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This technical documentation has been proofread by Claude-Sonnet-4 for readability, grammar, and formatting consistency. All technical content, project details, circuit designs, measurements, and conclusions have been verified and approved by the author.

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Project Overview

This is the GPSDO designed by my friend BH4GEX, who invited me to test on the device and give constructive feedback. It comes with an adorable CNC shell at only $30! You can contact me for purchase.

The GPS-Disciplined Oscillator (GPSDO) aims to create a precision frequency reference by locking a high-quality oven-controlled crystal oscillator (OCXO) to GPS timing signals. This project achieved at minimum 40 parts-per-billion (ppb) long-term frequency accuracy (limited by testing device).

System Architecture

The system consists of several key components:

Core Hardware

  • Microcontroller: STM32F030
  • OCXO: CTI OC12SC36 with ±20ppm initial accuracy (salvaged from used signal generator)
  • DAC: 12-bit DAC7563
  • Buffer: CDCV304PWR / LMK1C1104

The version I received is an engineering sample, which lacks a pull-up resistor on the buffer enable pin. There was NOT an internal pullup as expected... so a hotfix flywire there is.

Measurement Results

Testing was performed using Keysight CXA signal analyzer and EXG vector signal generator as reference.
All of the three buffered channel outputs stable signal at exactly 10.000000 MHz.
The on-board RGB led would indicate pll status, converging status, and PPS blink.
It generally takes ~5mins to lock and converge within ±20ppb (led turns green).

The spectrum analysis shows:

  • Center Frequency: 10.000000 MHz (exact)
  • Output Power: +12.06 dBm
  • Phase Noise: Better than -80 dBc/Hz at 1Hz offset (limited by testing equipment)

With a 10Hz span and 1Hz resolution bandwidth, the signal shows excellent short-term stability with no visible sidebands or spurious content.

Have questions about this project? Feel free to reach out via the Contact page.