[time-nuts] Thermal Compensation: Digital vs Analog

Bob Stewart bob at evoria.net
Wed Feb 26 12:05:12 EST 2014

Hi Atilla,

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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