[time-nuts] On some pitfalls of the dual mixer timedifferencemethod of horology

Poul-Henning Kamp phk at phk.freebsd.dk
Sun Oct 1 08:42:47 EDT 2006

In message <000001c6e54b$d13299a0$03b2fea9 at athlon>, "Ulrich Bangert" writes:
>Hello Paul-Henning,
>definitely uses no FFT but uses a theoreme from geometry to estimate the
>signal's frequency and the rest is a two dimensional non-linear fit for
>amplitude and phase. But i am starting to understand how a FFT might be
>helpfull too.
>Does it involve finding the maximum of the frequency spectrum by
>interpolating between frequency bins and then find the matching
>(interpolated) phase bin?

That would depend on your sampling rate.

If you sample a 1Hz signal 96k times a second, interpolation between
bins would probably just be a waste of time.

Alternatively, you could apply a really steep band-pass filter around
1 Hz.  Something like an 1131 pole FIR filter, and then find
the zero crossing geometrically using the three points around the
zero line.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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