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

Magnus Danielson magnus at rubidium.dyndns.org
Mon Mar 3 17:44:12 EST 2014

On 03/03/14 22:35, Poul-Henning Kamp wrote:
> In message <5314EF87.1020204 at rubidium.dyndns.org>, Magnus Danielson writes:
>> 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.
> Hmm, I have a 5359A as well, I din't consider that as a possible
> input source.

What you want is a synchronous trigger and means to steer the relative 
timing with sufficient precision. The 5359A and SR535 is there for 
exactly this type of exercise with their 50 ps and 5 ps step size.
I'm only fear they might have too much noise, but I need to check that.

> Detuning a Rb is obviously feasible, but being able to key in the
> phase you want on 0.1deg units is so much more convenient and
> repeatable.

Agreed. Just wanted to widen the scope of feasible solutions for setting 
up alternative solutions such that a multitude of approaches could get 
similar enough results.

>>> 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.
> Well, seeing how the pattern changed after I tuned A21, I'm pretty
> certain that a lot of issues are there, but maybe not all.
> It would be really interesting if the plot can be displayed in
> something approaching real time, so it would become feasible to try
> to tune A21 based on this plot, rather than a spectrum analyzer.

Which was what I was proposing earlier. One idea would be to dial in a 
suitable phase-relationship and then tune until the value goes "better" 
on the display and then tune to another phase-relationship. This assumes 
you just don't shift it around, but can actually work on it to become 

> Havn't quite figured out how to do that, but using Fast Binary
> mode on the 5370 and a 10,000,000.1 Hz signal from the HP3336
> should be able to do it in half a second...
> The one sensible idea I have on the 1.2GHz is that it comes from
> the BBB.  A run with the original CPU can resolve that.
> Absent that the "1.2GHz" signal simply doesn't make any sense, there
> are no frequencies that high in the 5370 anywere, it must be aliasing
> of a subharmonic somewhere.

Indeed. Let's assume that it's not the BBB causing the issue, but it's 
inherent to the 5370 design.

Did you know that the 200 MHz meets the trigger levels on the A18 board?
That's where the DAC for trigger levels of the START and STOP channels 
is located, as well as coarse counting for N0 occurs. The 200 MHz is fed 
into a tunable filter, and then into U15 MC10216 which is acting as a 
three-stage amplifier. Now, there's a high-slew-rate source of 200 MHz 
at the source-end of the trigger levels. There's filtering through a 
pi-filter as distributed between the A18 board (10 nF to ground, 680 nH 
in series) and A3 board (10 nF to ground, via resistor).

Would be interesting to "sniff" near those to see if the 200 MHz creeps 
into the trigger that way. It would sure explain a lot.

>> Those with a high quality RF generator could force-feed a 200 MHz into
>> the counter and see if it makes any major difference.
> Yes, that would be an interesting experiment.

It's a bit problematic thought, the signal is fed to three different 
places, A19, A20 (the interpolators) and A22.

>> BTW, have someone looked at how the 200 MHz is then used? Sure that no
>> "interesting" interaction happens there?
> It's squared up to ECL and fed to the digital side of things.

"digital" if I may. In these cases, "digital" is but a side-case of 
analogue. :)

> If anybody have a capable HP82xx that might also be an option.

I'll see how "clean" the 200 MHz is on my SIA.

>>> 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
> Yes, but there is so much to learn, and so little time...

That's why we hunt together and learn from each other.


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