14 September 2021

2000Bd/3250 FSK on 26.9 MHz

Interesting FSK signal spotted on 26.9 MHz by friend killer258 from raduioscanner.ru: modulation speed is 2000 bps and 3250 Hz shift (figure 1).

Fig. 1

ACF results show a kind of "interleaved" values of about 161.7 ms that make a 322-bit length period; indeed, looking carefully at the pattern, the stream is actually formed of a 161-bit sequence wich is sent alternatively in positive and negative polarity (figures 2,3).

Fig. 2
Fig. 3 - 322/161 bit period

Also interesting are the oscillations during modulation in the first part of the transmission which are visible in figure 4. Tones do not preserve their phase (figure 5).

Fig. 4

Fig. 5

Thanks to killer258 who kindly allowed me to use his recordings.


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