A digital servo with single frequency modulation for passive hydrogen maser

This paper introduces a digital servo using single frequency modulation for passive hydrogen maser. The main parts of this system include ADC, FPGA and DAC which sample error signal, process the signal and output two channels controlling voltage. The algorithm is used to separate two errors by single frequency modulation principle. Then response curves of two channels are gotten to calculate proper PID controller parameters. In the end, two incremental PID controllers are used to get two channels control voltage to control the resonant cavity and OCXO respectively. The influence caused by the speed and precision of DAC to frequency stability has been analyzed. The experiments prove that the precision of DAC had a little influence on the short and medium term frequency stability at high control speed. The frequency stability of passive hydrogen maser with this system has been tested. It has achieved at σ_y(1000)<;3×10^-14.