When an FM system is compared to an AM system with a modulation index
of 1 operating under similar noise conditions, then it can be shown
that the FM signal has a signal to noise ratio which is 3m2
better than the FM system. m here is the modulation index or deviation
ratio for the FM signal.
Preemphasis Transfer Function
In an FM system the higher frequencies contribute more to the noise than
the lower frequencies. Because of this all FM systems adopt a system of
preemphasis where the higher frequencies are increased in amplitude
before being used to modulate the carrier. The transfer function
sketched above is used for a preemphasis circuit for FM signals in
the FM band. The Time T = 75µs. For FM systems in the FM band
m ~ 5 resulting in a S/N improvement of 19dB. With preemphasis this
can be increased by 4dB for a total of 23dB.
Deemphasis Transfer Function
At the receiver the higher frequencies must be deemphasised in order
to get back the origunal baseband signal. The transfer function of the
deemphasis circuit is shown above.
Passive circuits are shown below. For deemphasis RC = 75µs. For
the Preemphasis CR1 = 75µs. To get unit
gain at low frequencies an opamp with gain(R1 + R2)/R2
must be added. The pole should be set at a frequency higher than the
maximum modulating frequency.
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COPYRIGHT © 1996, 2012 Cuthbert Nyack.