[GPS_Standard] VE2ZAZ

Wed May 26 19:23:16 EDT 2010

Indeed, I'm shocked at how temperature sensitive this oven is. If,
after trying some of Dave's methods and tests, I find the oven is
good, I suspect I will be placing everything in a very temperature
stable environment, possibly thermally bonding it to a Very Large
thermal mass, aka a 12" thick concrete slab ~90" below grade. :)

What are words of wisdom on a power supply? I have dozens of switchers
about that will do the job, but what of any noise?

On Wed, May 26, 2010 at 7:08 PM, Bert, VE2ZAZ <ve2zaz at sympatico.ca> wrote:
> Dave,
>
> The good thing with your setup is that it is very educational. You go
> one step at a time and try to see the effects that the changes have.
>
> Increasing the thermal resistance and the thermal mass will improve
> stability, no doubt about this. The LM78xx series of voltage regulators
> are pretty quiet, but their temp.co. (drift vs. temperature) is not the
> best.
>
> People tend to under-estimate the amount of drift that an OCXO can
> manifest when powering up after a long period of inactivity. Two
> identical units will show different behaviors. You also have to take
> that into account.
>
> Have fun,
>
> Bert, VE2ZAZ
> http://ve2zaz.net
>
> Dave Platt wrote:
>>> The system performance you are describing (1/30th to 1/300th of a
>>> second) appears to fall short of what most see. you should get better
>>> than 1x10e-9, with the mid to low 10e-10's a more common place. I don't
>>> know if you meant that your S sampling period is between 8 and 16
>>> minutes, but this is likely too short. An hour to a couple of hours is
>>> more common on this type of system. Of course, the stability of your
>>> OCXO may force you to go with shorter sampling period...
>>
>> Yes, I agree, it's too short for best accuracy (assuming that the
>> oscillator is stable).  I'm still in the early phases of getting
>> the controller settings "tuned in", and figuring out the factors
>> which limit this particular device's accuracy.  The current settings
>> favor a relatively rapid ability to track oscillator speed changes,
>> but very probably result in a system whose accuracy is dominated
>> by sampling error and GPS jitter.  A significantly longer averaging
>> period will help reduce this, once I figure out how much the oscillator
>> itself actually drifts (e.g. with ambient temperature) and how fast
>> I'll need to allow the controller to adjust.
>>
>> A few details on my particular implementation.  I'm using an Efratom
>> oscillator, which has a relatively large adjustment range (around
>> 10 Hz).  I've actually "padded down" the controller's adjustment range,
>> by using a voltage divider circuit with a separate, manual "coarse
>> adjust" cermet pot fed from a separate +5VDC regulated reference... I can
>> center the oscillator's rate with the coarse adjust, leaving between
>> 1 and 2 Hz of pull-range for the controller.  The oscillator is fed via
>> three separate voltage regulators (two 7812s for the oven and the
>> oscillator itself, and a 78L05 which regulates down from the oscillator
>> 7812 and feeds the cermet coarse-adjust pot).  The controller board has
>> its own on-board 7805, of course, and I added a separate 7805 sitting
>> on top of the output-driver chip (I call it the "mad hatter") to remove
>> the effect of output-load power variations from the controller's main
>> 7805 (which provides the DAC reference voltage).  The GPS and dual-port
>> Maxim serial-port chip are fed from another 7805... and all of the
>> 7805s are fed from a 7809 pre-regulator mounted at the power-entry
>> panel.  Lots of regulators :-)
>>
>> Over a period of a couple of weeks, I recorded the controller's
>> output reports, and ran 'em through some PERL scripts and then
>> into GNUPlot.  You can see the result at
>>
>>   http://www.radagast.org/~dplatt/hamradio/fll.pdf
>>
>> The interval from 3/31 through 4/4 was the initial turn-on
>> and lock-acquisition period.  I had set the coarse-adjust pot
>> using a decent HP counter as reference, and with a short averaging
>> period (a minute or so) the controller had indicated a lock, but
>> it doesn't seem to have actually approached stability until
>> around 4/2 or so (where the curve briefly flattens out).  On
>> 4/4 I manually tuned the controller to its mid-range,
>> tweaked the coarse-adjust pot until the controller re-established
>> a short-period lock, and then let the controller run (with an
>> 8-minute averaging period).
>>
>> The results are interesting.  For about a week thereafter, there's
>> a very pronounced 24-hour tracking cycle visible... which then
>> largely disappears.  The box was sitting in my unheated garage,
>> which goes through fairly pronounced temperature changes in good
>> weather (probably close to 30 degrees F on those days).  Rainy
>> weather arrived during the second week of April, with much more
>> constant temperatures, and you can see the effect on the
>> system's frequency stability!
>>
>> My next step (after finishing up the box bezel) is going to be
>> to try to reduce the standard's temperature sensitivity.  Although
>> the oscillator itself is in an oven, the other circuitry is
>> probably temperature-sensitive to some degree - in particular I
>> suspect that the various voltage regulators may be drifting a bit.
>>
>> I'll probably try sealing up the vent holes in the cabinet I've
>> used, perhaps adding some insulation or thermal mass, and perhaps
>> adding a sort of auxiliary-oven temperature regulator which would
>> keep the entire interior of the cabinet at a more constant
>> temperature.  Simply moving the standard into the interior of
>> the house would help, of course, but it's more convenient to keep
>> it in the garage/shop.
>>
>> If I can get the temperature sensitivity problem licked, one way
>> or another, I can then use a longer averaging period and gain
>> improved accuracy.
>>
>>
>>
>>
>
> ______________________________________________________________
> GPS_Standard mailing list
> Home: http://mailman.qth.net/mailman/listinfo/gps_standard
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:GPS_Standard at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>