[time-nuts] 5061 Cs frequency shift with long time constant?

John Ackermann N8UR jra at febo.com
Wed Apr 6 09:40:23 EDT 2005


Thanks, Tom.  That's another excellent point.  The long term stability 
impact of this cap is worth considering.

Following up on Brian's comment yesterday, I'm investigating sources for 
very low leakage caps.  If I find some, I'll let the group know, and if 
there's a minimum purchase quantity that's reasonable, I may just buy a 
bunch and make them available to the group here.

John
----
buehl wrote:
> John:
> 
> If circuit has an electrolytic, following may influence how/ when you 
> recalibrate.
> 
> Electrolytics change value and leakage with no voltage applied for long 
> periods.  So if the long TC cap has been switched out for months it will 
> need a long time with "in circuit" to stabilize again.  The electrolytic 
> barrier "forms" as a result of the electric field across it.
> 
> Tom Buehl
> 
> 
> At 07:41 PM 4/5/2005, you wrote:
> 
>> Thanks, Brian.  That's very helpful.  I looked at the schematic and 
>> the cap they are using for LTC is 8uF that's simply put between the 
>> input and the output of the op amp module.
>>
>> I guess the question is whether the leakage causes any stability 
>> problems, or just a constant offset.  Time for some experiments...
>>
>> John
>> ----
>>
>> wa1zms at att.net wrote:
>>
>>> John-
>>>
>>> I have seen the same effect on my HP-5060A.  I had attribute it to
>>> leakage in the loop filter capacitor.
>>>
>>> Should you find that you need to replace that cap in your standard,
>>> you might want to try some of the very low leakage current caps from
>>> WIMA in Germany.  If you're in a pinch I have a couple 1uF WIMA caps
>>> that I could send you.
>>>
>>> -Brian, WA1ZMS
>>>
>>> -----Original Message-----
>>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
>>> Behalf Of John Ackermann N8UR
>>> Sent: Tuesday, April 05, 2005 9:53 AM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: [time-nuts] 5061 Cs frequency shift with long time constant?
>>>
>>>
>>> This is very odd (I seem to be collecting these sorts of problems) 
>>> and I wonder if anyone else has seen something similar.
>>>
>>> I got my 5061A trimmed to the point where a 5 day run against GPS 
>>> showed an offset of less than 1x10e-13.  I then switched the unit 
>>> into the long time constant mode and now see that the offset (over 36 
>>> hours) is -1.7x10e-12.  Nothing else in the setup has changed.
>>>
>>> It's hard for me to see how simply changing the loop time constant 
>>> would change the frequency of the standard, especially by this much, 
>>> but stranger things have happened.
>>>
>>> Has anyone else experienced a shift like this when switching the time 
>>> constant?
>>>
>>> Thanks,
>>>
>>> John
>>>
>>> PS -- I was able to partially, but not completely, reproduce the 
>>> phase shift that I wrote about a couple of weeks ago by jiggling the 
>>> 1 PPS cable to the counter.  Replacing that cable seems to have 
>>> resolved that problem.
>>>
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>>
>>
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