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

[Matt Ettus]

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 :)


Thanks,
Matt