Thread: Homebrew GPSDO
View Single Post
Old 5th Mar 2021, 7:00 am   #11
Join Date: Apr 2017
Location: Buderim, Queensland, Australia.
Posts: 262
Default Re: Homebrew GPSDO

The "constant-temperature enclosure" is probably not required.
Particularly if the oscillator is already in a metal case with internal thermal insulation.

I am no expert, not even close on this subject, but after the latest post, I was quite curious about my own DIY GPSDO, and ran it overnight for some observations, which may be of interest.

My device uses a uBlox Lea5T RX and an NDK EG041 10MHz OXCO (cannot find a datasheet for this XO).
The whole device is in a 200x125x50mm plastic case.

So, starting from cold at 25 deg C, the case temperature of the XO runs up to about 50 deg, and stays there +\- 2 deg during stabilisation.
By this time the device has GPS stabilised with no alarms, and a temperature of 49 deg.
Most OXCOs have a temperature range over which they will work, typically -10 to +80 deg C.

I used a K1 type probe and digital meter for temp measurement directly on the XO metal case.

As the OXCO has its own temperature control, I don't see what an extra thermally insulated box will achieve, provided that the OXCO is within the manufacturer's operating range.

Testing my GPSDO
This is a literal dilemma, complete with garden paths, wormholes, and measurement uncertainties.
You get to a stage where you have to initially distrust everything.

I compared mine with two separate other working GPSDOs (permanently ON) at a mate's place while we had coffee and reminisced about radio for eight hours.
We used the CRO Lissajous comparison method.
After two hours, we had a stable circle, and it stayed like that until I went home six hours later.
If there is any doubt, if there is any slow circle wander, even very slow, there is a problem, and you have to rethink the process.
A GPSDO with an output frequency within a few Hz of nominal is not operating correctly.

After five minutes from ON, my OXCO seems to start about two hertz low, and drifts up to close to 10MHz, ie within 1 Hz, even without GPS action.
So some of these oscillators are already very accurate to start with, and quite possibly all that is required for most radio workshop use.
I say "close to 10MHz" because here in my workshop I cannot absolutely measure the OXCO frequency to say 0.1 Hz, without another separate GPSDO feeding 10MHz as EXT REF for my Philips Freq Counter.

I can live with this, as I can now use my GPSDO as EXT REF for my Freq Counter when measuring frequency of all other devices.

Very useful device to give measurement certainty.
Very worthwhile project.

Good luck with your projects.

Last edited by Radio1950; 5th Mar 2021 at 7:18 am.
Radio1950 is offline   Reply With Quote