Re: [Discuss-gnuradio] Question on IF filter design for FM demodulation

[Matt Ettus]

> I am amazed that even this design (wfm_rcv.py) works:

It does, quite well.

> Before the downconversion at the FPGA the entire FM band (88-108 MHz) is
> present inside the useful spectrum (with the desired station centered at
> 5.75 MHz).

Only 6MHz of it is presemt, but that doesn't matter

> However, decimating at the FPGA by a factor of 250 will essentially
> spread the entire spectrum of the desired station (not a problem),
> but also all other (neighboring) stations will be aliased on top of the
> desired one. These stations will appear in the desired band completely
> unattenuated.

> It seems to me that before decimation at the FPGA, a LPF is required
> (probably in two stages as Matt suggested).
> Can this be done in the FPGA of the USRP?

When we say "decimation" we mean "decimation with proper filtering first". 
There are 8
CIC decimators and 8 CIC interpolators in the FPGA.

Matt