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## Re: [Discuss-gnuradio] FM demodulation of a real signal

 From: Robert McGwier Subject: Re: [Discuss-gnuradio] FM demodulation of a real signal Date: Wed, 05 Apr 2006 16:56:26 -0400 User-agent: Thunderbird 1.5 (Windows/20051201)

It will not be possible to recover the signal if the real signal is at zero IF (and predetection) so we will assume that it is not at zero.
```
```
The simplest possible FM detector is a zero crossing counter. That is, you output a signal that is proportional to the distance between zero crossings. After filtration, this will provide a usable FM detection. It works much better if you do a bandpass filter first, count distance between zero crossings and then lpf the "distance" signals. The zero crossing should not be the number of samples.
```Given a sample N and N+1 where we have a zero crossing between,  one can do

(S(N+1)-S(N))
---------------     = slope
1

is the slope and the zero crossing then is the solution to:

slope*Xnew + S(n) = 0

or

```
Xnew = - S(n)/slope. Where we have decided to find the place where the connecting line crosses.
```
Xnew - Xold is the zero crossing distance and is output as the detected FM.

now set

Xold = Xnew and go do it again.

Bob

Eric Blossom wrote:
```
```On Wed, Apr 05, 2006 at 03:44:59PM +0200, Matteo Campanella wrote:
```
```Hello, I am quite stuck at a problem, that is, how to demodulate a FM
signal that is NOT in complex form using existing gnuradio blocks. The
quadrature demodulator works only with complex input; I have tried to
build an analytic signal out of my real one, but the result of the
demodulation is then quite poor.

Any suggestions?
Matteo
```
```
Is your real signal at a non-zero IF?

If so, just multiply by a complex sinusoid to translate to baseband,
then use existing code path.  We come from the complex is simple
school of signal processing...

```
```

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```