[Hackrf-dev] FM Receiver and offsetting frequency

[Jordan Kagan]

Greetings,

I was wondering why it is typical practice to select a center frequency
that is offset from your desired channel frequency or range of frequencies
when receiving a FM signal.  I've seen this in two particular cases:

The FM Receiver example from the "SDR with HackRF" lessons on
greatscottgadgets.com (attached image of the GRC block diagram).  In this
case the selected center frequency is selected 1.4MHz offset from the
desired channel frequency and then shifted to the channel frequency before
going into the low pass filter.

The RDS receiver example from gr-rds: https://github.com/bastibl/gr-rds
under apps/rds_rx.grc.  Here the received signal is fed into an FIR filter
that has a low pass filter tap.  The center frequency is chosen at a 250KHz
offset.

So in short I am wondering why one wouldn't just choose the center
frequency to be the FM channel frequency or a frequency in the range of
interest.

Thanks,

-Jordan
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