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
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