Drift Correction for Active Hydrogen MASERs

The drift characteristics of well-aged Hydrogen MASERs may be reasonably modeled as linear over periods greater than two weeks. There are several broad strategies used to manage drift: a) quantify and acknowledge drift in a system error budget, b) compensate using post-processing methods, c) apply a linear correction, or d) steer the MASER output frequency to a superior reference. The selection of drift management method greatly depends upon the requirements of a user’s system. Users, such as radio astronomers, may de-emphasize active drift management as their primary concern is 1-to-100 second stability. These users often prefer to compensate for frequency drift after passing a predefined frequency offset threshold. This removal takes the form of a manual frequency adjustment on the MASER. However, time-scale users, who require exact characterization and compensation of drift, will need to intervene using post-processing methods to avoid tampering with a characterized MASER which may be contributing to local UTC. This paper will discuss the underlying processes that may cause linear drift, how this drift is quantified, and then give an overview of the design implications of each drift handling method. For illustration purposes, a MASER’s drift will be characterized, and a linear drift correction will be applied. This illustration covers the use case when a MASER will perform as a standalone standard with moderate long-term stability requirements.