Re: [time-nuts] Introduction

[Magnus Danielson]

Enrico,

Nice additional labs.
For short cables, the start pulse can still affect the stop pulse, 
potentially give it worse slew-rate as the cross-talk superpositions on 
top of the actual signal. Such cross-talk should be less pronounced as 
you have lower slew-rate on the start-pulse, but then lower slew-rate on 
the stop-pulse would make it more sensitive so it would even out to some 
degree.

3 foot cable is 90 cm is 4.5 ns time-difference, which is where I expect 
such effects to show up.
10 foot cable is 3 m is 15 ns time-difference, which is where I expect 
such effects to be essentially gone.

Cross-talk between start and stop can be a bit annoying.

Cheers,
Magnus

On 02/25/2016 11:50 PM, Enrico Bellotti wrote:
Bruce, Mark and Magnus
thank you for your comments. Incredibly enough, I have found some time to
follow up and address some of the issues you outlined.

I measured the rise time of the start and stop signals for different cable
lengths and types of generators.

I have also used a HP8901A/8903A that generates square waves with ~350ps
transitions and 1.2V amplitude. This is the fastest  source i have.

I attach a pdf with the details and the Adev results.

It looks like that the length of the cable effects the Adev, probably
caused by the stop signal rise time degradation.

It is  interesting to notice that for short cables (~3ft on HP5370B #1) the
Adev increases. I wonder if, unless of other measurement errors, the cable
is too short to "decorrelate" start and stop.

I have just had time to try one different references (TB 10MHz) instead of
the internal one but i do not see much of a difference. I assume a large
set of measurements is needed.

Finally, HP5370B #2 is off anyway. I am going to look into the multiplier
10->200 section as it was suggested.

Best.
Enrico



On Wed, Feb 24, 2016 at 2:23 PM, Magnus Danielson <
mag...@rubidium.dyndns.org> wrote:

Dear Enrico,

On 02/24/2016 06:55 AM, Enrico Bellotti wrote:

Hello to all,

first of all, thank you for the great and useful work that the time-nuts
have done over the years.


Welcome!

I have finally been able to gather all my counters (HP5335A, HP51131A,
HP5370A, HP5370B #1, HP5370B #2) and try to do some "simple" measurements.

I have started testing and comparing the instruments I have available
using
the approach that was discussed a while ago on the list and also outlined
by John Ackermann (http://www.febo.com/pages/hp5370b/). Specifically, the
Adev for a time interval measurement on a (90ft of) RG58A/U cable.


It's a bit of a cable, so it can eat in on the rise-time somewhat. Did you
check the risetime?

I have attached a PDF file with the results and some additional details of
the test setup.


Nice setup.

  From what I have understood the Adev at one second is related to the
counter resolution. The results I have obtained seem to be reasonable
except for HP5370B #2. This instrument seems to be marginal at best. Does
anybody know if the measured value for HP5370B #2 is a symptom of a
multifunction or simple need for calibration?


Yes, it is somewhat related. The 1/tau slope you see is expected.
You can usually expect the slope to be in the neighborhood of
single-shot-resolution/tau, which is rule of thumb. It's more complex as it
depends on the experienced trigger jitter, which depends on the noise and
the slew-rate at the trigger point. You can thus optimize the jitter by
adjusting the start and stop trigger voltage.

The cable delay will act to decorrelate the triggers, but for most
designs, you don't need to go to 135 ns but can keep them tighter, of the
benefit of maintaining nice slope. However, when the stop trigger comes
just handfull of ns after the start trigger, then the remains of the
start-event can shift the stop trigger. The cable decorrelates this effects
so it behaves more as separate signals, so that is good.

The ripples you see for shorter taus for PPS signals would be interesting
to see the reason for, the phase plot should help to illustrate the reason.
However, it is curious how you provide measures from 0.1 s for a 1 Hz (1 s)
PPS signal.

However, it is nice to see the relative close correlation between the PPS
and 1 kHz signals. It would be nice to see if a slew-rate measurement of
the two sources could be related to the ADEV differences.

Thank you for any comments/suggestions/corrections you may have.


Hope you got some input from my ramblings.

Cheers,
Magnus

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