Principle of Pulsed-Interrogation Automatic Cavity Tuning for the ACES Hydrogen Maser

The output frequency of the Space Hydrogen Maser (SHM) developed by the Observatoire de Neuchatel (ON) as reference clock in the Atomic Clock Ensemble in Space (ACES) experiment on-board of the International Space Station (ISS) shall guarantee a relative long-term frequency stability of sigma_y(10000s) = 1.5ldr10^-15. For this purpose, its microwave cavity resonance frequency is to be actively stabilized at 0.1-Hz level, in order to compensate for the cavity pulling effect. The associated electronics developed by ON uses a novel Pulsed-Interrogation Automatic Cavity Tuning (PI-ACT) principle. Details of the design and performance results of the ACT shown on a laboratory demonstrator are given.