Cork Institute of Technology
Bachelor of Engineering (Honours) in Electronic Engineering – Stage 2
(Bachelor of Engineering in Electronic Engineering – Stage 2)
(NFQ – Level 8)
Summer 2005
Communications and Physical Electronics
(Time: 3 Hours)
Instructions
Answer FIVE questions, including at least TWO questions
from each Section.
Use separate answer books for each Section.
All questions carry equal marks.
Values for useful physical constants
q = |e | = 1.602 x 10-19 C me = 9.110 x 10 –31 kg
c = 2.998 x 108 m s-1 h = 6.626 x 10-34 J s
k = 1.381 x 10-23 J K-1 εo = 8.85 x 10-12 F m-1
Examiners: Professor C. Burkley
Mr. J. Ryan
Dr O. Gough
Mr. J. A. O’Doherty
Section A
Q1. (a) A possible technique for demodulating a DSB-SC signal is by coherent detection. Sketch
a diagram of such a detector and show that it will recover the modulating information. In
addition show that if the local oscillator has a phase error of θ, that the detected signal
voltage will be reduced by a factor of cos(θ). (12 marks)
(b) Show that, in order to avoid negative peak clipping in a diode detector, the maximum
permitted modulation index is given by
DC
AC
R
R
m≤
max . (8 marks)
Q2. (a) Explain how the Bessel zero method may be used to set the frequency deviation of an FM
system to a particular value. (4 marks)
(b) Draw a detailed circuit diagram of a Foster-Seeley Discriminator and explain how it can
be used to demodulate a frequency modulated signal. (10 marks)
(c) An FM modulator using a reactance FET is required to achieve a peak frequency
deviation of 50 kHz at a carrier frequency of 90 MHz. The inductor used in the oscillator
is 0.4 µH. Determine the total capacitance swing required of the reactance FET.
(6 marks)
