小型化是目前氢钟的发展方向之一。氢钟的小型化主要是谐振腔的改进， 它的结构尺寸决定了氢钟的最小尺寸和重量。目前采用的小型腔主要有电极负载腔、 磁控管腔、介质负载腔和TE111腔。新型的谐振腔虽然减小了体积减轻了重量，但是由 于它本身已经很难满足起振条件，所以在伺服电路上也不同于主动型氢钟。根据电路 原理的不同，将小型化的氢钟分为反馈振荡型和被动型。反馈振荡型氢钟使激射器工作在反馈振荡状态， 通过正反馈维持激射器振荡。被动型氢钟的激射器工作在振荡阈值之下，其作用与谐 振放大器相似。
Subcompact hydrogen clocks are developed in different institutes in the world. Different compact microwave cavities are used in order to reduce the size and weight, comparing to which of 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.