[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?

[Magnus Danielson]

Hi Erik,

On 2023-03-24 14:24, Erik Kaashoek via time-nuts 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.

So, back in the day, the 10 MHz clock was muxed in when existing, rather 
than using the internal clock. This was either through a very manual 
switching, or later through signal detection and steering of mux.

In most modern counters, a PLL is used. The benefit of a PLL is that it 
will also act as a clean-up oscillator so high frequency noise is 
effectively supressed. but lower frequency stuff will still affect 
performance.

Then, for instruments like TimePod and PhaseStation, you just digitize 
the reference and ends up looking at the fundamental as the overtones 
gets filtered out, with similar benefits as the PLL.

A PLL will not ignore noise at zero crossings, but may provide 
significant reduction of it through it's filtering actions.

What signal is best for a particular counter one should take a little 
bit of effort to study as one attempts to push the noise floor of ones 
abilities.

One should also ponder on the reference actually being properly 
terminated, and phase stability of the cable. At some point, it can 
become relevant things to consider. Ensuring termination to avoid reflex 
waves comes fairly cheap, so that one should be done out of normal 
RF-cleanness.

Cheers,
Magnus