John Miles wrote:
So why even talk about TC phase shifts at the ps / deg level? Who
cares and who needs it?
A picosecond is 1000 femtoseconds. When you're spending $100/ea. on parts
rated for 60-70 fs jitter, a picosecond starts to look like a long time
indeed.
Does not sound like you are doing the Nuts any service to address
things that are orders of Magnitude below usefulness.
When designing something like this, it's useful to understand where, and
why, your efforts fall short of the state of the art. That way you can be
sure that the compromises you're making are the right ones. The idea is not
to leave any obvious low-hanging fruit for optimization. You want to get
the most out of the money you're spending, whether it's $200 or $20,000,
right?
I do think there's a lot of room between "state of the art" timing
performance and conventional TIC-grade performance where corners can be cut
and costs can be saved. It's helpful (and more interesting) if you can make
those calls rationally, instead of just adopting circuits from papers and
hoping for the best.
Let's recall that the type of measurement varies. It may be that focus
is on phase noise and short-tau instability. The concerns for long-tau
instability does not apply to the same degree. The time-span of the
measurement is an important aspect. Averaging techniques assumes that
propeties is relatively stable over time, such that short-time noise can
be filtered. Calibrations may go out of tune. Being able to crank out
numbers of reasnoble reliability calls for stability of the measurement
rig, beyond short-term performance. Understanding the hidden errors is
important. Drift and environmental dependencies is among the issues to
consider.
You can have a perfectly good schematic, but still not be able to get
the performance. Choice of components, mounting, etc. all add up.
I find that I personal preferences towards various methods at various
times, but I also find that I need to reevaluate things over and over as
I get more influences. None of them is the best. Some of them has
inheret drawbacks, so I need to learn how they work and what
difficulties has been addressed. How is each defect handled? What does
that handling imply?
It's non-trivial stuff, so there is only one mode to handle it... learn
more all the time.
Cheers,
Magnus
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