[time-nuts] A simple sampling DMTD

[Gerhard Hoffmann]

Am 29.11.19 um 11:45 schrieb Jan-Derk Bakker:
> 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?

I admit that I did not follow this thread closely, but the Goerzel filter

is the single output line DFT , with O(n).

< 
https://www.mathworks.com/help/signal/ref/goertzel.html;jsessionid=8816f77eb76ad7dd1913c7021698 
 >

<   https://en.wikipedia.org/wiki/Goertzel_algorithm     >

If you need to simulate floats, fractional integers are easiest.

I/Q demodulation probably requires to recreate a clean carrier if you want

absolute phase and not only relative jumps. That sounds more like FPGA

than 8051, or whatever the 8 bit processor of the day may be.

regards, Gerhard



OK, in a previous life I did build a system for geophysics, where they fed

dangerous amounts of AC into the soil and measured the potential at

some 50 nodes. Rubber boots required.

Each node had a 8951 to control some switches and communicate setups.

They were most exited when I gave them the sources so they could

implement FFT pre-processing locally on each node themselves.

That required willingness to suffer.