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I have designed a receiver+FSK demodulator with an osmocom source and it works as it should (sometimes - as I'm finding the results to be somewhat incoherent, but I suspect that might be the transmitter that is at fault). Anyway sometimes the demodulated signal using the quad demod block isn't quite what I expect. With other SDR softwares I can usually change the bandwidth and get more appropriate results. With gnuradio, I find that changing the bandwidth field of the osmocom source block does nothing so I was wondering how does one actually change the bandwidth? And what is that field for if it has no effect? Thank you in advance

SomeEngineer100
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The bandwidth field of the Osmocom source is used to control an adjustable bandpass filter built into the receiver hardware, when one exists, as opposed to one which is fixed or only adjusts to the requested sample rate.

Probably what you are seeing in other software is the channel filter bandwidth, which is a software filter and usually much narrower than the hardware filter (particularly in SDR receiver software which displays a waterfall simultaneously with demodulation one signal within it).

The purpose of the channel filter is to fit the bandwidth of the signal you want to receive, so that it rejects as much out-of-band noise as possible while not discarding any of the actual signal. If you don't have one, add one. If you do have one, its bandwidth or cutoff frequency is the parameter you're looking to change.

Kevin Reid
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  • Hello Kevin, thank you for your reply, I think I got it :) I managed to solve it by using a band pass filter after the quad demod block. Although I had a bit of trouble setting the cutoff frequencies I think it works properly now, thanks! – SomeEngineer100 May 31 '21 at 09:54