[time-nuts] SRS SR620 External Source Issue -- Help Request -- A thought

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Feb 22 00:58:07 EST 2007


Didier Juges wrote:
> Dr Bruce Griffiths wrote:
>   
>> TheInfamousFlavio at hotmail.com wrote:
>>   
>>     
>>>> ...if my model is correct the apparent offset in the
>>>> mean will be increased by a factor of 10 over that for a a 1 second gate
>>>> time.
>>>>
>>>> Bruce
>>>>     
>>>>       
>>>>         
>>> Looks like you're right Bruce.  I did both 1, 0.1 and 0.01 gate times and 
>>> the error increased by a factor of 10.
>>>
>>> And it doesn't sound like this is a fixable in any easy sort of manner, 
>>> right? 
>>>
>>>
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>>>   
>>>     
>>>       
>> Flavio
>>
>> If the SR620 can do some simple arithmetic before displaying the result, 
>> then this calibration problem is easily corrected.
>> Otherwise you will have to do the corrections on a PC (or hand held 
>> calculator).
>> The correction will vary with time and temperature so you will need to 
>> measure it periodically.
>>
>> Corrected frequency = measured frequency*(nominal gate time)/(nominal 
>> gate time + differential delay).
>> In your case the differential delay appears to be about 150 picoseconds.
>>
>> Bruce
>>
>>   
>>     
> Bruce/Flavio,
>
> Could that be mostly corrected (at constant temperature) by inserting 
> the proper delay at the right place (in the form of a short piece of 
> coax cable)?
>
> It seems that the error could be easily reduced by a factor of 10.
>
> Didier
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>
>   
Didier

No.
Unless one inserts about 2cm or so inside the instrument between a 
synchronisers and the corresponding interpolator.

Better not to do this as it will vary with time and temperature.
Measuring this error sufficiently often an correcting in software is far 
better and more flexible.

Bruce



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