[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 <
> magnus at 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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