GW Instek GRF-1300A - Manual (Page 75)

RF Communication and Signals Experiments
carrier frequency. The amplitude of each component depends on
the Bessel functions.
Theoretically, FM bandwidth is infinite, but the energy of an FM
signal is mainly concentrated near the carrier frequency. The
sidebands of the FM signal only contain a small amplitude
component and are generally ignored in practice by engineers.
Provided that the amplitude at the sidebands is negligible, less than
10%, we can get the FM wave band as follows:
B=2(m
f
+1)F
From above analysis
Because
Therefore B=2(Δ F + F )
When Δ F >>F, it is wide band modulation,
m
f
>>1, B ≈ 2Δ F
When Δ F <<F, it is narrow band modulation,
m
f
<<1, B ≈ 2 F
The amplitude of the sideband components in an FM signal is
related to the frequency modulation index. This can be seen in the
comparison table in the appendix. Below we have a few examples
of the absolute magnitudes of the sidebands for signals with a
modulation index of 0.5, 2.41 and 5.
=0.5 m =2.41 m =5 m f f
f
0.5
The FM circuit in the GRF-1300A uses a phase-locked loop.
Using a PLL circuit for FM modulation not only solves the center
frequency stability problems in direct FM modulation but also the
narrow FM range limitations when using a crystal oscillator.
The spectrum of the modulating signal must be outside the of
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