5. In an FM system when the AF is 500Hz and the AF voltage is 2.4 V, the
deviation is 4.8 KHz. If the AF voltage is now increased to 7.2 V, what is
the new deviation? .If the AF voltage is raised to 10 V, while the AF is
dropped to 200 Hz, what is the deviation? Find the modulation index in
each case.
When Am= 10V, Δf= 2 X 10 = 20 kHz.
Note that the change in modulating frequency made no difference to the
deviation since it is independent of the modulating frequency.
So modulation indices
β1 = Δf1/fm1 = 4.8/0.5 = 9.6
β2 = Δf2/fm1 = 14.4/0.5 = 28.8
β3 = Δf3/fm2 = 20/0.2 = 100
The modulating frequency change have to be taken into account in the
modulation index calculation.
