[time-nuts] Checking accuracy of Rubidium standards

[Steve Rooke]

2008/11/9 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
> When one is measuring the beat frequency between an offset standard and
> the DUT, the sound card timebase doesn't have to be more accurate than
> the the DUT as it is only measuring the error in the small offset
> between the DUT and the offset standard.
> However the local standard against which the DUT is being compared does.

So as this is just measuring a beat, there is no requirement for
absolute accuracy in the measuring system, it just needs the reference
frequency to be as accurate as the desired measure of absolute
accuracy. Yes, I can see that.

> For example with a 10MHz DUT and a 10MHz local standard offset by 100Hz
> from 10MHz, the sound card only has to measure the  100Hz offset
> frequency ( between the DUT and the offset standard) to an accuracy of
> 1E-7 in order to determine the frequency of the DUT to an accuracy of
> 1E-12.

So 100Hz offset from 10MHz is 1E-5 and if I can measure the 100Hz to
an accuracy of 1E-7 that would give an overall measurement of 1E-12.
So that would mean that the sound card device would somehow have to
sample with at least 1E-7 accuracy. That would mean taking enough
samples of sufficient accuracy to determine this 1E-7 accuracy. A
sound card has a sample frequency of approx 44KHz but to see an offset
of 1E-7 would that not take a fairly long sample time? How would this
affect the ability of such a system to determine ADEV for small tau?

> When one uses a dual mixer system to compare 2 non offset 10MHz signals,
> most of the error contributions from the offset source and ADC sampling
> clock are common to both channels and tend to cancel on subtraction.

I can see that, assuming both channels are sampled at the same time.

Thanks Bruce.

73, Steve
-- 
Steve Rooke - ZL3TUV & G8KVD
Omnium finis imminet