Optimal Design Criteria for Millimeter-Wave Spectrometers

Abstract

-Optimal design criteria for millimeter-wave cavity-type spectrometeri, used in the measurement of molecular spectra, have been derived. Using an optimally designed spectrometer, the technique of molecular spectroscopy offers a highly sensitive and specific experimental research method for studying chemical kinetics and for monitoring gaseous pollutants and constituents of interest in pollution research. The method is also suitable for the study of the molecular properties of short-lived transient chemical species with lifetimes as short as one millisecond or less. Since the sensitivity of the spectrometer is determined by the total noise introduced during the prbcesses of microwave generation, absorption in the resonant cavity, and the detection and amplification of a spectral line signal. the spectrometer has been analyzed and optimized in detail with respect to the equivalent noise temperatures associated with these processes. The conclusions drawn from the optimum design criteria are applied in the design of a tunable high-Q resonant cavity spectrometer operating at 70 GHz.1. INTRODUCTION -Mi 11 imeter-wave rotati ona 1 spectroscopy measures the absorption in the millimeter region of the electromagnetic spectrum, caused primarily by transitions between pure rotational states of gaseous molecules possessing permanent dipole moments. The physical process leading to absorption is a coupling of the electrical vector of the incident microwave radiation with the permanent dipole moment of the rotating molecule. Using an optimally designed measuring s~stem, this technique is a highly specific and sensitive experimental research method for studying chemica 1 :klf:netit:s and for moni tori ng gaseous poll utants and constituents of interest in pollution research. Our investigations show that in addition to the detection and measurement of ordinary gases and vapors, the method is suitable' for the study of the molecular properties of shortlived transient chemical species with lifetimes down to a millisecond or. less. The technique supplements and may replace gas chromatography, mass spectroscopy, and various laser methods in the identification of trace-gas chemicals, and also makes possible fast, highly specific and sensitive measurements in the laboratory.

Noise Considerations in Millimeter-Wave Spectrometers

“…The effect of source noise has been discussed by Strandberg (6), and by Leskovar et al (7). Two contributions are respon sible for the total noise reaching the microwave receiver.…”

Detection and Measurements of Air Pollutants and Constituents by Millimeter-Wave Microwave Spectroscopy

“…system noise comes from the noise generated by the source oscillator. The effect of source noise on the sensitivity of microwave spectrmneters has been discussed by Strandberg 16 and by 7 Leskovar et al…”

Millimeter and submillimeter wave absorption by atmospheric pollutants and constituents

“…As in the case of spectrometers operating at lower frequencies, the primary sources of noise which limit sensitivity (25) can be attributed to the source generator which excites the molecules and to the receiving device which detects the radiation. The state of the art of these components has been discussed recently by Blaney (26) and the reader is referred to this excellent overview for a more complete review.…”