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

buehl buehl at superlink.net
Wed Apr 6 09:30:15 EDT 2005


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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