Q. 1. How fm transmitter differ from AM
Transmitters?
Ans. The FM
transmitters have a very large bandwidth as compared to AM transmitter. In FM
transmission ground way1 and sky wave propagation is not possible, and signals
in VHF and UHF bands are prpagated by line of sist propagation, which restricts
the range upto 50 km.
Q. 2. What are the methods of
FM generation/modulaiotransmission?
Ans. We can
generate FM signals by using various FM modulators. The basic requirement of fm modulator (FM signals generator) is to provide a variable output frequency with
varying proportions to the instantaneous amplitude of the modulating voltage.
The FM signals can be generated by two methods
1. Direct Method. The FM signals can be generated directly by frequency
modulating the carrier
2. Indirect Methods. The FM singals can be generated indirectly by
integrating the modulating and signal and then allowing it to phase modulate the
carrier.
Or by using the modulating signal first to produce a narrow band FM
signal and then by frequency multiplication, the frequency deviation may be
increased to the desired level.
Q. 3. What are the limitations of direct modulation
methods?
Ans. The direct
methods of FM generation suffers from the following
limitations:
In the direct methods of FM generation, it is difficult to obtain a
high order of stability in carrier frequency. This is because the modulating
signal directly controls the tank circuit which is generating the carrier. The
crystal oscillator cannot be used as it provides a stable but fixed
frequency.
The non linearity produces a frequency variation due to harmonics of
the modulating signal hence there are distortions in the output FM
signal.
Q. 4. Explain principle of indirect
methods of FM generation modulation.
Give the indirect modulation methods.
Ans. Indirect
Methods of fm radio transmitter Generation. The indirect method removes the limitations of direct
method.
The indirect method employs the phase modulation to obtain FM signal.
It is only necessary to integrate the modulating signal prior to applying it to
the phase modulator. The general block diagram for such a method is shown in
figure 1. This is used in VHF and UHF radio telephony.
The indirect methods of FM generation/modulation are
(i) Armstrong method, (ii) RC phase shift method.
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