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

[Mike Fahmie]

At 03:50 PM 2/21/2007, you 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-
>
>Another thing...It sounds like the Z3801A is being asked to drive 25 ohms 
>(two 50 ohm loads in parallel), perhaps the waveform is distorting in such 
>a way as to present a slow risetime edge to the counter, thus agravating 
>the effects of noise.

-Mike-