Phase-Locked Loops Used with Masers: Atomic Frequency Standards

Active atomic frequency standards, such as hydrogen and rubidium masers, are characterized by a very good frequency stability. However, they deliver a signal at a power level and at a frequency that are not practical to the users. This paper describes the techniques used to phase-lock to those masers a standard crystal oscillator at a cardinal frequency. A review of the various systems realized up to now is made.