[time-nuts] Yet another Arduino-based GPSDO

Jim Harman j99harman at gmail.com
Thu Apr 10 09:42:07 EDT 2014


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.
>




-- 

--Jim Harman


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