[time-nuts] DMTD Question1

Magnus Danielson magnus at rubidium.dyndns.org
Wed Jan 27 01:33:08 UTC 2010


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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