8 June 2016

CIS MFSK-16 175Hz (updated)

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:

pic. 1 - spectrum of the MFSK-16 signal
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 its speed 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)

pic. 3 - 14,66 Bd
pic. 4 - 33.33 Bd
pic. 5 - 66.66 Bd
pic. 6 - 133.33 Bd
Formerly the MFSK-16 phase was preceded by an MSK preamble with ~1866Bd and ~928Hz shift (pic. 7)
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)

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.

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