Optimal Design Criteria for Millimeter-Wave Spectrometers

Optimal design criteria for millimeter-wave cavity-type spectrometers, 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 processes 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.