This 16-tones signal occupy a bandwidth of about 2700 Hz, from 385Hz of the lower tone to 3010Hz of the top tone (dF = 2625Hz), with 175Hz increment steps as shonw in pics. 1,2:
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| pic. 1 - spectrum of the MFSK-16 signal |
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| pic. 2 - the 16 tones grid |
Since last January, this modem has been observed on several frequencies on USB and running at multiple speeds but all with the same constant frequency shift of 175Hz.
An interesting feature of this signal is itsspeed of manipulation,we have spotted 4 different waveforms with speed that increases at a '2x' rate, i.e.: 16.44, 33.33, 66.66 and 133.33 symbols/sec (pics 3,4,5,6)
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| pic. 3 - 14,66 Bd |
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| pic. 4 - 33.33 Bd |
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| pic. 5 - 66.66 Bd |
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| pic. 6 - 133.33 Bd |
Formerly the MFSK-16 phase was preceded by an MSK preamble with ~1866Bd and ~928Hz shift (pic. 7)
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| pic. 7 - MSK preamble before MFSK-16 transfer |
Although the preamble may appear as a QPSK modulation, a more carefull analysis indicates the MSK mode (pic. 8)
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| pic. 8 - preamble speed and mode |
The reason of the lack of the preamble in the current waveforms is unknwon, maybe the preamble could used in the set-up phase of the signal but it's only speculation.
