[time-nuts] Yet another Arduino-based GPSDO

Chris Albertson albertson.chris at gmail.com
Fri Apr 11 22:59:12 EDT 2014

On Thu, Apr 10, 2014 at 6:42 AM, Jim Harman <j99harman at gmail.com> wrote:

> I have not tried the dual PWM-based DAC, but my concern is that depending
> on the ratio of the resistors it might not be monotonic as the output goes
> from say 0FF to 100 hex. You would see this problem in the output every 256
> counts. Unless there is significant dither in the DAC values, this could
> cause the system to get stuck as it tries to track a slow change in the
> oscillator. The nominal resistor ratio is 255:1. I think it would be best
> to choose the resistors (including tolerance) so that the big one is sure
> to be more than 255x the smaller one. This would cause a jump rather than a
> kink in the output as the DAC varies. Lars uses 39k and 10 megs, which does
> not quite meet this criterion.

My plan was to eventually fix this in software.  Using ultra precision
resisters is not a good fix.  I'm using normal 5% 1/4W resisters.

I don't think I will get stuck.  if a step is to small it will simply move
up the next DAC value.  The I term in the PID controller will basically see
that the current DAC setting is not enough can add more.  I is (error *
time) and given enough time it will push the DAC in the right direction
even if the error is very small.

I don't have a good way to test the DAC.  My Fluke DMM can't see any
problems.  But eventually I want to add a self test or self calibration
function.  I figure that the OCXO should be a very good voltage meter.

> On the integrator, I am using Lars' circuit with a TDK C0G ceramic
> capacitor FK28C0G1H102J, which is supposed to be good to 30ppm/deg C, and
> no-name carbon resistors. I will fiddle with it and see if I can figure out
> where the sensitivity is coming from.
> On Wed, Apr 9, 2014 at 10:20 PM, Chris Albertson
> <albertson.chris at gmail.com>wrote:
> >  But the cheap 16-bit Lars used is
> > working very well.  PWM works because we only need less then 1Hz
> > bandwidth so we can filter the heck out of it.    The steps are less
> > than I can measure
> >
> > What TIC capacitor did you use.  If it is that temperer sensitive you
> > might want to replace it.
> >
> --
> --Jim Harman
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Chris Albertson
Redondo Beach, California

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