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Unformatted text preview: EEE206/274 Data Provided: None EEE206/EEE274
DEPARTMENT OF ELECTRONIC AND ELECTRICAL ENGINEERING
Spring Semester 2000—2001 (2 hours) EEE206: COMMUNICATION SYSTEMS
EEE274: SIGNALS AND COMMUNICATION SYSTEMS Answer THREE questions. No marks will be awarded for solutions to a fourth question.
Solutions will be considered in the order that they are presented in the answer book. Trial answers will be ignored if they are clearly crossed out. The numbers given after each section of a
question indicate the relative weighting of that section. 1. (a) Outline the reasons why vestigial sideband amplitude modulation is used for the
transmission of analogue television signals.
(b) An AM DSB CP transmitter has an output power of 1.5 kW when sinusoidally
modulated to a depth of 100 %. Determine the power output when
(i) the carrier is unmodulated
(ii) after sinusoidal modulation to a depth of 60 %, one sideband is
suppressed and the carrier component is reduced by 20 dB.
(c) Comment on the usefulness, or otherwise, of the scheme outlined in (b)(ii)
(2) (d) If the transmitter mentioned in (b) is sinusoidally modulated to a depth of 50 % and a
second sine wave, corresponding to 35 % modulation, is transmitted simultaneously,
determine the total radiated power. (5)
2. (a) Explain the difference between phase and frequency modulation.
(b) Describe, with the aid of diagrams, how a frequency-modulated signal may be
demodulated using a limiter circuit followed by a Foster-Seeley discriminator.
(7) (c) A voltage-controlled oscillator circuit contains a 15 pH inductor which is tuned by a
capacitor whose capacitance is a function of the applied control voltage. When the latter is
set to O V, the effective capacitance is 100 pF and this changes at 2 pF/V as the control
voltage is varied. Estimate the bandwidth of the oscillator output signal when the control
voltage has the form 5 cos(40007rt) V and the oscillator is followed by a x10 frequency
multiplying circuit. (10) EEE206/2 74 1 TURN OVER EEE206/274 3. (a) What are the advantages of digital communication systems over their analogue
u c (b) G1ve a comprehensive description of how a PCM signal made be generated from an
(3) (c) N binary coded telephone signals of 4kHz bandwidth, each having 8 bits per sample, are
time—multiplexed together. (i) If N = 12, ﬁnd the overall signal bandwidth.
(ii) If the frame length = 125 u sec, ﬁnd N. (iii) Why, in practice, are only 30 telephone signals sent in a 32 channel PCM frame ? (9) 4. (a) The maximum theoretical information-carrying capacity of a communication system is
given by the Hartley Shannon Law, i.e. S
C=Blog2[l+ﬁj (i) Deﬁne the various symbols contained in this formula. (ii) How well do practical communication systems obey this law? (4) (b) The transducer in a CD player sends out 216 equiprobable pulse amplitudes at a
sampling rate of 44.1 kHz. Estimate (i) the minimum bandwidth required
(ii) the channel capacity (iii) the signal to noise ratio of the recovered signal (6) (c) Since the highest frequency which can be heard by the human ear is less than
20 kHz, what purpose is served by oversampling in a CD player ? (2) (d) (i) Using the data listed below, estimate the minimum and maximum
bandwidths required to transmit a 625 line black and white TV picture if the received
signal-to-noise ratio is to be at least 30 dB. Picture aspect ratio = 4:3
Time for one line of the picture = 64 14 sec
Line ﬂyback time = 12.05 u sec (8)
EEE206/274 2 END OF QUESTION PAPER ...
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