Forum: GNU Radio Re: FM demodulation of a real signal

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635b8cf1ca106f565815ca16ccc438ae?d=identicon&s=25 Matteo Campanella (Guest)
on 2006-04-07 11:42
(Received via mailing list)
hello, thanks to your suggestions I have been able to obtain a quite
working demodulation, what I am actually trying to demodulate is AFSK
1200baud ax25 packet; the steps I am doing are

1) channeling from USRP stream - complex stream
2) fm demodulation - real stream

at this point I have an audio signal with 1200 and 2200 tones

3) mix the signal with a complex sine @ 1700hertz

this is an half complex mixer and it shifts my real spectrum 1700 hertx
down the frequency axis, with the result I have now:

a) one spectral line @ 500 (former 2200 tone)
b) one spectral line @ -500 (former 1200 tone)
c) one spectral line @ -2900
d) one spectral line @ -3900

4) 500Hz lowpass the complex signal to discard the negative unwanted
5) fm demodulation to restore the -1,1 symbol stream

actually I am getting some ringing, but I guess it depends on the
paramters of the lowpass filter that are not correctly choosen.

Is there any better way to demodulate this? Do you think that the result
would be better with the use of matched filters for mark and space
frequencies? Would it get better if I use a particular shaping of the
lowpass filter previous to final fm demod?

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