[time-nuts] A silly question ...

David G. McGaw David.G.McGaw at dartmouth.edu
Thu Sep 27 13:38:00 EDT 2018


David N1HAC

On 9/27/18 12:55 PM, Dave B via time-nuts wrote:
> ... Because I'm sure I should be able to figure this out for myself!
> I have (as many of you do also) one of the venerable Trimble Thunderbolt
> devices.  No problem with that.  All works fine, and is run 24/7, UPS
> backup power and all...
> I also have (again, as many of you do...) a free running OCXO for 10MHz
> used for "other" stuff etc.   Also left running for long periods, but
> only when I want to experiment with other stuff and don't want to
> disturb what the TB is keeping sane..
> Triggering a dual beam 'scope (Tek 465) from the TB on Ch1, and having
> the output of the OCXO on Ch2, the resulting display on Ch2 of course
> drifts in relation to the static waveform on Ch1.  (Both nice sinusoids.)
> If I time how long it takes for the OCXO to drift through one full cycle
> (co-incidence to co-incidence) relative to the TB on Ch1, how exactly do
> I turn that time, and knowing the base frequency of the TB at 10MHz,
> into a ppm discrepancy?
> "I think", that if for example, it takes 1 second to drift one cycle,
> that works out at 0.1 ppm.   If it takes 2 seconds, it's 0.05 ppm, if it
> takes 5 seconds, it's 0.02 ppm etc.   Is that correct?
> If not, please feel free to educate me!
> As I said, a silly question that I'm sure I would have answered myself a
> few decades ago, but age and medication etc...
> Interestingly, after "a lot" of googling, I see that anything like this
> using "analogue" or "CRT" scopes, has fallen off the radar and the
> interweb, and some of the practices using digital oscilloscopes seem to
> rely on the instrument itself to make the measurement, rather than from
> "observation" and common sense.  (That I seem to lack at times too!)
> But I did get diverted into reading up on some of the early history of
> CRO's.
> Regards to All.
> Dave B.

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