Forums › BB Series Discussions › BB60C 2FSK demodulation
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joeqsmith.
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joeqsmithParticipantI am attempting to demodulate a 915MHz ISM band radio using 2FSK with a 1200bps data rate. The only thing I can seem to do is increase the sample rate but there seem to be no way to decimate the data shown. Is there a way to view the digital data for low data rates like this?
AndrewModeratorHello Joe,
Are you using the digital demodulation measurements in the Spike software or zero span? I am attaching a quick image I took of the software configured for the settings you listed. This might help you configure the software for 2fsk demodulation.
The device decimation is selected based off of the chosen symbol rate. We do allow the user to customize the IF bandwidth a bit, but not enough to change the decimation of the receiver.
Let me know if you have additional questions.
Regards,
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joeqsmithParticipantI am attempting to use the digital demodulation as you have shown. Maybe a full screen shot would help so I could see all of your settings. I assume the problem is the operator. Maybe if I do a better job explaining, you can help me out.
The first plot showing a peak hold of the two tones.
Second plot showing the two devices. Slave device sends more data and has the lower amplitude.
Third plot zooming in to see the bit rate which appears to be roughly 1200bps.
Now when I try to decode this with the digital I need to use much higher sample rates than 1.2K before the spectrum shows the two spurs. If I use 50X over sample (60K) the eye diagram looks correct and I can see the preamble but a 0 will be represented with 50 zeros sort of thing.
Any help in what I am doing wrong would be greatly appreciated.
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joeqsmithParticipantOne with persistence showing the two devices.
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AndrewModeratorThe signal looks like an unfiltered FSK signal with 12kHz deviation and 1.2baud? Our FSK demodulation expect a deviation which is a fraction of the overall symbol rate. For example, at 1.2k baud, the maximum deviation our software could handle is ~1.2kHz. Our algorithms work at an oversample rate of 16 which is going to limit the bandwidth of your signal.
Your best options working within the limitations of our software, would be to use zero-span to visualize the data in the FM plot, or select a sample rate in the demod mode that is much higher (as you did with 50kHz) and work around the repetitive symbols.
A.J.
joeqsmithParticipantThis makes sense. Yes, the signal is +/-12KHz at 1.2K BAUD.
If I record say a second of data in zero span, it looks like it’s all there. Which leads me to believe that is really is a limitation of the software and not the hardware.
I am curious why there is a limitation like this? It looks like you are pretty close to being able to just pull the digital data directly. Of course, then I would want to store the digital data rather than the copy to the clip board.
AndrewModeratorThe 2FSK measurements are definitely geared more towards GFSK/GMSK signals with a 0.5 modulation index.
joeqsmithParticipantLooking at the data a little closer, it appears it is actually 2.4K, not 1.2. I had assumed the preamble was 101010… With 4X I can see the data. There is no way to scroll the symbol table which would have been nice. Also just adding a way to post trigger (pattern search) and decimate the data would be helpful.
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AndrewModeratorHi Joe,
Thanks for the feedback. I’m currently working on a scroll bar for the symbols and a video trigger delay, which delays the measurement ‘n’ symbols after a video trigger is seen. I believe both of these will benefit your use case. Expect these next release.
Regards,
A.J.
joeqsmithParticipantTried the new features. Thanks.
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