Demodulating the signal

At this point, GFSK and MSK are still possible candidates (since we had amplitude variations in the pulses). Let's adjust our filtering to just see the signal. Add a file sink to your GNU Radio flowgraph (grab a file sync block in the GUI and route the output of the final block to the input of the file sink; the filename is in the file sink block options) and capture an emission.

Open your output file in Audacity (File | Import | Raw data | 32-bit float) and adjust your sample rate to the one you used in your flowgraph. The file in Audacity looks as follows:

Figure 9.14 – Signal in Audacity

You can now trim the file to keep just the emission. Export it as Other uncompressed file | RAW headerless | 32bits float.

Now, let's work on this isolated sample to try to demodulate it.

GFSK is frequency-based, so if we try to demodulate the cut sample with a quadrature demod block, we should see something significant...