Dear Magnus, Removing the slope between the two sample end points (or: trimming/adding the fractional sample part of the period) is the point of the estimator I posted earlier ( http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2019-November/098450.html ).
In general: as much as I like having it in my toolbox, I don't see how using an FFT would be the best tool for the job in a zero-crossing detector for a DMTD, let alone this particular sampling DMTD. For one, this 8-bit processor doesn't have the spare cycles to run FFTs on the 32-bit data I get from my CIC^2 decimator; besides that, I would only be interested in a single bin (the beat frequency), where it would be more efficient to simply I/Q-demodulate the samples in software (O(N) vs O(N log N)). While I admit that in the latter case windowing would help, at this point I/Q demodulating (effectively calculating only a single bin of the DFT) does not appear to have advantages over least squares fitting the arcsine of the incoming samples. Am I missing something here? Sincerely, JDB. On Fri, Nov 29, 2019 at 2:32 AM Magnus Danielson <mag...@rubidium.se> wrote: > Hi, > > On 2019-11-28 21:18, Joseph Gwinn wrote: > > JD, > > > > I'll have to think about it, but I will mention that with batch > > processing using window functions, it's common to precede the FFT using > > a simple FIR filter to eliminate the low-frequency energy (due to > > clutter, DC leakage/offset et al), the problem being that the FFT alone > > may not have sufficient rejection to prevent low-frequency energy > > breakthrough. > > > > An alternate approach is to compute the mean of the windowed data and > > subtract that mean from all samples in the window before computing the > > FFT. In analog terms, this is like a big coupling capacitor blocking > > DC. > > You also wants to remove the slope between the two sample end-points, or > else that slope represents an in-phase sawtooth function. A > window-function tends to do that. > > Cheers, > Magnus > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.