Miniature passive hydrogen maser at Shanghai Observatory

A miniature passive hydrogen maser is developed at Shanghai Observatory which employs compact cavity design and square wave frequency modulation. It is 704 mm high, 410 mm wide, 526 mm deep and weighs 40 kg. Preliminary measurements indicate a frequency stability of approximately σ(2, τ)=1.0×10^-11 τ^-1/2 (1s<τ<5×10⁴ s). Electrode-loaded resonant cavity and 4-pole state selector are used in passive hydrogen maser to reduce the size. Other design in the physical part is similar to the SOHM-III active hydrogen maser produced by Shanghai Observatory. Electronic part of the passive hydrogen maser comprises two frequency lock-loops: 10 MHz crystal oscillator frequency is locked to the hydrogen emission line and resonant cavity frequency is locked to the crystal oscillator frequency. Square wave frequency modulation is used in both loops in order to avoid the even harmonics in the sine wave modulation. At present, the initial test is finished. Next we will do some work to improve the performance of the miniature passive hydrogen maser