[time-nuts] Yet another Arduino-based GPSDO
albertson.chris at gmail.com
Sat Apr 12 00:59:12 EDT 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
> > up the next DAC value. The I term in the PID controller will basically
> > 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
> > 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
> 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
> 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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