I've been doing some research for a project (that'll probably never happen)
where the main constraint is to measure a time interval with nanosecond
(preferably picosecond) resolution and repeatability. Obviously we're not
talking about timer0 here :o)

I've found some exceptionally high speed counters ( OnSemi mc10E016 -
better than 1 GHz) That could reach 1 nanosecond, but finding a GHz clock
precise enough adds another challenge (<20 PPB). Even if I had a suitable
clock, calibrating it adds more challenges.

There's also the option to measure it with a R/C time constant. Let a known
capacitor charge (or discharge) through a known resistance for an unknown
time and measure the resultant voltage. This option adds some complications
of its own. I doubt that I could find (or measure) an adequately precise
capacitance to derive the time completely mathematically. But I would
expect that high quality components in a controlled environment should be
able to achieve that level of repeatability - then it's just a (not so
trivial) matter of comparing it to other instruments and deriving
calibration data.

Anybody have any input on this one? Better clocks and counter options? What
kind of capacitors have the best characteristics (ageing - tempco, etc),
20+ bit A/D converters (about 10 samples per minute, max)?

-Denny


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