[GPS_Standard] VE2ZAZ

[Bob Bownes]

So, 24 hours later, I still can't keep this thing locked if I crank
the value of S over 16. Right now, it is set as follows:

S: 8
F: 8
L : 4
H: 10
W: 20
N: 3

The coarse dac value swings from one end of the scale to the other.
When I happen to catch it in the unlocked state, I may make a coarse
DAC adjustment of 150-200 to get it lock again.

The DAC is changing the output of the oven, just not fast enough to catch up.

Any hints?


On Tue, May 25, 2010 at 4:26 PM, Dave Platt <dplatt at radagast.org> 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.
>
>
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