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

[WarrenS]

>this thread has wandered a bit.
The thread was originally for "Simple"...

Bottom line is that electrolytic caps can be made to work fine for a 
"SIMPLE" analog controller built for home NUT use,
Not recommended for space or critical life support applications, or any 
production thing.

Besides putting the crappie RC inside a closed loop the other thing that 
seems several are missing is to limit the correction range.
If one sets up the simple loop to give say a 100 to 1 improvement, then all 
the other concerns become non-issues.

>"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..."
Loop Gain is not a problem when making a frequency lock loop, even with a P 
only controller, using any kind of phase detector because the gain is 
infinite.

The leakage current noise I measured was way below insignificant when things 
are properly scaled.
Such as when you scale it so you only care about a 1% of 5 volt change and 
not uv.

If you're depending on a specific time constant for a SIMPLE controller 
using electrolytic caps then the problem is the design and not the caps.
If you're want to make a 1e-10 to one correction with a simple controller 
the problem is not the cap but the configuration and the expectations.
But making a 1e-13 correction to a 1e-9 Rb is no problem  (1000 to one 
improvement)


>Hal Murray Posted:
>"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."
Interesting question, so I tried it.
No effect on the One I tested.
I charged a cheapie 1000uf, 50V cap to 5 volts then changed it's temperature 
which did changed it's capacitance and leakage, but had no effect on it's 
charge voltage.
I guess the charge is not Fixed, so not the same thing as changing the value 
by paralleling the cap.

ws

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[time-nuts] Thunderbolt? (re simple gpsdo.) >> capacitors
Bob Camp lists at rtty.us

Hi

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...

Bob

*******************

On Behalf Of Chris Albertson

> 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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