Demodulate FSK

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Demodulate FSK

Post by NN3694 » Sun Mar 26, 2017 2:59 pm


I have recorded the FSK modulation (using SDR-sharp) in baseband (I/Q data) and now I would like to demodulate the signal "0" or "1". But I do not know how demodulate this. Using any LPF and HPF?

You can see my the picture...

Thank you for your help.
IQ_FSK.png (49.79 KiB) Viewed 23636 times

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Re: Demodulate FSK

Post by Username » Tue Jun 20, 2017 10:49 am

Do you have or know an Application who can decode it?

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Re: Demodulate FSK

Post by rtlsdrblog » Thu Jun 22, 2017 4:48 am

You'd need to use something like GNU Radio to turn it into a bit string. Have a look at, and Google for Micheal Ossmann's GNU Radio tutorial series.

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Re: Demodulate FSK

Post by hotpaw2 » Thu Jul 20, 2017 4:05 pm

If you want to do this in code (your own program), and you know the bit or baud timing plus the two FSK frequencies, then two running Goertzel filters will work. They are roughly equivalent to narrow band-pass filters. Just take the difference of the two Goertzel filters magnitude outputs and look for the maxima and minima for your bit decoding.

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Re: Demodulate FSK

Post by bobasaurus » Wed Dec 27, 2017 7:44 pm

You'll need an initial filter for your desired bandwidth, a FM discriminator, and a frequency comparator (using the results of two bandpass filters) to get digital bits from the resulting audio differences. Here is a good reference: ... s/Lab6.pdf

Aussie Susan
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Re: Demodulate FSK

Post by Aussie Susan » Thu Dec 28, 2017 2:39 am

(A bit off topic i know but...)
Is there a similar reference for mPSK (especially QPSK) as is used for LRPT?
I've been looking for such a thing for a while as I've successfully FM demodulated the NOAA POESs and I'd like to try to METEOR M2 satellite signal as well.
Thanks for the above reference - it has helped clarify a few things for me as well.

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Re: Demodulate FSK

Post by rtlsdrdev-user » Sat Jun 16, 2018 6:59 am ... -gnu-radio

GNU Radio
Universal Radio Hacker

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