The design of mono frequency modulator for passive hydrogen maser control

Southeast University of China began to research small passive hydrogen maser since 2005. In respect of frequency control, in order to control both voltage control crystal oscillator (VCXO) loop and automatic cavity tuning (ACT) loop, the mono frequency modulation method is adopted rather than dual frequency modulation method or timeshare modulation method used widely. The 1.420405GHz microwave signal modulated by a mono frequency square or sine wave is injected into the microwave cavity. After interacting with the hydrogen atoms in the cavity, the amplitude modulation (AM) signal is coupled out and processed to get two different error signals to control VCXO and ACT loop respectively. In this paper, the principle of mono frequency modulation for passive hydrogen maser control is introduced and a digital modulator based on direct digital synthesizer (DDS) and digital signal processor (DSP) is designed to generate error detecting signal, which is the core of the mono frequency modulator to interrogate the error signal of VCXO and ACT for passive hydrogen maser control.