Functioning of EPR Automatic Frequency Control in the Presence of the Circulator’s Spurious Leakage

This article presents an analysis of spurious signals’ influence on automatic frequency control (AFC) in EPR spectrometers. The primary source of spurious signals is leakage across the circulator in the microwave bridge. Additionally, the signals reflected from connectors in the line between the circulator and the resonator modify that signal. Spurious signals may degrade the performance of AFC, significantly offsetting lock points from the center frequency of the resonator. The offset’s size depends on the parameters of the resonator and the amount of the circulator isolation. It can be minimized by the appropriate use of a phase shifter located in the line between the circulator and the resonator. Another way to avoid these problems is to introduce a leakage compensation arm to the microwave bridge.