locked Re: FST$4W in a PIC #FST4W

Bill Somerville

On 16/03/2021 21:54, Kenneth Williams wrote:
Yes, I caught the issue that this chip is designed for two-tone FSK.  However, after thinking about your idea, it is not clear that this would work without knowing the undocumented internals of the chip.  Very specifically, it is not clear that if both F1 and F2 hold the same frequency, that there would be no phase discontinuity when flipping the FSK input.

So, in the method you propose, at some point in the operation, set F1=F2 and change FSK.  What should be the setting for ramped or non-ramped?  If ramped, what rate? And then, the big question, how well is phase continuity guaranteed?  This phase continuity is not clear to me from the chip specs.

This is a good and interesting topic.


the DDS is phase continuous when doing FSK frequency shifts. The ramping can be disabled in ramped-FSK mode by clearing the TRIANGLE bit if necessary. I did say that any F1/F2 swapping would need to be done in dwell time mid-symbol, probably just prior as at mid-symbol in ramped-FSK mode it is time to flip the FSK input to start the next frequency change if there is one. So I don't think any of your questions above are relevant or unanswered by the data sheet.


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