[time-nuts] Checking accuracy of Rubidium standards

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Nov 9 07:24:16 UTC 2008


Steve Rooke wrote:
> Bruce and Joe,
>
> I have been enjoying this discussion too.
>
> 2008/11/9 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
>   
>> J. L. Trantham wrote:
>>     
> ...
>   
>>> That is something that I have not yet had a chance to contemplate as far as
>>> how to measure.  It would appear to require a particularly stable (pure)
>>> source as a reference though.  Various multiplying or dividing protocols
>>> would seem to introduce a host of other variables that would seem to be
>>> difficult to account for though they might accentuate an impurity in the
>>> signal in question.  I have read Bruce's comments and I still do not
>>> understand the basics of time stamping or how a sound card might provide
>>> this.
>>>       
>
>   
>> There is sufficient information available from the sound card samples to
>> calculate the input signal at any time between 2 samples and in
>> particular derive the time at which the signal crosses zero.
>> This is the time stamp for that zero crossing.
>> The frequency and ADEV of a signal can then be calculated from such a
>> sequence of time stamps.
>> However it is necessary to either calibrate the sound card sampling
>> frequency or lock it to a known frequency.
>> The method used to interpolate between samples is called WSK (Whittaker
>> Shannon Kotelnikov) interpolation.
>>     
>
> Given Joe's comments, doesn't making ADEV measurements rely on highly
> accurately spaced sound card samples and the nature of system based
> upon this hardware affect the accuracy of any such based system? Does
> the accuracy of the measuring system not have to meet or exceed the
> accuracy of the DUT?
>
> 73, Steve
>   

Steve

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.

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.

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.

Bruce



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