10 MHz Frequency Reference

Background

There are a number of example projects on the net that try to lock an ovenized crystal to a GPS reference. The idea is that the GPS clock will have good long-term stability, but poor short-term stability An ovenized crystal will have good short-term stability, but worse long-term stability. Combine the two and you have a perfect reference!

What’s so cool about this project? The ovenized crystal is built onto the main PCB, there is no fancy or expensive extras needed. This oven contains both the crystal oscillator, and the DAC that provides the control voltage for the crystal. This minimizes potential sources of error.

Internal Pictures

1. Rockwell Jupiter GPS Module is mounted here
2. Voltage reference for DAC is isolated to minimize error due to PCB bending
3. Main Atmel AtMega164P
4. Heater voltage and current are monitored, so the MCU can maintain constant power into heater, regardless of input voltage.
5. Oven chamber, detailed below
6. Bridge allows AC or DC

1. CPLD located inside oven runs DAC, also provides PLL to compare GPS frequency to scaled version of 10 MHz output
2. Voltage-Controlled Temperature-Compensated Crystal Oscillator (VCTXCO)
3. Transistor is heater element
4. DAC located insided oven to reduce thermal drift
5. Shift register (for extra I/O pins)
6. Clock buffer, again located inside oven
7. Frequency output

Step Response of Heater