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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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