The Amplitude Modulation schemes like AM,DSB-SC and SSB-SC systems can not handle inherent Non-linearities in a really good manner where as FM can handle it very well.
Let us suppose un Modulated FM carrier
By considering un modulated FM carrier in terms of frequency(by neglecting phase) i.e has been interfered by a near by interference located at a frequency where is a small deviation from .
the nearby inerference is
when the original signal got interfered by this near by interference , the received signal is
now the phase of the signal is
As the demodulated signal is the output of a discriminator
, which is the detected at the output of the demodulator.
the detected output at the demodulator is in the absence of message signal i.e, .
i.e, when message signal is not being transmitted at the transmitter but detected some output which is nothing but the interference.
As ‘A’ is higher the interference is less at t=0 the interference is and is a linear function of , when is small interference is less. That is is closer to interference is less in FM.
Advantage of FM :- is Noise cancellation property , any interference that comes closer with the carrier signal (in the band of FM) more it will be cancelled. Not only that it overridden by the carrier strength but also exerts more power in the demodulated signal.
This is known as ‘Capture effect’ in FM which is a very good property of FM. Over years it has seen that a near by interference is 35 dB less in AM where as the near by interference in FM is 6 dB less this is a big advantage.
Two more advantages of FM over AM are:
- Non-linearity in the Channel ,FM cancels it very nicely due to it’s inherent modulation and demodulation technique.
- Capture effect( a near by interference) FM overrides this by .
- Noise cancellation.