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

[Steve Rooke]

2008/10/12 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
> 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.

That would surely just make it nutty then...
-- 
Steve Rooke - ZL3TUV & G8KVD
Omnium finis imminet