[time-nuts] Thermal Compensation: Digital vs Analog

cheater00 . cheater00 at gmail.com
Wed Feb 26 12:34:35 EST 2014


Hi Bob,
better use an FIR.

Your rolling average didn't smooth out enough because it doesn't have
a cutoff low enough.
Hysteresis is not going to help here that I know of.

Cheers,
D.

On Wed, Feb 26, 2014 at 6:05 PM, Bob Stewart <bob at evoria.net> wrote:
> 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.
>
> http://www.evoria.net/AE6RV/TempComp/GPSDO.png
>
> 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.
>
>
> http://www.evoria.net/AE6RV/TempComp/ADEV.png
>
> 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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