A Frequency Meter for Microwave Spectroscopy

Abstract

A frequency meter using known absorption lines as microwave frequency standards has been developed. By mixing signals of unknown microwave frequency f_x, standard microwave frequency f₀, and known radiofrequency signals nf_c from a crystal‐controlled harmonic generator, it is possible to obtain a set of ``beat'' frequencies f_r which can be detected by a calibrated radio receiver. From the relation $$f_r{\mathrm{=\pm m(f}}_x{\mathrm{-f}}_0{\mathrm{)\pm nf}}_c$$ it is possible to determine values for the unknown microwave frequency f_x. Tests on methanol absorption lines with ammonia lines as microwave frequency standards show that microwave frequencies can be determined to ±0.03 mc/sec. Advantages of this device over other frequency meters are discussed; the accuracy is ultimately limited by the uncertainties in the value of the standard frequency f₀.