Study on high stability frequency equipment based on double disciplined loops

The Chinese BD-2 Navigation system can be used to provide a precise time and frequency reference to control various local oscillators such as an Oven Controlled Crystal Oscillator (OCXO) or a rubidium clock. In order to get a frequency source with high long term stability and short term stability, double disciplining loops are developed. Based on the analysis of The BD-2 receivers´ timing signal - One Pulse Per Second (1PPS), the design of the hardware and control strategy for converting the time interval between the BD-2 output and the local oscillator to the frequency difference is provided. Unlike traditional design, in order to achieve a high performance, the double closed feedback loop is adopted. BD-2 receiver´s 1PPS is adopted to act as the reference signal to discipline the local rubidium, it serves as the first loop. The disciplined rubidium acts as another frequency reference to discipline the local low noise OCX, it serves as the second loop. In order to overcome the BD-2 timing signal frequency jitter, the adjacent averaging numerical filter is designed. The disciplining algorithm uses Least Squares method to overcome the phase error derived from the receiver. Both the loops have the same strategy, they are called digital phase locked loops, including digital phase error detector, digital loop filter and the voltage controlled oscillator. Illustrations of the model and experiment result are also presented. From the experiments, the normalized frequency accuracy of the disciplined OCXO could be improved from about 2×10^-8 to about 1 ×10^-11, keeping better long term stability and phase noise.