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

[Magnus Danielson]

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

On 2022-02-03 09:36, Erik Kaashoek wrote:
> To prepare for the implementation of dead zone countermeasures I did 
> some measuring of the dead zone band width versus frequency of the 
> subharmonic
> The test setup use a generator with two outputs, one fixed at 10MHz 
> and one variable to test the dead zone. The fixed 10MHz was send to 
> one input of the timestamping counter. The variable frequency output 
> was send to the other input.
> The reference clock used for the timestamping was set to 200 MHz and, 
> through its VC-TCXO,  locked to the fixed 10MHz using a SW control 
> loop updating the voltage to the VC-TCXO once every 10 seconds.
> As the generator used was only able to set frequency at 0.1 Hz 
> resolution there where some limitations in this assessment.
> The dead zone was observed on the sub harmonics of 200MHz and its 
> harmonics. The size of the dead zone was very much dependent on the 
> used frequency
> Below 1MHz the width of the dead zone was below 0.1Hz and thus not 
> observable
> At 10 MHz the width was about 1 Hz
> At 40 MHz the width was about 2 Hz
> At 80 MHz the width was about 10Hz
> This makes implementing dead zone counter measures doable as with 
> lower frequency subharmonics the width of the dead zone decreased thus 
> putting a limit on the amount of subharmonics to include in the 
> calculations and this makes it unlikely there is a  scenario where the 
> two input frequencies used make it impossible to find a reference 
> frequency avoiding the subharmonics.
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