[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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