The Frequency Stability of a Pair of Auto-Tuned Hydrogen Masers

Abstract

An automatic tuning circuit for correcting the resonant frequency of the cavities used in a pair of hydrogen masers is described, with particular emphasis on measurements of the relative frequency stability obtained using the system. The measurements show that improved long-term stability of the two NRC masers can be obtained by the use of the auto-tuner. The stability of the measured frequency difference between the masers is now typically 1.5 parts in 1014 over periods up to one week. Correlation has been found between changes in the frequency corrections supplied by the auto-tuner and changes in atmospheric pressure, indicating that the cavity frequency of each maser is sensitive to pressure changes. The magnitude of this effect and modifications made to the masers to reduce it, are discussed.