[time-nuts] Thermal Compensation: Digital vs Analog
bob at evoria.net
Wed Feb 26 12:05:12 EST 2014
The GPSDO is VE2ZAZ's circuit with new code. It detects phase crossings to change the DAC, so it has a phase crossing for every update. I'm working on a TIC design but haven't started on the hardware. In the interim, I hooked up an LM34 thermistor and have been playing with that. In the 8 hour plot below, there are no frequency updates, only temperature updates. I've tried a rolling average, but it doesn't smooth it enough, so I'll have to try hysteresis next. The orange/green/blue line is the DAC. The red line is the thermistor. The cyan smear is the phase plot of 1PPS from my Adafruit (MT3339) against the OCXO (Trimble 34310-T). The units on the right correspond to the temperature - 100 degrees at the EFC divider directly beneath the OCXO. Also, they correspond to the wrapped phase, where 0-20 is 0-360 degrees. The OCXO is limited to a swing of about +/- 1.1Hz at the moment.
In the plot below is the ADEV. Hopefully it's self explanatory. The phase has varied a bit more than 180 degrees during the test.
The biggest influence on temperature seems to be the low quality divider resistors in the EFC divider chain. I have new low TempCo resistors but I couldn't resist playing with these first. Without temperature compensation, phase would vary through about one cycle every change to the red (thermistor) line.
Bob - AE6RV
> From: Attila Kinali <attila at kinali.ch>
>To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
>Sent: Wednesday, February 26, 2014 10:23 AM
>Subject: Re: [time-nuts] Thermal Compensation: Digital vs Analog
>On Wed, 26 Feb 2014 08:09:44 -0800 (PST)
>Bob Stewart <bob at evoria.net> wrote:
>> I've been experimenting with digital thermal compensation on my GPSDO.
>> The results have been favorable for a 14 bit dithered PWM-based DAC, but
>> leaves a bit to be desired in the big picture. And it takes up a lot of
>> program bytes on my PIC.. What's the general consensus on this? Should
>> thermal compensation be completely analog?
>Out of pure interest. Could you elaborate what results you got?
>Ie. what does your GPSDO look like? How do you compensate for the
>temperature coefficient? How much did that improve performance
>compared to non-compensated operation? Did you try any other approaches?
>Why? Why not?
>Yes, i'm a curious mind :-)
>Thanks in advance
> Attila Kinali
>The trouble with you, Shev, is you don't say anything until you've saved
>up a whole truckload of damned heavy brick arguments and then you dump
>them all out and never look at the bleeding body mangled beneath the heap
> -- Tirin, The Dispossessed, U. Le Guin
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