[time-nuts] Yet another Arduino-based GPSDO

Hal Murray hmurray at megapathdsl.net
Sat Apr 12 00:20:01 EDT 2014

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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