[time-nuts] an interesting problem
Hal Murray
hmurray at megapathdsl.net
Sun Feb 6 06:14:00 UTC 2011
> I've got a system at work with an internal clock oscillator that I want to
> get some statistics on, but there's no direct visibility for the
> oscillator, nor do I have a convenient test point that I can probe.
...
Fun problem. Thanks for tossing it out.
I see two approaches. Are there others?
One is to generate something like a PPS pulse and capture timestamps. Then
crunch the data and hope you see N buckets so you can ignore anything that
isn't in bucket 0. (or correct them by shifting by N ticks)
The other approach would be to measure the time between pairs of pulses. You
can probably do that much faster than once per second. This should give you
2*N buckets.
I can't quite figure out how far apart the pulses should be for best results.
(It will probably be simple after I see it.) I expect it will depend on the
ADEV of the measuring system and the ADEV of the clock you are trying to
measure.
I assume you can get a rough ADEV of the clock you want to measure by
measuring a similar part on a typical lab setup.
It's probably worth sanity checking the divide step to make sure it's
dividing by M rather than M-1 or M+1. (Digital geeks are often off by one,
especially if nobody has checked carefully.) I'm not sure how to do that.
Probably something like divide by 2*M and see if it matches. Or divide by a
small M and measure the frequency.
-----------
Plan B would be to use an inconvenient test point. (or make one)
Years ago, my boss gave a neat talk about how to prototype hardware. Step 0
was hire a good technician. :)
--
These are my opinions, not necessarily my employer's. I hate spam.
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