A microwave spectrometer with a frequency control system employing a frequency ‘‘scanning window’’ locked to the rotational absorption peak

Publisher Website
Google Scholar
Abstract
This article reports a microwave spectrometer with a novel technique of locking a frequency ‘‘scanning window’’ [F1,F3] in the radiation source to a microwave rotational absorption peak. The center (F2) of the scanning window is initially locked to the resonant frequency (Fr) of a tunable resonant cavity so that the center of the scanning window may be tuned by changing the resonant frequency of the cavity. Microwaves are then input to a nonresonant gas cell containing the analyte gas at low pressures and by means of Stark modulation the lower sideband (F1,F2) and upper sideband (F2,F3) of the scanning window are amplitude modulated at different phases. After demodulation of the two sidebands, an error signal is derived through addition of the two demodulated signals (being zero when the scanning window exactly overlaps with the absorption peak) and this is used to lock the resonant frequency (Fr) of the tunable cavity (and thus the center (F2) of the scanning window) to the center (F0) of the absorption peak of the analyte gas. For quantitative measurement, a signal is derived through subtraction of the two demodulated signals (being maximum when the center of the scanning window is approximately at the center of the absorption peak). This spectrometer system has been used to measure ethylene oxide gas (absorption line 23.123 GHz) at concentrations up to 100% and water vapor (absorption line 22.235 GHz) at concentrations from −40 °C dewpoint (127 ppm) to 50 °C dewpoint (138,736 ppm). The detect limit of the spectrometer has been tested and found to be approximately 3×10−9 cm−1 (signal‐to‐noise ratio equal to 2).