Today was DMTD Rev.3 day:
As can be seen by the changed (and changed again, and again...) resistors, I had to play with the mixer input attenuators
and the gain of the opamp output stage. The SA612 datasheet isn't really communicating which kind of levels to expect
at the pins, at least not in a language I can successfully parse. But with a bit of experimentation, it got worked out.
Also had to supply the opamps with more than 5V (see bodge wires) because the input has a bias close to 5V.
This is the whole mess:
The yellow trace on the scope is the cycle-to-cycle phase difference between the 10MHz references.
(On the counter is the same phase difference, but measured with the help of the DMTD)
The scope measurement shows a periodic jitter of about 2us - not sure what to make of that
But in general I am very pleased: the new circuit alone has pushed the noise on the counter one digit to the right!
And changing Offset-OCXO's showed the previous one had more noise than the new one, which almost gained another digit!
Thanks again to Mounty for the OCXO's - they are a lot better than what I had before!
Now I can watch Oscillator phase drift
almost by 1/100 of a degree - only downside is,
I don't have Oscillators that are stable like that
(Or rather, the trimpots are too sensitive...)
I can vouch for this rabbit hole, it sure is cozy down here
Edit: Oops, forgot a zero there
I'll post schematics/layout later on the Metrology DMTD thread.