[time-nuts] GPSDO using 100Hz
Mike Monett
XDE-L2G3 at myamail.com
Tue Nov 25 00:06:27 UTC 2008
Bruce wrote:
> Analog time constants of several hours are generally considered
> impractical due to the lack of suitable low noise components
> principally high value resistors and capacitors.
> So how do you propose to get around this with an analog control loop?
> Bruce
Not that I'm recommending this approach, but for general info,
Digikey sells a 1 Farad 5.5V cap for about $7.00CAD in singles:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=131084;keywords=1%20farad
A time constant of 1 hr would need a 3.6k resistor. That's not bad.
I have some 5 Farad caps that might be even better.
The typical leakage spec for electrolytics is
I = K * C * V
where
C = capacitance in Farads
V = applied voltage
K = 0.002 for low leakage electrolytic caps, 0.02 for standard
I have measured K values of 7e-7 for double layer caps. This is
much, much better than any electrolytic I have ever seen. This
enabled some advanced silver ion generators using differentiation
that could not be done with electrolytics.
The typical frequency response rolls off above a few hundred Hz.
This is of little consequence in timing circuits intended for
several hours duration.
But I'd go with a digital approach for a low frequency PLL.
Mike Monett
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