Well, SRS hasn't been much help. Either I'm getting the run-around or the lowest tier of help.
Anyway, here is my solution-- use it at your own risk, any and all comments are welcome. What I did is I changed the value for the TAC offset calbyte, calbyte 01. Like I mentioned earlier, I took the 10MHz output of my Z3801A, split it via a bnc-T-connector and ran one cable to the external clock source on the rear of the unit and the other cable to channel A. I also hooked up a XY monitor to the counters output so that I can monitor the effects of the adjustment in real time in the "mean" display screen. I adjusted calbyte 01, which is the start TAC converter offset, down so that the averaged frequency over several readings becomes 10,000,000.0000 I started off with a 1sec gate and 10 samples. then I moved to 0.1s gate, then finally to a 0.01s gate and 5 samples. As the gate time decreased the offset increased, so moving to a faster gate speed enabled me to "fine tune" the adjustment. In the end, I could go from a 1s gate to a 0.01s gate and the reading would average out to 10,000,000.0000Hz exactly (less resolution for the faster gate times of course). I told SRS that's what I did, I got a "send it in for calibration" non answer. So I don't know what effect my adjustment made to the rest of the unit. I asked SRS for the procedure for manually calibrating calbytes 0-3 but I got the same non answer. Calbytes 0-3 are supposed to be handled in the autocal function, but my counter does seem to be doing the autocal correctly. Calbytes 0-3 would be changed with a autocal or and cold reset so the adjustment is not permanent. I was told SRS will burn a new eeprom for there calibration so that would make the change permanent. ----- Original Message ----- From: <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" <[email protected]> Sent: Thursday, February 22, 2007 15:40 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Possible Solution >I have been able to correct the offset problem. The solution fixed the > offset problem for both channels A and B and all gate times. > > I've have written to SRS tech support to ask them if my solution is > correct. I will post the solution once they reply-- I don't want people > to > start playing with there SR620's and then find out they are just screwing > them up because of my solution > > Thanks for everyone's input, especially Bruce's gate time observation! > > > ----- Original Message ----- > From: <[EMAIL PROTECTED]> > To: "Discussion of precise time and frequency measurement" > <[email protected]> > Sent: Thursday, February 22, 2007 14:32 > Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request > > >> Bob, >> >> I see what your saying about the frequency error spec on pg vii, but I'm >> still reading as a fluctuation not an offset of the mean. In my mind, I >> don't see why there should be some sort of adjustment for this. I really >> doesn't make sense that the would make a counter that doesn't display a >> mean >> that is accurate. >> -Flavio >> >> >> ----- Original Message ----- >> From: "Robert Crawford" <[EMAIL PROTECTED]> >> To: "Discussion of precise time and frequency measurement" >> <[email protected]> >> Sent: Thursday, February 22, 2007 11:58 >> Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request >> >> >>> Flavio, >>> >>> Your experience exactly parallels my surprise and disappointment when I >>> bought my first SR620. In fact, that's why I bought my second one: I >>> just couldn't believe the results, even though it had been freshly >>> calibrated by the factory. >>> >>> However, the Specifications on page vii of the manual tells the story. >>> The Frequency Error is listed as "< +/- ((100 ps typ) [350 ps max])/Gate >>> + Timebase Error) x Frequency". If you make timebase error zero and use >>> a 1 second gate, for a 10 MHz signal you get +/- 0.001 Hz, or +/- 0.0035 >>> Hz max. For a 0.1 second gate, the typical error goes up to 0.01 Hz, >>> which is exactly what you are seeing. >>> >>> I haven't played with the CALBYTE 4 value since I sent both of mine to >>> SRS for calibration. If I remember right you need to move a jumper >>> inside the unit to enable adjustment of this value, and I didn't want to >>> break the factory calibration stickers. I thought this was simply the >>> internal time base frequency adjustment. >>> >>> Bob Crawford >>> >>> >>> >>> [EMAIL PROTECTED] wrote: >>> >>>>I read the same spec on pg 63 but I read it as a fluctuation not an >>>>offset. >>>> >>>>I'm curious if you try the test Bruce suggested and set the gate time to >>>>0.1s and then 0.01s and see if the offset increase by a factor of 10 for >>>>each change? >>>> >>>>I'm finding it hard to believe that this top of the line instrument >>>>can't >>>>handle this rudimentary task when my old Fluke 6680 with 500ps one shot >>>>precision (25 times slower than the SR620) handles it perfectly. >>>> >>>>I'm still not buying this offset is correct. >>>> >>>> >>>>----- Original Message ----- >>>>From: "Robert Crawford" <[EMAIL PROTECTED]> >>>>To: "Discussion of precise time and frequency measurement" >>>><[email protected]> >>>>Sent: Wednesday, February 21, 2007 23:13 >>>>Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request >>>> >>>> >>>> >>>> >>>>>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 >>>>>>[email protected] >>>>>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>_______________________________________________ >>>>>time-nuts mailing list >>>>>[email protected] >>>>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> >>>>> >>>>> >>>> >>>> >>>>_______________________________________________ >>>>time-nuts mailing list >>>>[email protected] >>>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> >>>> >>>> >>>> >>> _______________________________________________ >>> time-nuts mailing list >>> [email protected] >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >> >> >> _______________________________________________ >> time-nuts mailing list >> [email protected] >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> > > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
