Cavity modulation coupled with asynchronous source modulation in a microwave cavity spectrometer

Abstract

This paper reports a novel modulation technique, namely, cavity modulation coupled with asynchronous source modulation, employed in a simple microwave cavity spectrometer system. The feasibility of the technique was successfully demonstrated by determining the output of the system whilst examining one of the absorption peaks of ethylene oxide gas (with centre frequency F0 = 24.923 GHz for the level transition 202 to 211). The variation in the output signal with frequency was as expected to within a standard error of 8.3%. Also, a good linear correlation was demonstrated between the magnitude of the output signal and the concentration of ethylene oxide gas in the gas cell (to within a standard error of 4%). The operation of this system is such that the microwave source frequency, F, sweeps across a given range (F1 to F2) which contains one of the absorption peaks of the analyte gas whilst the cavity resonant frequency, Fr, is oscillated within the profile of the absorption peak. Signal enhancement is achieved by adding a relatively small magnitude high-frequency 'dither' signal to the source frequency sweep pattern. The salient information of the gas absorption due to the oscillation of the resonant frequency of the cavity is carried by the 'dither' signal and then amplified and extracted by a series of tuned amplifiers and demodulators.