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

[Eric Blossom]

On Fri, Jan 07, 2005 at 06:34:16PM -0500, Achilleas Anastasopoulos wrote:
> 
> I am amazed that even this design (wfm_rcv.py) works:

A couple of other comments (from wfm_rcv.py):

    # usrp is data source
    src = usrp.source_c (0, decim)
    src.set_rx_freq (0, IF_freq)

set_rx_freq sets the center frequency of the digital downconverter.
decim sets the decimation factor of the CIC filter.

The downconversion is carried out in the FPGA.  In this example what
comes across the USB is a complex baseband signal at 256 kS/sec
containing only the FM station of interest.

In wfm_rcv_many.py, we feed multiple DDCs in the FPGA from the same
A/D and use them to extract the relevant stations.  The complex output
samples from each DDC are interleaved on the USB.  On the host side we
deinterleave them and pass them to the relevant software pipeline.

Eric