[time-nuts] Yet another Arduino-based GPSDO
bobdarby at triad.rr.com
Thu Apr 10 15:24:42 EDT 2014
This article might be of interest to those building 16 bit PWM DAC's
Mr. Woodward deals with the monotonicity issue quite thoroughly.
On 4/10/2014 9:42 AM, Jim Harman 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.
> 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.
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