[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?

[Bob Camp]

Hi

Assuming that “modern measurement device” = “typical lab instrument” ( and not something 
more exotic …).

If you dig into the typical counter or signal generator today, it is not all that different than one
from a decade or three ago. They depend on some sort of internal reference for achieving
the noise levels at the required “goodness”. 

Why?

They have no idea at all just how good or (or far more often) bad the external reference input
will be. Their spec sheet might define an input level range. Past that, it’s tough to find one
that digs into reference noise specs. 

Since the assumption is that it’s a pretty poor source (noise wise), they tend to do some pretty
basic lock setups. About the only thing they do is to run a narrow-ish bandwidth loop. That
might be 10 Hz, It could be a bit wider. The parts used are typically pretty standard stuff. 

You could indeed do a *much* narrower loop with digital techniques these days.  It would not
take a lot. It *would* assume that the external reference is still worse than the internal down to
whatever the new bandwidth is.  Just how they would make that determination is unclear. 
Doing anything more than the basics doesn’t seem to fit in their budget …..

If you are talking about a phase noise test set that does A to B comparisons (= a PhaseStation
ref input ) then indeed “that’s different ….”…...

Bob

> On Mar 24, 2023, at 9:24 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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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