[Hackrf-dev] FM Receiver and offsetting frequency

[Dominic Spill]

On 20 August 2015 at 15:42, Jordan Kagan <jrkagan2 at gmail.com> wrote:
>
> 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.

A great question and one that causes confusion for many people.  It's
discussed in the HackRF FAQ, but from a different angle (i.e. "how do
I solve problem X?"  "offset the centre frequency").

If you start gqrx or any other tool that shows an FFT of the received
data, you will notice a large spike in the centre of the display.
This is the 0Hz component of the received signal, also known as the DC
offset or DC bias.  It's a side effect of the quadrature sampling
technique and is just something that we have to live with.  This is
discussed here:
https://github.com/mossmann/hackrf/wiki/FAQ#what-is-this-big-spike-in-the-center-of-my-received-spectrum

The following FAQ question is about how to deal with it:
https://github.com/mossmann/hackrf/wiki/FAQ#how-do-i-deal-with-the-dc-offset

The tl;dr of that question is: offset the centre frequency of the
radio and then shift it back in software.  This is what those
flowgraphs are doing.

I hope that answers the question, although if you would like more
detail, I'm sure someone on this list can answer it better than I can.

Thanks,
  Dominic