[time-nuts] Phase, One edge or two? (was Digital mixing with a D Flip Flop)

Poul-Henning Kamp phk at phk.freebsd.dk
Wed Oct 22 17:37:06 EDT 2014

In message <0D2DB2B131E5461BB087713B3E49BEA9 at pc52>, "Tom Van Baak" writes:

> Consider measuring a 10811 for a year. Do you need to follow its
>phase or frequency every 100 ns? Or second? Or minute? Maybe as
>little as one data point per day is more than enough to make a
>perfectly accurate long-term frequency drift plot.

Just to pick a nit here:  That depends precisely on what and how
you measure.

If you measure phase, then no, you probably don't need to measure
more often than one phase difference per hour or even day, as long
as you can reliably predict (from the frequency including noise)
exactly how many periods were in that hour or day.

This is basically what timelabs do:  They measure against some radio
signal (GPS, Two-Way, etc. etc.) every so often, trusting their
stability between measurements.

If you measure frequency, you MUST measure the frequency continously
at all times without any deadtime between the measurements to get
the precise result.  The advantage is that you make *no* assumptions
about the frequency or its stability at all.

>3) Every instant on a sine wave is actually a data point, not just
>the zero crossing(s). So in reality there is near infinite information

Sorry, but no.

If you tell me it is a sine and give me the time of two zero crossings
I can tell you everything there has or ever will be to know about any
point on that sine-wave.

Where looking at the whole curve makes sense is if it is not a
sinewave, either because it is a complex signal (Loran's 3rd crossing)
or because the sinewave is distorted in a way (ie: non-harmonic)
which can be averaged out by looking at the entire curveform
(locking onto a received radio signal.)

But for pure sine signals or good approximations, measuring the
zero crossing tells you all you can ever learn.

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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