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

[Tom Van Baak]

>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.

Three comments that may help.

(1)
0.0015 Hz out of 10 MHz is 1.5e-10 which seems a little
high but not too bad. But do not expect exactly 10 MHz with
this sort of "test". What you are giving to the channel A
input is the most highly phase correlated signal you can
imagine relative to the internal clock and the interpolators.
This won't happen in real life with real input frequencies.

(2)
Try different lengths of the channel A cable and see if the
number changes. My prediction is it will.

(3)
Run the setup menu self-calibration if you haven't in a while.

/tvb