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 <[email protected]> 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 > <lesson1-grc.png><RDS_GRC>_______________________________________________ > HackRF-dev mailing list > [email protected] > https://pairlist9.pair.net/mailman/listinfo/hackrf-dev _______________________________________________ HackRF-dev mailing list [email protected] https://pairlist9.pair.net/mailman/listinfo/hackrf-dev
