[time-nuts] Counter Internal Oscillator Importance with External Reference?

Poul-Henning Kamp phk at phk.freebsd.dk
Tue May 9 09:55:39 EDT 2017

In message <54ee7bdd-0a49-cc49-540d-2812e70dd4f3 at rubidium.dyndns.org>, Magnus Danielson writes:

>Wither it is proper side bands or other systematic noise, it is bad 
>indeed, [...]

It's more subtle than that, it can be things like thresholds in
the counters trigger being sensitive to the phase of the internal
clock because of (very slight) cross-talk.

I reported data on this a couple of years ago, as I recall my setup
were something like this:

House-10MHz --> HP3336C clk-in and HP5370B clk-in.

HP3336C output -> HP5370B start then 3m coax then HP5370B stop.

Measure TI(start->stop), for different settings of output phase angle on the HP3336C.

Theoretically that plot should be a flat line.

In practice it is not even close.

I think I convinced myself that the majority of the problem was trigger-noise the HP5370B,
but my notes are not accessible at this time.

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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