[GPS_Standard] VE2ZAZ

[Bert, VE2ZAZ]

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.
> 
> 
> 
>