ANAND INSTITUTE OF HIGHER TECHNOLOGY
KAZHIPATTUR-603 103
DEPARTMENT OF ECE
Monthly Test II
Staff Name
Subject/Code
Year/Sem
Branch
1.
2.
3.
4.
5.
6.
: S.Lakshmi Narayanan
Date of Exam: 11-03-2014
: Communication Theory/EC2252
: II/IV – ‘B’ Section
Time: 8.40am - 10.10am
: ECE
Total marks: 50
Part-A
(6*2=12)
Distinguish between Narrow band and wide band FM.
A carrier of frequency 100MHz is frequency modulated by a signal 20sin
(200∏x103t).What is bandwidth of FM signal if the frequency sensitivity of the
modulation is 25 KHz/Volt.
List the non linear effects in FM.
What is white noise? Give its characteristics.
Define thermal noise. Calculate thermal noise voltage across the simple RC circuit
shown with R=1KΩ and C=1µF at T=270C.
Part-B
[(16*3 = 48]
a) (i) . Explain the working principle of ratio detector with the help of neat circuit
and phasor diagrams, and also compare with foster-seeley discriminator.
(10)
(ii) A 100 kHz carrier i s frequency modulated to produce a peak deviation of 800
Hz.This FM signal is passed through a 3 by 3 by 4 frequency multiplier chain, the
output of which is mixed with an oscillator signal and the difference frequency
taken as the new output. Determine the frequency taken as the new output.
Determine the frequency of the oscillator required to produce a 100 kHz FM
output and the peak deviation of the output.
(06)
(or)
b) (i) Explain with a block diagram, how the PLL is used to demodulate the FM
wave.
(8)
(ii)An
angle
modulated
wave
is
described
by
an
equation
s(t)=10cos(2X106πt+10cos2000πt). Determine the power of the modulated signal,
maximum frequency deviation and Bandwidth.
(8)
7. a) (i) Write notes on shot noise and thermal noise.
(8)
(ii) A mixer stage has a noise figure of 20 dB and this is preceded by an amplifier
that has a noise Figure of 9 dB and an available power gain of 15 dB. Calculate the
overall noise figure referred to the input.
(8)
(or)
b) (i) Derive the expression for signal to noise ratio for DSB-SC system.
(8)
(ii) Consider two amplifiers are connected in cascade. First stage amplifier has
gain and noise figure as 10 dB and 2 dB. Second stage has noise figure of 3 dB.
Calculate total noise figure.
(8)
8. a) Derive the expresssion for narrwoband noise and also represent the narrow band
noise in terms of envelope and phase components.
(or)
b) Explain Narrowband noise. Show that a narrow band noise n(t) can be represented
in terms of its inphase and quadrature phase components.