[time-nuts] Why using HP5370 ext-ref is (maybe) a bad idea

Poul-Henning Kamp phk at phk.freebsd.dk
Sun Mar 2 17:29:54 EST 2014

I have spent another evening playing around with the 5370 and the
conclusion is pretty ironclad now:

Running a 5370 with ext-ref locked to input frequencies is simply
a bad idea and should not be done.

Running it on the internal OCXO works fine.

Running it on another frequency *not* locked to the input frequenc
also works fine.

In both cases the errors are statistically well-behaved, and can
be treated with normal statistical methods, including the built-in
STD-DEV function.

But feeding ext-ref a frequency which is locked to the input frequencies
causes the errors to become systematic, and they can no longer be
treated as statistically well-behaved.

For instance:  The length of the coax to ext-ref suddenly affect
your TI measurements, because it shifts the phase between the 200MHz
and the input signal.

I tried tuning up the A21 200MHz synthesizer to the best of my
ability, and it clearly made a difference, the phase pattern
of errors shifted around, but the errors did not get any smaller,
they just moved.

I also tried disconnecting the "10 MHz present" circuit, that
didn't change the magnitude of the errors either, but did shift
the phase of the peak noise a couple of degrees.

Looking at some old notes from years past which just didn't make
sense, does now.

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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