[time-nuts] Setting Osc Frequencies

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Mar 3 17:02:16 EST 2007


Tom Van Baak wrote:
> Don,
>
> Yes, using an oscilloscope in this way is a time-honored
> trick to observe slow changes in relative phase over time.
>
> It gets even better if you have access to a storage 'scope
> or variable persistence because then you can get a rough
> sense of short-term jitter as well.
>
> Either way, the math is the same. Frequency error is just
> phase change over time. You have two things working for
> you: 1) you don't even have to wait for full cycles, and 2)
> there is no limit on how long you can wait.
>
> If you have a fast sweep of 10 ns/div you can easily eyeball
> fractions of a 100 ns cycle. High-end 'scopes allow ns and
> sub-ns resolution. Add that to long observation times (many
> minutes, even hours) and you've get quite good frequency
> resolution.
>
> Again, as an example, if you observe that it takes about
> 3 hours for it to move 1/10 a cycle (10 ns), that's about
> 1e-8/1e4, or 1e-12. You see, even using the old CRO
> method, you get fine results. The downside is that you
> have to keep an eye on it now and then to make sure the
> relative phase changes continue to be gradual and that
> you aren't cycle slipping.
>
> Now you ask if there is a better method. If you intend to
> automate it, to collect a lot of data, then the CRO method
> is maybe not ideal. For example, to see how frequency
> changes over time, from week to week, or month to month,
> so that you can compute the aging rate, you might want
> something more digital and less eyeball. Or if you want to
> measure oscillator stability (rather than accuracy) you'll
> want to collect a series of periodically spaced readings.
>
> Your 5245L would partially help here. The trouble is they
> don't have a modern digital output; it's a parallel BCD
> interface if I recall.
>
> Does anyone else have ideas for Don?
>
> /tvb
>   
Tom

One could always use an HP K34-5991A (or equivalent phase comparator) 
connected to a strip chart recorder.
Failing that an equivalent device should be easy enough to build and it 
could be connected to a DVM or other low resolution ADC which in turn is 
connected to a computer for data logging purposes.

Bruce



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