[time-nuts] Setting Osc Frequencies

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Mar 3 17:12:00 EST 2007


Dr Bruce Griffiths wrote:
> 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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>   
Tom

Additional info on K34-5991A
Manual available at:
http://www.to-way.com/k34599a.pdf

Since the circuit uses 2 line receivers, a dual flip flop an exclusive 
or gate and an opamp, designing/building an equivalent instrument should 
be fairly simple.

Bruce



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