transmitted FSK, sniffed something PSK-looking

Question

I am transmitting a FSK signal on 433,941 ± 32 kHz at 19200 baud rate. From what I read I should be able to recover my signal with a a modem centered on my carrier and having a bandwidth of 32 × 2 + 19.2 kHz. So I have defined my modem carrier-centered with a 84kHz bandwidth:

but am getting this result:

which to me looks wonky because of no frequency changing throughout the signal (I made sure I am not transmitting only 0 or only 1). Was expecting to read back something like:

What am I missing?

Answer 1

You say you set up your frequency shift as ± 32 kHz and that your receiver is “carrier centered” (I assume you mean at the midpoint of the two frequencies).

Therefore, the shift changes the sign but not the magnitude of the frequency, which looks similar to a phase shift but is not the same thing.

You have two options from here:

• If you want to get a signal like in your last diagram, you need to set your receiver LO (“carrier”) frequency so that the magnitude of the two FSK frequencies is different (i.e. do not be centered between them). Then you will see the "slow and fast" look in the output.

  This is what you should do if you want to do further processing as "mono audio", such as you would get from a SSB receiver.

• The data you are looking at in your second picture is complex-baseband in the two stereo channels. If you take the two channels as a complex number or a point in 2D space, you will find that the value is moving in a circle about the origin. When the frequency changes from +32 to -32, it will start moving in the opposite direction. This is really easy to demodulate — just look at the sign of the difference in angle between successive samples.

  This is what you should do if you're trying to write a straightforward SDR demodulator.