ELEC 202 COMMUNICATION SYSTEMS MOCK EXAM PAPER 2017/18
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ELEC 202
MOCK EXAM PAPER 2017/ 18
COMMUNICATION SYSTEMS
1.
a) Draw the block diagram of an FSK modulator, sketching the signals in the time domain at the output of each block.
b) Distinguish between simplex, half-duplex and full duplex communication protocols.
c) Sketch the spectrum of an AM signal with modulation index 0.5, and explain why this form of modulation is also referred to as DSB-LC.
d) Envelope detectors are commonly used to demodulate AM signals.
Explain whether this is a form of coherent or non-coherent demodulation? Where else can envelope detectors be used in communication systems.
e) Design a suitable envelope detector for the demodulation of a 5 kHz audio message modulated with a 100 kHz carrier, making particular reference to any component values selected.
Total 25
2. A musical performance containing frequencies of up to 15 kHz is digitised for PCM transmission along an optic fibre.
a) The signal is sampled at a rate of 44.1 kHz, and then quantised into 65,536 levels and subsequently binary coded for PCM transmission. State whether this is critical sampling, over-sampling or under-sampling, and mention any advantages or disadvantages of this choice of sampling rate. Sketch the spectrum of the sampled waveform.
b) Determine the minimum number of binary digits required to encode one sample, and the minimum number of binary digits per second required to encode this audio signal.
c) Suggest how this digitisation process could be modified to increase the sensitivity to lower amplitudes, but without increasing the bandwidth required to transmit the signal.
d) Compare TDM and FDM, and briefly discuss which would be more suitable for bandpass communication.
e) Give 3 examples of line coding formats used for baseband pulse communication, commenting on at least one feature of each.
Total 25
3. The FM output signal of an angle modulator is given as:
V(t) = 20cos(1.9 ´108 πt + 2π sin1000πt)
b) Determine the carrier frequency.
c) Estimate the effective frequency bandwidth of this signal.
d) Is this narrow-band FM or wide-band FM?
e) If the same signal V(t) were instead the PM output of an angle modulator, what would the original message signal be?
Total 25
4. a) Draw the block diagram of an FSK demodulator, sketching the signals in the time domain and frequency domain at the output of each block.
b) List four bandpass modulation schemes, and four baseband transmission formats.
c) An analogue audio signal of bandwidth 10 kHz signal is transmitted in the following manners:
- It is modulated using DSB-SC with a particular carrier frequency and transmitted wirelessly
- It is suitably sampled and digitised using 8 bits and then modulated using binary ASK at the same carrier frequency, and transmitted wirelessly.
Calculate the minimum bandwidth required to transmit each signal.
d) An ASK-modulated waveform can be coherently demodulated by multiplying it by a local carrier, and then low-pass filtering. Show how the demodulated waveform will be distorted if the local carrier suffers a frequency error of Δw compared to the original carrier frequency.
Total 25
2023-08-29