PAPER CODE....................ELEC202...............PAGE....1........OF..............8..........................CONTINUED PAPER CODE NO. EXAMINER: Dr W Al-Nuaimy ELEC 202 DEPARTMENT: EE&E TEL. NO: 44512 SECOND SEMESTER EXAMINATIONS 2017/18 COMMUNICATION SYSTEMS TIME ALLOWED: Two Hours INSTRUCTIONS TO CANDIDATES The numbers in the right hand margin represent an approximate guide to the marks available for that question (or part of a question). Total marks available are 100. Answer ALL Questions. The use of a calculator IS allowed. PAPER CODE....................ELEC202...............PAGE....2........OF..............8..........................CONTINUED 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 PAPER CODE....................ELEC202...............PAGE....3........OF..............8..........................CONTINUED 2. Figure Q2 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 5v t 2v PAPER CODE....................ELEC202...............PAGE....4........OF..............8..........................END 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 PAPER CODE....................ELEC202...............PAGE....5........OF..............8..........................CONTINUED 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?)? Figure Q4 5 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 Question 4 continues overleaf. PAPER CODE....................ELEC202...............PAGE....6........OF..............8..........................END Question 4 continued. 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). Determine the number of bits per second resulting from the transmission of this audio signal. 5 Total 25 PAPER CODE....................ELEC202...............PAGE....7........OF..............8..........................END Q2 Solutions. a) There are 10 carrier cycles for each message cycle, so the message signal will have a bandwidth of 100kHz / 10 = 10 kHz. b) Modulation index = (5-2)/(5+2) x 100% = 43%. c) Percentage of power in upper sideband !" # $%"&$%' × 100% = 4.2% d) Any practical/suitable values for R and C such that 1/100k << RC << 1/10k e) The signal would need to be sampled at a rate of fs > 2x10kHz and quantised using n bits per sample. The bandwidth would then be in the region 2 x n x fs. If fs were chosen to be 30 kHz and n to be 8, the bandwidth would be in the region of 480 kHz. Q3 Solutions. a) A frequency error of 2Hz would introduce a 2Hz “beating” distortion to the recovered message, but would be more favourable than a π/2 phase error as this would completely attenuate the recovered signal to zero. b) A coherent demodulator could recover a message from DSB-LC modulation. Accept any mathematical working out to show multiplication of cosines and effect of low pass filter to block high frequency components. PAPER CODE....................ELEC202...............PAGE....8........OF..............8..........................END Q4 Solutions. a) For critical sampling (i.e. at 20 kHz) and a quantiser bit depth of n, the bit rate would 20,000 x n , and the bit duration would be 1/(20,000n). In Figure Q4, the bit duration is 5 µsec , from which n can be calculated to be 10 bits/sample. b) Power = amplitude2/2 = 32/2 = 4.5 c) This appears to be (B)PSK. Sinc-shaped spectrum centred at fc to include both sidebands. d) 180 kHz (b = 75/15 = 5, BW = 2x15x(1+b) e) n = log2(65536) = 16. Bit rate = 44.1k x 16 = 705.6 kbps































































































































































































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