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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2011/
Module Title: Communications Systems 2
Module Code: ELTR
School: Mechanical, Electrical and Process Engineering
Programme Title: BEng in Electronic Engineering – Award BEng (Hons) in Electronic Systems Engineering – Year 3
Programme Code: EELXE_7_Y EELES_8_Y
External Examiner(s): Dr A. Donnellan Mr I Kennedy Internal Examiner(s): Dr O. Gough
Instructions: Answer four Questions.
Duration: 2 Hours
Sitting: Summer 2012
Requirements for this examination:
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.
Q1. (a) Explain the following terms
(i) Nyquist Rate. [4 marks] (ii) Natural Sampling. [3 marks] (iii) Reconstruction Filter. [3 marks] (iv) Aliasing. [4 marks]
(b) The following waveform is sampled using Flat-top sampling.
Vm ( t ) 5 cos 2 250 t
The sampling rate is fS = 1000 Hz with a pulse duty cycle of 20%. (i) What is the Minimum Sampling rate for this signal as determined by the Nyquist Criterion. [2 marks] (i) Sketch the amplitude spectrum of the sampled waveform. [6 marks]
(iii) What is the cut-off frequency required for the reconstruction filter? [3 marks] Total: 25 marks
Q2. (a) Draw a block diagram of a linear PCM encoder and briefly explain the function of each part of your diagram. [10 marks] (b) Show that the quantization noise power in a uniform quantizer of step size is given by
^2
Nq . State any assumptions used.
[10 marks] (c) A sampled data converter is required for a linear position sensor used in an adaptive suspension. A low pass filter with a cutoff frequency of 180 Hz is used to condition the sensor output. The system is required to convert signals with a dynamic range (SQNR) of 60 dB. Determine the word length and data rate required. [5 marks] Total : 25 marks
Q5. (a) Explain why NRZ signalling is undesirable in an AC coupled system. Select an alternative signalling scheme and explain using time and frequency plots how it alleviates some of these problems. [10 marks] (b) Draw a clearly labelled block diagram of a clock recovery system suitable for use with Manchester coded data. Explain the principle of operation of the system you have chosen. [9 marks] (c) Clock synchronisation is assisted by using zero substitution codes. Give an example of one such code and explain the encoding procedure. [6 marks] Total : 25 marks
Q6. (a) Draw block diagrams for a Binary FSK transmitter and receiver and briefly explain their operation. [12 marks] (b) Draw the constellation of a QPSK transmission system and explain how noise can lead to symbol errors. What is the phase margin associated with such a system? [6 marks] (c) A communications channel is required to have a transmission capacity of 100 kb/s Determine the bandwidth necessary when using QPSK and 16-QAM modulators assuming raised cosine filtering with roll-off factor of 0. [7 marks] Total : 25 marks
Useful Formulae for Communications Systems 2
Nyquist Sampling: fs =2B
SQNR for sinusoid
SQNRdB N b
SQNR M
SQNR for uniformly distributed signal
SQNRdB Nb
SQNR M M
Slope overload onset:
fs
^2 fA
Delta Modulator SQNR for sinusoid.
f B
f
N
S s
q 2
3
Fourier coefficients (spectral amplitudes) of rectangular pulse train.
sin
0
0
n
n
T
a V
n
