[GPS_Standard] ocxo recommendations

[Dave Platt]

On 10/16/2014 10:05 AM, Phil wrote:
> What ocxo's has the group used? (other than the HP unit)

I build mine around an Efratom 10 MHz VCOXCO.

The one I bought was on a sub-board, mounted on a larger
telecom board which generated a 15 MHz (I think) disciplined
signal.

There are a bunch of similar Efratom oscillators and boards
available on eBay - I see at least two suppliers - see
eBay auctions 280558808335 and 261327219062.  The latter
looks like a sub-board very similar to (perhaps identical
to) the one I incorporated into my VE2ZAZ project.  For
$20 (delivered) it seems like a good candidate.

They do have a disadvantage, compared to the HP - this
oscillator family does not seem to provide a tightly-
regulated 5-volt (or other) reference output for the
control voltage.  The VE2ZAZ design is such that the
5-volt supply to the PIC is effectively the reference
for the control voltage, and a 7805 isn't the most stable
voltage reference in the world... using a precision 5-volt
regulator to drive the PIC would be a nice upgrade.

I found that the temperature stability of mine wasn't as
great as I had hoped - there was a perceptible shift in the
control voltage over the course of a day, as the temperature
in my garage drifted around.  I improved things a lot by
building a small temperature-controller-and-heater board,
mounting it inside the project-box enclosure, and then
insulating the whole box with foam - basically turning
it from a single-oven to a double-oven architecture.
This cut the control-voltage changes by about 50% and
smoothed out the curve a lot, to the point where my
modified (PI algorithm) control logic is keeping the
tracking quite good.

http://snulbug.mtview.ca.us/dave/fll.png

The drift may have been my own fault, though.  When I built
my VE2ZAZ, I researched the Efratom oscillators and could not
find any specific information about their voltage-supply
requirements.  I experimented, and found that mine seemed to
heat up the oven and stabilize OK when powered from 12 volts,
and I was afraid to drive it harder and perhaps burn it out,
so that's what I gave it... I'm using a laptop-type 19-volt
DC supply and regulating down to 12 volts.

Both of the auctions now up on eBay show an input voltage
of 24 volts.  So, I may very well have been under-powering
my oscillator's oven and making it harder for it to maintain
the proper operating temperature.

Maybe I'll dig up a 24-volt supply and rewire things a bit,
and see if that improves the performance.