[Hackrf-dev] FM Receiver and offsetting frequency

[Sukandar Kartadinata]

Hi,
I was wondering the same thing and found an answer in the FAQs: https://github.com/mossmann/hackrf/wiki/FAQ
I think the third&fourth apply to your question.
In short it’s about a DC offset.

However, I’m really new to this myself, so there might be other reasons.

best,
Sukandar



> 
> On 20.08.2015, at 16:42, Jordan Kagan <jrkagan2 at gmail.com> wrote:
> 
> 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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