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

[Steve Rooke]

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...

:)
-- 
Steve Rooke - ZL3TUV & G8KVD
Omnium finis imminet