GPS disciplined 10MHz Oscillator

For applications requiring a very precise reference frequency, the use of an oven-controlled quartz oscillator (OCXO) is recommended, as it provides excellent frequency stability by maintaining the crystal at a constant temperature. The frequency stability of, for example, a 10 MHz OCXO (Oven-Controlled Crystal Oscillator) from Morion, Inc. (MV89-B01E-10.0 MHz) is already very good.

 

However, it can be further improved:

OCXOs usually provide a control input (UIN) that allows fine adjustment of the output frequency within a limited range. With this capability, OCXOs effectively become voltage-controlled OCXOs (VCOCXOs). To generate the control voltage required for frequency regulation, methods such as a phase-locked loop (PLL) or the so-called “disciplining” approach can be used.

 

Since the phase noise performance of a PLL is not optimal for this application, the disciplining method was chosen. This approach requires a highly accurate external timing reference. A GPS receiver module (e.g., GY-GPS6MV2) can be used to provide such a precise timing pulse.

The exact function of the function is best explained with a Block Diagram:

The GPS 1-PPS pulse serves as the sampling trigger for the triangular waveform. This means that once per second, the instantaneous value of the triangular waveform is captured using a sample-and-hold circuit. A triangular waveform is a linear ramp, i.e., its voltage increases linearly with time. As a result, the phase information is directly encoded in the voltage.

 

This produces a voltage that is proportional to the phase difference between the local oscillator and the GPS reference signal. The sampled signal is then low-pass filtered to obtain a smooth control voltage.

 

The 10 MHz signal is distributed across four decoupled outputs.