[time-nuts] Re: Timestamping counter techniques : dead zone quantification

[Bob kb8tq]

Hi

Pretty much *any* counter can measure 1x10^-9 if you leave the gate
open long enough. Gate time matters. One can *assume* a 1 second 
gate time, but that’s not always the case. ADEV suffers from this same
“unstated time” issue. 

In “HP” terms 1x10^-9 at 1 second is roughly a 5335 from back in the 
1970’s. The 5345 / 5360 go back even further, but are not as common 
on eBay ( at least not in the sub $100 price range ). 

Like it or not, to be “accurate” to 1x10^-9 at 1 second for a range of
input types and waveforms, you need a 1 ns resolution timebase in 
the beast. Without that, there will be a whole range of signals that 
do this or do that when the counter sees them. 

For certain signals under certain conditions, it is possible to get a bit
of an improvement by doing this or doing that. The more sampling
and math you use, the more limited the usable signal set becomes. 
The gotcha is the customer(s) who have those signals will be going
a bit nuts when the math isn’t doing its thing. Once they start posting
“do this and see how it fails” …. yikes …..

If you look at the current Keysight specs, they are pretty careful about
what they do and don’t claim for their math. They also publish a lot of 
paperwork explaining how this or that breaks down. There’s a lot of 
engineering man hours there …. Even with all that, a lot of folks still
take the “try it and see” approach, even on their newest and greatest.

Yes, you also need a time base that is quite good to get accuracy. In
the era of GPS, that’s not as hard as it once was. Even there, you do
need a GSDO setup to do the job. The most likely candidate would 
be OCXO based. In this era of $5 OCXO’s from China, that may not 
be a big deal. 

Bob

> On Feb 4, 2022, at 5:49 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> Magnus,
> Thanks, good input. To check if there is "pulling" between the two counter inputs I used two signals generated by two PLL's from the same OCXO. First measurement is both at 10MHz. The ratio of these two signals was measured in the two counters using a shared 10MHz reference with a 0.1 s gate time. The ADEV behaves well and starts below 1e-9.
> When one of the signals is shifted with 0.1 Hz (ratio change 1e-8) or 0.2 Hz (ratio change 2e-8) the ADEV starts to show oscillations and the frequency difference shows a pulling pattern that repeats every 10 s for 0.1 Hz difference and 5 s for 0.2 Hz difference.
> Both ADEV and frequency difference plots are attached
> The difference between the 10MHz from the signal generator and the 10MHz reference in the counters was large enough to not create any visible pulling using a 0.1 s gate time but when I brought the 10MHz from the signal generator close (within 0.2 Hz) to the 10MHz reference in the counter the interaction became very visible and the repetition rate nicely varied with the measured frequency difference.
> This clearly demonstrates the cross-talk you mentioned, both between the two counter inputs and between the inputs and the counter reference OCXO
> As my goal is to create a dual input timestamping counter that can reliably measure with 1e-9 accuracy (both short and long term) there is clearly some work to do.
> Erik.
> 
> On 3-2-2022 17:14, Magnus Danielson via time-nuts wrote:
>> Erik,
>> 
>> You should be aware that cross-talk of transitions is a factor here. It "pulls" the transition to the time-base clock.
>> 
>> It can be worth evaluating this by delaying the time-base clock in controlled manor and measure non-linearity of the time-stamps.
>> 
>> A similar test is done between two inputs, as the trigger inputs can cause cross-talk from one another. This is known to be the issue of several vendors counters.
>> 
>> As you push the limit for the resolution, these effects tends to increase in relative size, but for other work they can be fairly ignored.
>> 
>> For some reason I have built a collection of pulse-generators and delay mechanisms to increase the ability to test this. :)
>> 
>> Cheers,
>> Magnus
> <Pulling.gif><pulling_ADEV.gif>_______________________________________________
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