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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 1 Examinations 2011/
Module Title: Communications Systems 1
Module Code: ELTR
School: Electrical and Electronic Engineering
Programme Title: BEng in Electronic Engineering
Programme Code: EELXE_7_Y
External Examiner(s): Dr A. Donnellan, Mr I. Kennedy Internal Examiner(s): Dr O. Gough
Instructions: Answer any four questions
Duration: 2 Hours
Sitting: Autumn 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) Briefly explain the terms (i) White Noise (ii) Flicker Noise [10 marks]
(b) Two amplifiers have the characteristics: G 1 = 8 dB, F 1 = 2.1 dB G 2 = 20 dB, F 2 = 8 dB Determine the overall power gain and overall noise figure for the system. [10 marks] (c) A sensor with resistance 100 kΩ is used in a cryogenic system at a temperature of 77 K. If the measurement bandwidth is 50 MHz determine the open circuit noise voltage disturbing the measurement. [5 marks]
Q2. (a) (i) Sketch a diagram of a diode detector and using waveform sketches, explain its operation. [10 marks] (ii) A diode detector may suffer from Negative Peak Clipping, explain how this arises and the limitation it imposes on the modulation index. [6 marks]
(b) An AM station transmits an unmodulated carrier power of 40 kW. (i) What is the maximum permitted power for this system? [4 marks] (ii) If it uses an average modulation depth of 40%, how much power will be transmitted? [5 marks]
Q5. (a) Explain the terms (i) Antenna Gain (ii) Free Space Path Loss as they can be applied to antennas. [10 marks]
(b) A WiMAX service operating at a frequency of 3.5 GHz is to be established over a cell of 30 km radius The receiving antenna has a gain of 6 dB and transmit antenna has gain of 18 dB. If the transmit power is 25dBm, determine the received power. [15 marks]
Q6. (a) (i) Why are standards important to Communications Engineers? [5 marks] (ii) Briefly outline the principal functions of the divisions of the ITU. [5 marks] (b) The ITU recommendation K.52 is used as a guide to compliance with limits on human exposure to electromagnetic radiation. Explain the following concepts with reference to this recommendation. (i) Normally Compliant and Inherently Compliant Sources [10 marks] (ii) Controlled/Occupational Exposure [5 marks]
Useful Formulae for Communications Systems 1
AM Wave v am ( t ) V c Vm sin( mt ) sin( ct ).
Power in AM wave:
m^2 PT PC
Total modulation index: mt m 12 m 22 m 32
Diagonal Clipping:
m
RC ^1 m^2
Negative Peak Clipping: DC
load AC R
R
R
m RR
FM Wave:
( ) sin^ cos( t ) f
v t V t f m m
fm c c c^
Carson’s Rule: B 2 ( mf 1 ) fm
Image Frequency Rejection Ratio:
si
s s
si f
f f
f
IRR Q
Antenna Gain: 2
A (^) eff G
Friis Formula: 2 4 r
PGA
PR T eff
Noise Figure of Cascade: 2 1
3 1
G G
F
G
F F F
