[time-nuts] PRS-10 findings

Poul-Henning Kamp phk at phk.freebsd.dk
Wed May 30 09:33:36 EDT 2007

In message <v04210100c2831acb5ca5@[]>, Peter Vince writes:

>	I have seen something similar with my 53132A.  I was checking 
>on the delay variation of an amplifier distributing 10 MHz, and 
>noticed a regular sinusoidal pattern, about a third of a nanosecond 
>peak-to-peak, with a period of about 70 seconds.

Almost any kind of interference will cause such anomalies and the
closer the frequencies are to a multiple of each other, the longer
the period will be.

It is quite common for the frequency difference between the counters
internal X-tal and the measued frequency to show up like that once
you start to measure down in the nanosecond end of things.

The HP5370 has a rather heavyhanded piece of electronics that
eliminate this effect with a jitter based approach and as far as I
have been able to measure, it works.

"normal" counters don't have this, as they are not designed to measure
in that domain of disturbances.

The easiest way to determine if this is indeed the problem, is to
feed the counter an external frequency which can be varied a bit
up and down.  If the period of the artifact changes accordingly: QED.

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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