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Re: [Discuss-Gnuradio] IF filter design for FM demodulation

From: Matt Ettus
Subject: Re: [Discuss-Gnuradio] IF filter design for FM demodulation
Date: Thu, 6 Jan 2005 15:53:53 -0800
User-agent: Internet Messaging Program (IMP) 4.0-cvs

Quoting Achilleas Anastasopoulos <address@hidden>:

> Dear all,
> I am looking at the FM demodulator example
> provided in the gnuradio examples (in the mc4020 directory)
> and I have a question on the
> the design parameters of the filter used to separate one
> FM station before demodulating:
> the sampling frequency is 20 Msps
> the desired station is centered at 5.75 MHz
> the designed filter (lowpass version)
> has a cutoff frequency at 250 KHz
> and a transition band of 800KHz
> The signal is filtered and IQ demodulated and then
> decimated by a factor of 125.

> It seems that this filter is too wide:
> an adjacent FM station (at 5.75 MHz + 200 KHz)
> is not attenuated at all and its spectrum (after decimation)
> is folded on the desired station spectrum.
> This is clearly seen in the simple example I attach.

> What am I missing here?

Nothing.  You are correct -- the filter is way too wide.  This was done to
reduce the computation load on the PC.  The correct way to would be to break
this up into 2 steps -- decimate by 25 in the first step, with a wide
transition, and then do another decimate by 5 with the desired passband.

We would definitely welcome a patch to do the above :)


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