I'm trying to analyse a signal from a HVAC remote control (864 MHz) with my HackRF, but I'm facing an interrogation about the used modulation:

  • Is it BFSK (frequency modulation) or BPSK (phase modulation) ?

Here's what the signal looks like, once visualised from the HackRF raw data:

(with both I and Q waveforms, each one shifted from 90°)


NB: I can notice two different frequencies but sometimes it looks like both are shifted apart from 90° between transitions. Or maybe the ratio (bit width / modulation period) is really low in this example?

Here is a zoom on a part of the sample that gave me headaches:


EDIT: Like pointed out in the comments, the question was missing a lot of information and visualisation about the signal. Thanks for helping me guys... here is more:

Spectogram along time:


Zoom-in part of the first "symbols" with FFT, Amplitude, Phase and Frequency


FFT visualisation with max detection enabled (under GNU radio companion, with a DC blocker):

I've recorded the samples close to the signal frequency, I realised I might have done something wrong...


Constellation plot (under GNU radio companion, with a DC blocker):


  • 1
    $\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Aug 25 '20 at 17:05
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    $\begingroup$ Thanks a lot @rclocher3 ! $\endgroup$
    – Rascafr
    Aug 25 '20 at 18:57
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    $\begingroup$ Trying to understand a signal by looking at a time-domain plot of the I and Q components is like trying to find a driving route by looking at the SVG source of a map. $\endgroup$ Aug 26 '20 at 4:11
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    $\begingroup$ Without getting out my ruler to test anything: 1. you need better DC offset removal. 2. I don't think it's either; with slightly improved phase recovery, you might see this clearer, but my standard guess for these kinds of signals is OQPSK. $\endgroup$ Aug 26 '20 at 7:57
  • 1
    $\begingroup$ @MarcusMüller OQPSK Wikipedia article, for those of us that didn't know what that meant. :-) $\endgroup$
    – Mike Waters
    Aug 26 '20 at 11:46

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