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 [EMAIL PROTECTED] 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. > >_______________________________________________ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts