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

Bob Camp lists at rtty.us
Tue Jan 3 12:57:50 EST 2012


No argument there, but this thread has wandered a bit. 

If you are depending on the capacitor to provide a specific time constant,
then you will have issues. If the control loop is not impacted by the
changes, then they will track out. Often it's not quite an either / or, but
a some of this and some of that.

In any case the noise created by the leakage current in an electrolytic will
be an issue outside the loop bandwidth and only will be reduced by the
available gain...


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chris Albertson
Sent: Tuesday, January 03, 2012 12:35 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Thunderbolt? (re simple gpsdo.) >> capacitors

On Tue, Jan 3, 2012 at 9:14 AM, Bob Camp <lists at rtty.us> wrote:

> 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......

In general you are right.  But in this case the electrolytic cap is inside
a closed loop so as the temperature changes and the voltage in the cap
changes, the loop will correct it, as long the temperature changes slowly
compared to how frequently we measure the phase of the PPS signal.

You could always place the entire system inside box and control it to a
constant temperature.


Chris Albertson
Redondo Beach, California
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