system for the same baseband modulating signal in part (a) above. Using your design, estimate the bandpass bandwidth required for such a digital transmission. 5

Total

25

3.    a)     Draw a block diagram of a DSB-SC modulator and a coherent/synchronous detector, sketching the spectrum of the signal at each point. 10

b)    A coherent DSB demodulator suffers a sudden fault which introduces a

constant frequency error of 2 Hz in the carrier recovery circuit. What is    the effect on the receiver output? How would this compare with a π/2 rad phase error in the local carrier? 5

c)    Using mathematical expressions as necessary, discuss whether a coherent

demodulator could be used to recover the message signal from an AM (DSB-LC) signal. 5

d)    Compare the AM (DSB-LC) and WBFM modulation schemes in terms of  power and bandwidth efficiencies, range and vulnerability to noise. 5

Total

25

4.    a)     The digitally modulated signal shown in Figure Q4, of amplitude 3 V, is

the result of critically sampling a 10 kHz audio message, followed by a  uniform quantiser. What is the bit-depth of the quantiser (i.e. how many

bits are used to encode each sample?)?

b)    Referring to the signal described in part (a) above and shown in Figure Q4, determine the power of the modulated signal. 5

c)    What is the name of the digital modulation scheme shown in Figure Q4? Sketch the spectrum of this bandpass signal. 5

d)    An FM transmitter with a carrier frequency of 98 MHz has maximum frequency deviation of 75 kHz and reproduces audio signal up to 15 kHz. Find the minimum channel bandwidth required for this bandpass signal. 5

e)    A musical performance containing frequencies of up to 15 kHz is

digitised for PCM transmission along an optic fibre. Given the channel

can support a data rate of up to 800 kbps, design a suitable sampling and   quantisation scheme (by suggesting a suitable sampling rate and quantiser bit depth). 5

Determine the number of bits per second resulting from the transmission of this audio signal.

Total

25