[time-nuts] Thunderbolt? (re simple gpsdo.) >> capacitors

[Bob Camp]

Hi

Using electrolytic caps in timing applications is a bit exciting. Their
leakage current changes each time you change the voltage on them. It's
enough of a change to significantly impact long time constants. In some
cases the capacitance changes with voltage as well. 

Temperature stability of capacitance for most processes is in the 10 to 20%
change over 0 to 50C.  Leakage at least doubles every 10C. 

Many ceramic bypass caps have similar TC and change in cap with voltage
issues. NPO ceramics or *good* film capacitors are the stuff you make your
analog computer out of. (Yes, I'm old enough that you had to check the
course description to see if the "computer" course was analog or digital...)

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Hal Murray
Sent: Monday, January 02, 2012 8:15 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Thunderbolt? (re simple gpsdo.)


> Time constant is just R*C.  If you have a 1000uF cap and a 1K resistor you
> have 1 second.  In theory you could build 100s just by using a 100K
resistor
> but I think real world components are not perfect enough. 

Does anybody know anything about the temperature coefficients of large caps?

I found data for ceramic caps, but when I added "electrolytic" all I got was

lifetime stuff rather than capacitance change with temperature.

I'm not interested in the frequency shift of the filter as the temperature 
but the voltage shift due to a fixed charge as the capacitance changes.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.




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