[time-nuts] SRS SR620 External Source Offset- My Possible Solution

TheInfamousFlavio at hotmail.com TheInfamousFlavio at hotmail.com
Fri Feb 23 00:07:38 EST 2007


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: <TheInfamousFlavio at hotmail.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
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: <TheInfamousFlavio at hotmail.com>
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> 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" <avitek at wwnet.net>
>> To: "Discussion of precise time and frequency measurement"
>> <time-nuts at febo.com>
>> 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
>>>
>>>
>>>
>>> TheInfamousFlavio at hotmail.com 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" <avitek at wwnet.net>
>>>>To: "Discussion of precise time and frequency measurement"
>>>><time-nuts at febo.com>
>>>>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
>>>>>
>>>>>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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>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>_______________________________________________
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>>>>>
>>>>>
>>>>
>>>>
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