[time-nuts] Yet another Arduino-based GPSDO

Chris Albertson albertson.chris at gmail.com
Sat Apr 12 04:59:12 UTC 2014

Yes I agree.   My long term plan with the software is to write a C++ class
called "DAC" where you send it a voltage.  The class hides the way the DAC
works.   I was also thinking like you to treat this as two DACs where you
use one for fine tuning and the other to make a course adjustment to place
the desired voltage in the center of the fine adjustment DAC's range.   And
not treat this like a 16-bit DAC.  This moves the discontinuity out to some
place where it will never be used.  But this is low on my list because by
luck I not operating near the transition.

But as I wrote before I first need a good way to test this and get the test
data into a computer.

On Fri, Apr 11, 2014 at 9:20 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> albertson.chris at gmail.com said:
> > 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.
> The way to get in trouble is to have the step size of the big DAC be bigger
> than 256 x the step size on the small DAC.  That would leave a gap between,
> say, 3FF and 400 that you can't get to.
> [I see lots of opportunities to get the words wrong here.]
> If I was doing it, I'd try making the step size on the big DAC be 1/2 of
> the
> range of the small DAC.  The idea is that you don't plan to change the big
> DAC.  If you need to go up from 3FF, you will end up at, say, 440.  That
> will
> leave you room to go back down without having to change the big DAC.
> Of course, that only works if the active/working range is a fraction of the
> total range of the small DAC.
> > 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.
> Using the osc was the first thing that came to my mind.
> You might be able to see it on a scope if you took out the big caps and set
> up the software to toggle between two values and flap a pin to trigger on.
> --
> These are my opinions.  I hate spam.
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Chris Albertson
Redondo Beach, California

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