[time-nuts] an interesting problem

jimlux jimlux at earthlink.net
Sun Feb 6 17:01:44 UTC 2011


On 2/5/11 10:14 PM, Hal Murray wrote:
>
>> 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.

Yes.. and from an analysis standpoint, either would give you the same 
ultimate answer after you've crunched the data.

>
> 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.

Yes. it's not intuitively obvious, which way to go.

The ADEV of the measuring system (depending on whether you consider the 
Spacewire time ticks part of it) can be arbitrarily good (an advantage 
of working at JPL..There's a 1pps and 100 MHz from a hydrogen maser 
piped into the lab via fiber)

>
> 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 a nothing special CPU crystal oscillator.. tvb's data is probably 
as good as any.
>
> 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.
>

Oh, that we've checked for other reasons.  This is popped up because 
someone asked "hey, can we measure the oscillator performance?" and I 
thought.. "should be able to"..

In a couple years, I'll be able to do it against a GPS receiver in the 
same device... the FPGA implementation of the GPS receiver provides a 1 
pps, so I can compare it directly.  But the GPS software won't exist 
until next winter.

> -----------
>
> 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.  :)

There is understandable resistance to "modifying" the hardware when it's 
going into space and has already been through all the environmental 
tests. The interest in ADEV is mostly curiousity, and opening up the box 
to satisfy curiosity is out of the question.  So I was trying to think 
of ways of doing it using the interfaces I've got.






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