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

[Ulrich Bangert]

Hello Paul-Henning,

www.tmo.jpl.nasa.gov/progress_report/42-121/121G.pdf

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?

Best regards
Urich Bangert

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Poul-Henning Kamp
> Gesendet: Sonntag, 1. Oktober 2006 13:05
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] On some pitfalls of the dual mixer 
> timedifferencemethod of horology
> 
> 
> In message <000001c6e547$554bd670$03b2fea9 at athlon>, "Ulrich 
> Bangert" writes:
> 
> >> All the repeated steps of amplification/limiting to find 
> the = zero 
> >> crossing can be almost entirely replaced by a a single FFT.
> >
> >If dsp methods are a choice one can even do better (at least 
> for simple
> >sines) as described by Mr. Greenhall in
> >
> >www.tmo.jpl.nasa.gov/progress_report/42-121/121G.pdf
> >
> >The clue here is that not only the zero crossings and its 
> surroundings 
> >are used for interpolation but ALL samples are used for a comlete 
> >signal reconstruction and noise removement. For a simple 
> sine this is 
> >of course easier done as for the more complex Loran-C signal. =
> 
> That is exactly what the FFT does for you.
> 
> Loran-C is not that much more tricky, it has a well defined 
> frequency spectrum as well.  Not as simple as a sine-wave, 
> but well defined all the same.
> 
> -- 
> 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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