Thread: FM "deviation"
View Single Post
Old 31st Mar 2010, 1:47 pm   #32
jimmc101
Heptode
 
jimmc101's Avatar
 
Join Date: Mar 2009
Location: Portsmouth, Hampshire, UK.
Posts: 674
Default Re: FM "deviation"

This is FM, nothing is that simple!
The carrier amplitude is not a simple function of the modulation index (m), it varies as a Bessel function.
At certain values of m the carrier disappears, the so called Bessel nulls, just past these nulls the carrier increases to some value before again decreasing to zero at the next null. The values of m for the first 3 nulls are 2.405, 5.520 & 8.654.
Incidentally this provides an accurate method of calibrating deviation if you have a spectrum analyser.
See for example http://cp.literature.agilent.com/lit...988-5677EN.pdf (hint 6)

Oops Dave beat me to it.
I should add that for FM the total power is constant so as Dave has said the total sideband power must come from a reduction in carrier power.

Jim

Last edited by jimmc101; 31st Mar 2010 at 1:52 pm. Reason: Last para
jimmc101 is offline