On Thu, Apr 10, 2014 at 6:42 AM, Jim Harman <[email protected]> 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 > <[email protected]>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 > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > -- Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
