UNIT II
Two
marks questions
1.
Why
is frequency modulation preferred for voice transmission? (AU-MAY14)
Voice
signal requires the frequencies upto 10KHz. For such a small range of voice
frequencies FM does not have wide bandwidth. Hence FM is preferred.
2.
If
the maximum phase deviation in a PM system is 0.1 radian when a modulating
signal is applied is 10V. determine the value of phase deviation constant.
(AU-MAY14)
K=
Ѳ(t)/e(t) = 0.1/10 =
0.01 rad/volts.
3.
What
is the need for pre emphasis? (AU-MAY14)
The
noise has greater effect on higher modulating frequencies and less effect on
lower ones. Hence higher modulating frequencies should be artificially boosted,
so that effect of noise can be reduced. This is done by pre emphasis.
4.
Define
frequency discriminators. (AU-MAY11)
The
frequency discriminator produces the signal whose amplitude is proportional to
the deviation in the frequency of FM signal. Thus the discriminator is similar
to frequency to voltage convertor. It is used in FM detection. Balanced slope
detector is an example of frequency discriminator.
5.
What
do you understand by FM stereo multiplexing? (AU-MAY09)
FM
stereo multiplexing is used for stereo transmission. It is basically frequency
division multiplexing. It is used for FM radio broadcasting. The left and right
channel signal are used to generete sum and difference signal and carrier are
combined together and sent. Such FM multiplexed signal can be coherently
received by stereo as well as mono receiver.
6.
State
carson’s rule. (AU-MAY13)
It
is given by BW=s(δ + fmax).
7.
Differentiate
between narrowband FM and wideband FM. (AU-MAY11)
In
narrowband FM, the frequency deviation is very small. Hence the frequency
spectrum consists of two major sidebands like AM. Other sidebands are
negligible and hence they can be neglected. Therefore the bandwidth of
narrowband FM is limited only to twie of highest modulating frequency.
If
the deviation in carrier frequency is large enough so that other sidebands
cannot be neglected, then it is called wideband FM. The bandwidth of wideband
FM is calculated as per carson’s rule.
8.
What
are the advantages of FM over AM? (AU-MAY10)
a)
The
amplitude of FM is constant. It is independent of depth of modulation. Hence
transmitter power remains constant.
b)
Since
amplitude of FM is constant, the noise interference is minimm.
c)
The
depth of modulation have limitation in AM. But in FM the depth of modulation
can be increased to any value.
d)
Since
guardbands are provided n FM, there is less possibility of interference.
e)
Since
space waves are used in FM, the radius of propagation is limited to line of
sight. Hence it is possible to reuse frequency.
f)
Since
FM uses UHF and VHF ranges, the noise interference is minimum compared to AM.
9.
Draw
the simple schematic of a PLL demodulator. (AU-DEC13)
Refer
book
10. Can we call phase
modulation as time modulation? Relate. (AU-MAY11)
Time
modulation and phase modulation are not same. In PM phase shift takes place
with respect to amplitude. In time modulatio, the time component in both wt and
theta(t) must be modulated.
8
& 16 marks
1. Discuss in detail FM
stereo multiplexing (AU-DEC07) 8
marks
2. Explain the methods
of demodulation of FM signal (AU-DEC11, DEC13 and MAY 2011) 16 marks
3. With neat diagram
explain Armstrong method of generating FM signal (AU-MAY13 and NOV 2013) 10 marks
4. Mention any 5 points
to differentiate AM and FM wave (AU-MAY14) 3 marks
5. Differentiate
between narrowband and wideband FM. (AU – MAY13)3 marks
6. Derive the
expression for wideband FM in terms of Bessel functions. (AU-DEC12) 10
marks
7. Derive an equation for
FM signal. (AU-MAY13) 10
marks
8. Derive an expression
for spectrum of FM signal with single tone modulation using Bessel functions.(AU
–DEC11) 6 marks
9. With neat diagram
explain the operation of foster seelay discriminator for demodulating FM signal
(AU-MAY2012) 16 marks
10. With neat diagram
explain the operation of Ratio detector for demodulating FM signal (AU-MAY2012)
16 marks
11. With neat diagram
explain the operation of slope detector for demodulating FM signal (AU-MAY2011)
16 marks
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