Sensitivity and response time improvements in millimeter-wave spectrometers

Abstract

A new version of a microwave spectrometer for the detection of gaseous pollutants and other atmospheric constituents is described. The spectro meter, which operates in the vicinity of 70 GHz, employs a Fabry-Perot resonator as a sample cell and uses superhetrodyne detection for high sensitivity. The spectrometer has been modified to incorporate a frequency doubler modulated at 30 MHz to permit operation with a single Gunn oscillator source. As a result, faster response time and somewhat greater sensitivity is obtained. The spectrometer is capable of detecting a minimum concentration of 1 ppm of S0~ diluted in air with a 1 second time constant. For OCS diluted in air, the minimum detectable concentration is 800 ppb and with a 10 second time constant 300 ppb.