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TruePosition GPSDO build question
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Squiddaddy:
I put a TruePosition GPSDO and an Atmega 328 controller in an aluminum case, similar to the BG7TBL units on Ebay.
Those units are not ventilated, so I didn't either, to keep temp fluctuations down.
The room temp is 22-23C or 73F. The GPSDO climbs up to about 38C. At least I haven't been letting it go beyond that. I end up removing the cover.
I'm worried about this running away with heat.
What temperature should I really start worrying, or to keep the frequency stable?
Anyone with a BG7TBL who can tell me how hot they run?

Some pictures of my project.
I have tried with and without the foam cap on the OCXO.
Thanks

edpalmer42:
Do NOT put a foam cap over the OCXO.  It's designed to expect a certain amount of heat to escape.  If you prevent that, the unit heats up to a point where the oven throttles back, but the temperature keeps going up!  The oven turns off completely, which means that there's no regulation of the temperature, but the temperature keeps rising just from the normal operating power of the oscillator!  Worst case, the temperature keeps going up until the circuit fries.  Best case is that the unit operates above its design temperature and the performance suffers.  The oven controller could even start oscillating which would really mess up the stability!

Most OCXOs operate their ovens in the 50C to 80C range.  Each unit has its own sweet spot for temperature.  The controller will try to maintain the temperature within a very tight range.  Very high performance units might have a range of 0.001C.   Maximum operating temperature is typically something like 10C below the oven temperature.

Protect the OCXO from drafts (hot or cold), sunbeams, etc., and leave some ventilation room around the outside of your GPSDO, but otherwise, leave it alone.  I see that the NVG47B oscillator is available in three operating temperature values from 50C to 85C.  Unfortunately, there's no easy way to tell what the maximum operating temperature of the NVG47A oscillator in the TruePosition is.  But 40C near the oscillator shouldn't be a problem.  Temperatures over 50C would require some careful investigation.

Ed
Squiddaddy:
I'll ditch the foam cap. I thought I read somewhere that a foam cap would help. I guess you can't believe everything you read on the internet.  :)
There is a CTS 196 OCXO on this TruePosition as it's a newer 2011 model.
That temp reading is what's reported from the board in the datastream. I'll put a temp sensor inside the case to get the surrounding air temp, and monitor for a while.
These things are pretty slick for the price. After I did a 12 hour survey to get position, it has stayed locked for several days, usually getting 5 to 9 satellites. 5 or 6 at night and 8 or 9 during the day.
Took me a while to get the software written the way I wanted it, but it worked out pretty good.

I appreciate your advice Ed!
edpalmer42:

--- Quote from: Squiddaddy on January 07, 2019, 12:54:30 pm ---I'll ditch the foam cap. I thought I read somewhere that a foam cap would help. I guess you can't believe everything you read on the internet.  :)
There is a CTS 196 OCXO on this TruePosition as it's a newer 2011 model.
That temp reading is what's reported from the board in the datastream. I'll put a temp sensor inside the case to get the surrounding air temp, and monitor for a while.

--- End quote ---

When I got started in this stuff I thought a foam cap would be a good idea.  But when I started reading the Time-Nuts mailing list I saw a message that explained why it was a bad idea.  If you were running an OCXO in a cold environment, the foam cap might be a good idea, but not in a normal temperature environment.


--- Quote ---These things are pretty slick for the price. After I did a 12 hour survey to get position, it has stayed locked for several days, usually getting 5 to 9 satellites. 5 or 6 at night and 8 or 9 during the day.
Took me a while to get the software written the way I wanted it, but it worked out pretty good.

--- End quote ---

A properly installed and operating GPSDO should stay locked forever.  Any loss of lock should be investigated.  Of course, if your antenna location and satellite visibility are crappy you might have issues.


--- Quote ---I appreciate your advice Ed!

--- End quote ---

Glad I could help.

Ed
jpb:
By way of comparison, my Star4 GPSDO reports (via Lady Heather) its temperature at around 48 to 49 degrees C which I was quite happy with though it is getting a little high - OCXOs generally are specced to either 0 to 50 I think or -25 to 75, that sort of range.

If an OCXO is put into a smallish metal box then it will get quite warm.

I think the crystal itself (within the oven) is kept at around 70 degrees C (though obviously higher for OCXOs specced to go to 75 externally) - it is what ever temperature presents an optimal turning or inflection point.
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