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The frequencies for the fsk messagge are in the range 3Khz to 20Khz, so for example a message could be modulated with 4050 for F0 and 4450 for F1 and another message could be modulated with 18000 for F0 and 18400 for F1.
the data baud rate is up to 200baud (i.e. from 40Hz to 200Hz)
I think i need 2 iq demodulator one for F0 freq and another for f1 signal,is it right?
At the moment the sampling rate is 200khz...
The last question is not so clear...
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ok now it is better (I also correct my questioning of baud as it will be low since each symbol will have many samples).
For the case of f0 at 4050Hz and f1 at 4450 I will use demodulator to downconvert the signal midpoint [(4450+4050)/2 = 4250Hz) to zero frequency i.e. use -4250Hz (or +4250).
I assume your input is real so you have a mirrored copy at +/- 4250 centres.
thus the output of demodulator will consist of two images
one copy around zero: 4250 - 4250 = 0 i.e. f0 becomes -200Hz, f1 becomes +200Hz)
and second copy at -4250-(-4250) = -8500Hz centre
You need one IQ demodulator but two filters (I/Q) to remove second copy.
now you need to cut somewhere between 200Hz and 8500Hz on your sampling rate of 200Khz. For example cut at 210 and let transition go towards 8300Hz
However this applies to one case of f0/f1. The worst case at 3Khz will move to 6400 so a filter that cuts at 210 and transition band till 6200 will do.
I will see if I can get a sensible filter.
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