[time-nuts] What is the best counter for a Time Nuts?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Oct 12 06:02:42 UTC 2008


Steve Rooke wrote:
> 2008/10/12 Myers, Charlie <Charlie.Myers at ps.net>:
>   
>> Hello to the Time Nuts,
>>
>> I have been reading the mail on this topic for the last week or so with
>> great interest.  Lots of interesting ideas have been put forth for
>> measuring frequency to a high degree of precision and for comparing a 10
>> MHz clock's frequency to a highly accurate 10Mhz frequency "standard".
>>
>> The way I measure the frequency of a 10 MHz clock is to compare it to a
>> second 10MHz clock of known accuracy and stability, not only with a
>> frequency counter but also with a phase meter.
>>
>> I have several GPS disciplined OCXO's, one GPS disciplined Rubidium
>> oscillator, and several free running rubidium oscillators.  I measure
>> the frequency of an unknown 10 MHz clock using a 2 step process.  First
>> I measure the unknown 10 MHz clock using an HP 5384A reciprocal counter
>> that employs my known 10 MHz clock as its external timebase.  I set the
>> gate time to 10 seconds and the counter delivers a frequency measurement
>> with a resolution of less than 3 mhz (3 millihertz).  So, assuming my
>> known timebase is "bang on", I know the frequency of the unknown 10 MHz
>> source to an accuracy of roughly 3e-10 or 3 parts in 10 billion.
>>
>> To get a more precise measurement of the frequency difference between
>> the two 10 MHz clocks, I supply the known 10 MHz clock to the Channel A
>> input of an HP 3575A Gain-Phase meter and the unknown 10 MHz clock to
>> the channel B input of the Gain-Phase meter.  I measure the change in
>> the phase angle between the 2 input clocks over some convenient time
>> interval (e.g., 10, 100, or 1,000 seconds) and compute the frequency
>> difference using the formula:
>>
>>
>> Frequency Difference = [Change in Phase Angle (in degrees) / Measurement
>> Duration (in seconds)] X [1 / 360]
>>
>> The frequency difference can then be converted to frequency accuracy
>> using the formula:
>>
>> Accuracy = Frequency Difference / 1e7
>>
>>
>> This seems like a pretty straight forward technique.  Am I missing
>> something?
>>     
>
> So what's time nutty about this method...
>
> :)
>   
Using ~40 year old boat anchor rackmount equipment to measure something 
that can be done more accurately with a handful Of ICs.

Bruce



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