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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