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

[Magnus Danielson]

Hi Poul-Henning,

On 03/03/14 14:41, Poul-Henning Kamp wrote:
> 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.

Cross-talk and ground-bounce through common inductor is known to cause 
this issue. Seems to recall that Bruce mentioned such a note on the 
5370, which I think I have lying around here somewhere.

I know that one vendor found out that putting input channel comparators 
in separate ICs reduced ground-bounce through power-leads between 
signals. A big issue as you try to get further down the precision scale.

>> 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.

All good ideas. I have some more below.

>> 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...

Got a HP3325B, HP5370B/C/D but also 5359A and SR535.
Another approach is to set a rubidium for a *slow* scan over 
phase-relationships.

> 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.

If that is where the issue is.

> 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.

Those with a high quality RF generator could force-feed a 200 MHz into 
the counter and see if it makes any major difference.

BTW, have someone looked at how the 200 MHz is then used? Sure that no 
"interesting" interaction happens there?

> 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.

Yes, but Poul-Henning, we are time-nuts, we dive deep just for the fun 
of it, to see what we can learn. :D

Cheers,
Magnus