ANALYTICAL MICROWAVE SPECTROMETER EMPLOYING A GUNN OSCILLATOR LOCKED TO THE ROTATIONAL ABSORPTION-LINE

A means of locking the frequency of a Gunn effect oscillator operating in the 18-26 GHz range, to the spectral line of a molecular gas which exhibits the quadratic Stark effect, has been developed. The gas is confined within a Fabry-Perot semiconfocal cavity resonator at a reduced pressure of the order 1-10 Pa (0. 01-0. 1 mbar). A sinusoidal Stark voltage of 6 kv peak-to-peak at 4. 5 kHz is impressed upon the gas in the cavity which is coupled to the Gunn oscillator operating at a frequency near to the line frequency. Working with the ammonia 25. 056 GHz (J equals 6, K equals 6) spectral line, frequency stability of the order plus or minus 30 kHz standard deviation was achieved over the working day. Double logarithmic calibration curves obtained at this and the 23. 870 GHz (J equals 3, K equals 3) lines were linear from the limits of detection (2. 5 and 3 PPM ammonia) to 10% volume fraction ammonia in air.