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

Bob Camp kb8tq at n1k.org
Thu Oct 23 07:44:50 EDT 2014

> On Oct 23, 2014, at 2:01 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> --------
> 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
>>>> available.
>>> 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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Looking at both zero crossings would give you a lot of information about the duty cycle of the input waveforms. If that’s what you are after - there are easier ways to do it. If that’s not what you are after, it’s just going to mess up the readings.


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