Re: [Discuss-gnuradio] Measure FM frequency deviation

[Anders Hammarquist]

In a message of Thu, 25 Feb 2016 05:15:58 +0100, Tom Theisen writes:
>Could you please give me some hints on how I could implement this in 
>GNURadio? Modulation index would be nice to have, but I'm really after 
>the freq. dev.

Just a sketch of how I would do it: You need to find out the bandwidth
of the modulate FM signal, as well as the maximum frequency of the
modulating signal. So demodulate it, and average the spectrum of
both the modulated and demodulated signal for a while (how long a while
is will obviously depend on the signal. If it's a steady sine wave a
while is a cycle, if it's a broadcast FM station without RDS and other
subcarriers, probably a few minutes to get both speech and music).

Once you have that, figure the bandwidth (bw) of the FM signal, and the
maximum frequency (fmax) in the demudulated signal. The bandwith is
(approximately):

bw = 2*fmax + 2*dev

so deviation is:

dev = (bw - 2*fmax)/2

modulation index is just:

h = dev/fmax

HTH
/Anders