I'm just so excited by my first results, I just had to tell someone...
This GPSDO is inspired by the project in Silicon Chip magazine, September 2018:-
http://www.siliconchip.com.au/Issue/2018/October/GPS-synched%2C+lab-quality+frequency+reference+(Part+2)(select #5 at left)
(I won't reveal any details of that project not visible above, out of consideration for the magazine. If you're interested, the article can be purchased )
I've used the voltage controlled, oven stabilized oscillator part of the project, and will use it's multiple output frequency synthesizer in the final build.
But I've used a different processor (Arduino Nano - that may change), and implemented the counter function in external hardware.
The oscillator produces 40MHz nominal, which is counted during the GPS PPS pulse. After the count, the control voltage to the oscillator (with quasi 24 bit resolution) will adjust the frequency until a count of 40,000,000 is obtained. The counter can operate over multiple PPS periods (up to 106 seconds) to reduce jitter effects, and get very fine resolution measurements of the output frequency.
I'm excited because the first test, with no oven cover (and a ceiling fan blowing cool air all over the place), and with the heater turned off, shows only one count deviation. Which is *far* better than I expected.
The measured frequency is 39,999,469 to 39,999470 - I've seen only those two numbers after operating for well over an hour now.
(And that variation could simply be because the actual oscillator frequency is 39,999,469.5Hz)
This is my development rig:-
Top left PCB: Oven and oscillator, with cover removed.
Bot left PCB: Counter and gating logic
Top right PCB: Just a 32 bit divider to get a bunch of frequencies (not completed)
Bot right PCB: Output frequency synthesizer (not completed)
Middle: Bodge board with oscillator buffers, ambient temp sensor, shift register to read counter output.
Bottom: Small GPS module (covered by wires), and Arduino Nano
(if there's interest, I can share details of the counter and gating logic used here)
Here is the first page of output:-
This setup could be used for any voltage controlled oscillator, but I really like the idea of the SC design, since it's a TCXO built from scratch, using all brand new parts, and therefore no need to use whatever can be found on eBay.
Earlier development of the oven control shows a very stable 35C, with only +/-1 resolution unit of the DS18B20 sensor (0.0625C):-
Now to work on the software combining the oven control and get to adjusting for 40,000,000Hz...