[time-nuts] Re: Is the practical quality of a 10 Mhz reference determined by the quality of the fundamental or by the quality of the zero crossings?

[Mattia Rizzi]

Hello,

> When using 10 MHz reference in a modern measurement device, is the lock
on the reference done by direct conversion to a square wave (some simple
digital circuit like a limiting amplifier) or are more advanced clock
recovery approaches used that look only at the fundamental and use all
information in the 10 MHz fundamental, just like the Phase Station is doing?

The phase detector in a PLL has strict requirements in terms of low latency
due to loop stability. PhaseStation can take all the time it needs to
extract the phase information. You might use it with a very ultrastable
reference source and very stable local oscillator, but in this case the
ultrastable source is already very clean (see next lines) so I don't see
any real improvement.

>   ignoring noise on the zero crossings

There's no more noise on the zero crossing than in any other point of the
waveform. The issue with using zero-crossing detectors is that you are
effectively using a sampled time system, and therefore limited by the
sampling frequency. If you're locking a PLL with a 10 MHz reference signal,
and the phase detector is running at 10 MHz, any phase noise on the
reference signal modulated above a frequency offset of 5 MHz will fall back
towards the "baseband" (i.e. the carrier frequency, 10 MHz).

Il giorno ven 24 mar 2023 alle ore 14:28 Erik Kaashoek via time-nuts <
time-nuts at lists.febo.com> ha scritto:

> For one of my projects I was requested to make a presentation about
> measuring phase and frequency
> Part of the presentation is about measuring phase and frequency for
> which I could use a lot of excellent material from various sources.
> I did run into one small problem when trying to explain why the
> PhaseStation phase measurement method (decimated I/Q down mix to zero
> Hz) is ok compared to previously zero crossing methods such as used in
> interpolating reciprocal counter.
> When using 10 MHz reference in a modern measurement device, is the lock
> on the reference done by direct conversion to a square wave (some simple
> digital circuit like a limiting amplifier) or are more advanced clock
> recovery approaches used that look only at the fundamental and use all
> information in the 10 MHz fundamental, just like the Phase Station is
> doing?
> In what category would a PLL for clock recovery fall? Is the PLL looking
> to the fundamental and ignoring noise on the zero crossings by using all
> available information or is it plagued by the same problems as a zero
> crossing clock recovery?
> I hope someone with knowledge on clock recovery could help out here.
> Many thanks in advance.
> Erik.
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