An STM32 GPSDO

Designing, assembling, programming and testing an ultra-low-cost GPS-disciplined oven controlled crystal oscillator / lab clock based on an STM32 32-bit ARM microcontroller

Keywords: STM32, GPS, GPSDO, crystal oscillator, frequency standard, atomic clock

André Balsa, April 2021

The Project

I decided to build my own version of a 10MHz GPSDO with the following features:

• Very low cost i.e. < 30 euros ($35), short and easily purchased BOM.
• Acceptable performance (+/- 1ppb) "out of the box", can be fine tuned over time.
• Uses an STM32F411CEU6 "Black Pill" microcontroller module, programmed in C/C++ using the Arduino IDE (STM32duino).
• Requires only a 5V @ 1A power supply, supplied through a USB C cable.
• Compact and portable.
• Optionally battery powered (allows taking it outdoors for optimal satellite signal reception).
• An optional suite of environmental sensors (temperature, humidity, air pressure, voltages, OCXO current/power consumption).
• An optional Bluetooth serial interface for wireless communication with a PC.
• Extensive logging of various operating parameters to allow further software tuning.
• An optional small OLED displaying room temperature, UTC time, uptime, operating status, measured frequency.

The first breadboard prototype:

In normal operation this is displayed on the small OLED: