Is this signal FSK or PSK??

[emre2blue]

Hello, I'm trying to read my TPMS system with CC1101. Now I'm using URH in windows 11 with RTL-SDR. I'm working on my car with TPMS, I've successfully capture the signal and decoded in UHR. Sensor catalog says that it is FSK modulated 433.92Mhz - 76khz deviation , 9600 baud rate. And Manchester coded. All seems fine . But is this signal really FSK ?




UHR Decoded the signal without problem. And data is there but how is this a FSK signal. I'm asking because when I want to capture it with CC1101, I get random bits and I could not manage the capture any real data since I started the project. But every time I use UHR it captures perfectly.



Here is the data for UHR https://drive.google.com/drive/folders/1yoGfmySBvS7HPjrfBjRb0RMwVOsWrdtn?usp=sharing


Additional information :



if you look at the wave from of BPSK. it is almost same as the data I've capture from the URH. how can it solve the wave form as FSK??? also I've done a spectrum analyzer in URH. When I check the form it is like FSK this time.




I'm so confused. Please help

[radiogeek381]

It looks to be FSK.

But really really really awful FSK. It would be really really awful PSK too.

The abrupt changes in phase where the first derivative goes from positive to negative more or less instantaneously is vile. Such modulation schemes went out with polar relays and 88 mH toroids.

Civilized FSK is not if (D == 0) x = sin(f1 t) else x = sin(f2 t)
The digital waveform is *shaped* with some kind of filter and then converted to a continuous function m(D) between -1 and 1. Then the output is x = sin(f0 + m(D) * delta_F)

That’s not what we see here. Instead we see the instantaneous phase changes that cause enormous clicky sidebands. Those are all the spikes in the spectrum.

But this may be an artifact of your collection scheme?  I see that you’ve limited the RX bandwidth to 9.6 kHz.  Am I reading that right?  For a signal with a shift of 76 kHz deviation, you really want a sample rate above 100 kHz (higher if possible) and you want to open the filter up to at least 50 kHz on either side of the carrier.

Beyond that, one can do a fairly simple FSK demodulator by “beating” the carrier down to baseband with a complex sinusoid tuned to the midpoint between the two tones. This gets you an I and Q representation of the signal.  Looking at the IQ values as a vector in the complex plane, the “high” tone is represented by a counter-clockwise rotation of the vector, a “low” tone by a clockwise rotation. This scheme is substantially easier to implement than a BPSK demodulator that needs to track the carrier phase reasonably well. For the FSK demod that I just described, you don’t even have to get the carrier frequency correct, just close.

From eyeballing your traces, this is an FSK signal. But a pretty crappy one.