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

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Mar 3 08:41:49 EST 2014

In message <C542ADEE-19DD-4DAB-A1BF-FB842C0775FB at rtty.us>, Bob Camp writes:

>The likely answer is that the trigger point must be changing.

Yes that would be my first theory as well.

> The question is whether it's changing because the 
>3336 is doing something (small waveform changes) or because the 5370 is 
>doing something. 

Yes, and obviously there are many experiments that can be performed
to flesh out the details of that:

Changing the 3336 output amplitude.

Exchanging the signals, so the 3336 feeds ext-ref,
and lab-standard feeds start+stop.

Using a different lab-standard.

Measuring opposite polarity etc.

>I would make up / dig up a coax cable that is 8 degrees at 10 MHz. 
>Something around 1/2 meter long should do the job. 

As I said, I'm not really kitted out for RF work, so my selection
of coax isn't that versatile and I don't have the crimp-tools or
routine to make my own.

>Next step would be some sort of filtering between the 3336 and the 5370. 
>That would help rule out harmonics and spurs from the generator as the 
>source of the problem.

The 3336 delivers pretty clean output, so I expect a couple of sanity
checks will exonerate it.

Unfortunately my HP33120 does not have an external clock input, so I
can't use that for the experiment.  Anybody with a HP3325 or later
HP33* with an external clock input can participate in this game...

But to be honest, I'm not sure how much more work is really warranted
for me, given that I don't think I can tune the 200MHz multipliers
filters much better than they presently are.

The really interesting experiments, in my mind, would be to ditch the
200MHz multiplier and feed 200MHz from a good generator with high
purity instead.

But then again:  It is so much easier to just run the HP5370 on the
internal clock and that solve^H^H^H^H^Hhides all the problems.

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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