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

[Dr Bruce Griffiths]

Mike Fahmie wrote:
> At 02:57 PM 2/21/2007, you wrote:
>   
>> I have an SR620 counter that I set up with a Z3801A as an external 
>> reference.  If I put a bnc T connector at the output of the Z3801A and use 
>> two equal length bnc cables, one to the ext. ref input on the back and the 
>> other to channel A then do a frequency measurement, I get a mean that is 
>> about .0015 Hz below 10,000,000.0000Hz.
>>
>> Does any know why this might be happening? I would expect it to read 
>> 10,000,000.0000 exactly give or take a couple on the last digit.
>>     
>
> Perhaps you're running into some sort of meta-stability problem since your 
> clock and input are synchronous. Try lengthening one of the cables a foot 
> or so and see if it makes any difference.
>
> Also, it could be the compound result of noise in both the reference input 
> and signal input amplifiers.
>
> -Mike-
>
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>   
Flavio

It could also, depending on the design, be something quite mundane like 
the difference in propagation delay for an ECL gate for rising and 
falling edges.
Or the difference in propagation delay from clock to output of an ECL 
fliflop between 0-1 and 1-0 output transitions.

Bruce