It is great we have got here more threads on different approaches to the GPSDO design!!
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To the discussion on "old" Miller design (or any MCU-less one) - could we somehow formalize which one is better for a specific use?
...
Imo, to answer your questions:
1. As I wrote before, just because a GPSDO has an MCU means nothing. It all depends on how the MCU is used, in other words, on the program.
2. Please define "better". In the case of a DIY GPSDO, this could mean many, many things.
3. I am not at all critical of the Miller GPSDO design, quite the contrary actually. But technology has evolved while his design did not. So, in 2021, it's obsolete. Note that obsolete does not mean it was a bad design or didn't do its job many years ago, similar to the Apollo Guidance Computer on the lunar lander that put men on the moon some 50 years ago.
https://en.wikipedia.org/wiki/Apollo_Guidance_Computer4. There are so many
potential advantages to using an advanced MCU in a GPSDO (
or any lab instrument) that I can't really make a list, it would just be too long. However, as I noted above, these potential advantages translate into reality if and only if the
software/firmware implements them. Conclusion: the software really matters when you add an MCU to a GPSDO (
or, again, any lab instrument).
Let's see some
advantages of the STM32 GPSDO vs other designs:
1. Inexpensive.
2. Can be assembled on a breadboard in two~three hours.
3. Easily purchased components.
4. Informative OLED display.
5. Low power consumption (< 5W).
6. Full suite of sensors.
7. Modular.
8. Bluetooth interface to a smartphone.
9. Full GPS status reporting.
10. Open source hardware and software.
11. No adjustments, works "out of the box".
12. Hardware implements both PLL and FLL control loops. (Note: the PLL software is still work in progress)
13. Upgradable firmware. Flashing a new firmware takes less than a minute.
14. Uses a powerful 100MHz 32-bit ARM Cortex-M4 MCU.
And there are some
disadvantages:
1. No PCB yet - this is coming ASAP.
2. Not a commercial project, you can't buy it assembled or as a kit.
3. People with no programming background may be put off by the GPSDO software, or how to flash the STM32 MCU.
4. Still an evolving project (Beta stage).
5. There is no user manual yet.
6. Even though it is probably capable of better performance, the claim is 1ppb (10E-9) short term accuracy/stability. That translates into 0.01Hz frequency or 10ns phase accuracy/stability; or 1 second every 32 years, but I honestly doubt anybody is going to keep a GPSDO running continuously for 32 years...
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