The block diagram of FM Receiver in the presence of noise is as follows
The incoming signal at the front end of the receiver is an FM signal got interfered by Additive noise , since the FM signal has a transmission band width ,the Band Pass filter characteristics are also considered over the band of interest i.e from to .
The output of Band Pass Filter is is passed through a Discriminator for simplicity simple slope detector (discriminator followed by envelope detector) is used, the output of discriminator is this signal is considered over a band of by using a LPF .
The input noise to the BPF is n(t), the resultant output noise is band pass noise
phasor representation of Band pass noise is where and .
are orthogonal, independent and are Gaussian.
– follows a Rayleigh’s distribution and is uniformly distributed over . are separate random processes.
substituting Equations (1), (3) in (2)
now the analysis is being done from it’s phasor diagram/Noise triangle as follows
is the resultant of two phasors and .
is the phase of the resultant signal and when this signal is given to a discriminator results an output.
the second term in the Equation where – denotes noise after demodulation.
this can be approximated to , which is a valid approximation. In this approximation is Quadrature-phase noise with power spectral density over
the power spectral density of will be obtained from Equation (6) using Fourier transform property
the power spectral density functions are drawn in the following figure
, from Carson’s rule
the band width of v(t) has been restricted by passing it through a LPF.
To calculate Figure of Merit
Calculation of :-
output Noise power
The output signal power is calculated from tha is
From Equations(I) and (II)
Calculation of :-
input signal power
noise signal power
from Equations (III) and (IV)
Now the Figure of Merit of FM is
to match this with AM tone(single-tone) modulation is used i.e, then the signal power and
since for tone(single-tone) modulation .
when you compare single-tone FM with AM
the modulation index will be beneficial in terms of noise cancellation, this is one of the reasons why we prefer WBFM over NBFM.