[time-nuts] SRS SR620 External Source Issue -- Possible Solution

Thu Feb 22 20:40:50 UTC 2007

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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>>>
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