Methodological approach to GPS disciplined OCXO based on PID PLL

The Global Positioning System (GPS) can provide a precise frequency reference to control local oven controlled crystal oscillator (OCXO). Based on the analysis of the GPS receiver´s timing signal-one hundred pulses per second (100PPS), this signal is adopted to act as the reference signal. Instead of a normal phase detector or a professional frequency counter, a high resolution time to digit converter (TDC) is used to act ac the phase detector to measure the frequency error of the pairs. In order to overcome the frequency jitter of the GPS timing signal, the averaging and adjacent averaging numerical filter have been designed. The phase error estimates are processed to overcome the frequency jitter by using least square fitting(LSF), a proportional-integral-differential (PID) control loop and converted to digital-to-analog converter (DAC) digital codes to provide the controlling voltage to the local OCXO. The designed phase locked loop (PLL) based on PID control has a good performance at a very low frequency about 1~10Hz. Illustrations of the system structure and the experiment results are also presented. It shows that the normalized frequency accuracy of the disciplined OCXO could be improved from 2times10^-8 to about 2times10^-11 over 1 hour.