[time-nuts] A simple sampling DMTD

[Jan-Derk Bakker]

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 <magnus at 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
>
>
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