[time-nuts] Introduction

[Magnus Danielson]

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