Subcompact hydrogen clocks

Subcompact hydrogen clocks are developed in different institutes in the world. Different compact microwave cavities are used in order to reduce the seize and weight, compared to conventional hydrogen clocks. Electrode-loaded resonant cavity, magnetronic cavity, dielectric-loaded cavity and TE111-mode cavity are often used in the subcompact hydrogen clocks at present, which will lead to failure of resonant condition. Two approaches in compact hydrogen clock design are presented. One design, the Q-enhanced maser oscillator, employs a hydrogen resonance to sustain oscillation in a microwave cavity which quality factor is enhanced electronically. The other design, the passive maser, uses the atomic resonance as a narrow bandpass amplifier.