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

Poul-Henning Kamp phk at phk.freebsd.dk
Thu Oct 23 02:01:30 EDT 2014

In message <9A96CAA5BA7B467D9A106EC858EA0DCE at pc52>, "Tom Van Baak" writes:
>>>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.
>What about phase noise measurements?

Yes, what about it ?

The CE mark requires that you meet various EMI/EMC rules, one of which
is in essence a phase-noise requirement for a 0Hz carrier.

But it didn't take long for vendors of clock generator chips to solve
that issue:  The FM modulate the clock so that if you follow the prescribed
measurement procedure there is no spikes above the red line.

However, if you measure it without the heavy averaging usually required
for phase noise measurements, you find that it is as noisy as always,
but at varying frequencies which average out.

All statistical treatments of signals are subject to this kind of
effect because the entire point of using a statistical treatment is to
reduce the amount of information to something we can cope with.

ADEV throws a lot of information away (otherwise we wouldn't need
phase noise), phase noise throws a lot of information away (otherwise
we wouldn't need ADEV) etc.

There's no way around thinking critically about what information
you throw away and why in each specific application.

In the context where this issue came up, the trowing away was about
how much of a signal you should feed into a PLL that steers an *XO.

There we want to throw as much noise away from the input signal as
possible while still keeping the *XO at frequency.

Therefore you only want to feed the minimum amount of information
about the input frequency necessary for the frequency steering into
the PLL, anything above that just adds noise.

In this application any information excess to the frequency of the
input signal will be noise, and that includes any spurs, harmonic
or not.

That's why we usually throw all information about amplitude away
and focus on one direction of  zero crossing, which is (or with a
trivial capacitor can be made) well defined for any signal.

And to return to the original question:

The only reason to look at both zero crossings would be to double
the frequency of the input signal to the loop (ie: 2Hz from a 1PPS
instead of a 1Hz), at the cost of adding a whole lot of noise
in the process.  Don't do it.

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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