3 June 2020

NATO 75Bd & 50Bd FSK: F1F2 phase

I decided to replicate the analysis of the phase of the two FSK tones shown in the  previous post  by taking a look at the NATO 75Bd & 50Bd FSK transmissions just because only in the latter the shift (850 Hz) is an integer multiple of the bit rate (850/50 = 17). The The difference between manipulation with a break and without a phase break during switch-over is also visible in SA program in Wave Form mode (Fig. 1): signal "a" is 75Bd/850 FSK, signal "b" is 50Bd/850 FSK; all NATO transmissions.

Fig. 1 -
As expected, both the tones of the signal a (75Bd/850 FSK) do not preserve their phase after each switch-over (Fig. 2) while the tones of signal b (75Bd/850 FSK) preserve their phase (Fig. 3).

Fig . 2 - 75Bd/850 FSK
Fig. 3 - 50Bd/850 FSK
To be precise, 75Bd FSK is from NAU Isabela on 16121 KHz and 50Bd FSK is from NSY Niscemi on 8203 Khz: therefore it must be considered that generally the data are not generated in the same place where the FSK signal is formed. Don't know if the same modem is used in both the waveforms (...and in both the two TX sites), anyway the two tones of the French-Ny 50Bd FSK too have the same behavior i.e their phase is preserved (Fig. 4).

Fig. 4 - French-Ny 50Bd/850 FSK

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