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

[Robert Crawford]

I ran into this same problem with my first SR620.  I thought it was 
defective, so I sent it to SRS for a cal and refurbishment.  It got 
better, but still had an offset of 0.0004 Hz.  Later I bought a second 
one, and it, too, had an offset similar to yours.

A careful reading of the manual, page 63, Performance Tests, Accuracy 
(where the 10 MHz output from the back panel is measured at the A input) 
reveals that a +/- 0.0035 Hz offset on a 10 MHz input is acceptable and 
within spec.  I think this also applies to your test configuration.

I have a photograph of my two SR620s, both using an external clock 
(PRS-10/GPS from an FS710 Distribution Amp), and both measuring the same 
signal on their A inputs.  One reads 10,000,000.00096 Hz and the other 
reads 9.999,999.99827 Hz.  I used a gate time of 1 second averaged for 
100 readings.  Both SR620s had fresh factory calibrations. 

Bob Crawford

TheInfamousFlavio at hotmail.com 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.
>
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