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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Autumn Examinations 2008/
Module Title: Communications Systems 2
Module Code: ELTR 7008
School: Electrical and Electronic Engineering
Programme Title: Bachelor of Engineering in Electronic Engineering – Award
Programme Code: EELXE_7_Y
External Examiner(s): Mr. D. Deneiffe Dr. P. O’Sullivan Internal Examiner(s): Dr. O. Gough
Instructions: Answer four Questions All questions carry equal marks.
Duration: 2 Hours
Sitting: Autumn 2009
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 difference between natural and flat top sampling using time and frequency diagrams. Are there any forms of distortion present and if so explain? [12 marks]
(b) A signal
Vm ( t )= 1. 5 cos ω 0 t + 3 cos 2. 5 ω 0 t
is sampled by a pulse train represented by the following series,
∞
0
() cos n
St an n ω st
where f 0 is 5kHz.
(i) Determine the minimum sampling rate for the given signal. [2 marks] (ii) Show that the modulated signal consists of multiple side-frequency pairs. [4 marks] (iii) Draw the amplitude spectrum of the sampled signal when the sample rate is twice the minimum rate. [4 marks] (iv) What is the cutoff frequency required for the data recovery filter in this case? [3 marks]
Q2. (a) Explain the use of companding in PCM systems. [10 marks]
(b) Show that the quantization noise power for a linear PCM system is given by 12
∆^2
Nq = [10 marks]
(c) A stereo PCM codec is required for an audio mastering application. If the audio signal is to be band limited to 20 kHz, and the dynamic range must exceed 120 dB determine the resulting data rate for a linear converter operating at the Nyquist rate [5 marks]
Q6. (a) Draw block diagrams for an FSK transmitter and receiver and explain their operation. [12 marks] (b) An FSK signal consists of bursts of frequency of 800 kHz and 900 kHz with the higher frequency used to transmit a mark and the lower to transmit a space. The bit rate is two kbps. Sketch the spectrum of the signal and calculate the bandwidth. [7 marks] (c) A radio channel is implemented with a raised cosine response having a roll-off factor of 0.4 and absolute bandwidth of 850 kHz. If 16 QAM is used to transmit 8 bit samples, determine the maximum sample rate and the maximum signal bandwidth. [6 marks]
Useful Formulae for Communications Systems 2
Nyquist Sampling: fs =2B
SQNR for sinusoid ( ) 6. 02 1. 76
SQNRdB N b
SQNR M
SQNR for uniformly distributed signal SQNRdB N b
SQNR M M
Slope overload onset: f s
∆ ≥^2 π fA
Delta Modulator SQNR f B
f N
S (^) s q 2
3 8 2
π
Fourier coefficients (spectral amplitudes) of rectangular pulse train.
sin 2 0
0
n
n
T
a V n
