TRIPLE MODULATION TECHNIQUE APPLICABLE TO MICROWAVE SPECTROMETERS WITH CAVITY GAS CELL

A triple modulation technique, applicable to microwave cavity spectrom eters, is described, which enables Stark, Zeeman, double resonance or cavity modulation in the cavity gas cell to operate at a relatively lo w frequency, while satisfying the criterion that the microwave crystal detector should operate at a relatively high frequency. This techniqu e is such that the microwave source is frequency double-modulated by m eans of a sweeping signal of frequency f1 and a relatively high-freque ncy sinusoidal signal of frequency f2 applied to a tuning varactor, wh ile an additional relatively low-frequency modulation such as Stark, Z eeman, double resonance or cavity modulation (frequency f3) is applied in the cavity gas cell. The low-frequency modulation signal (f3), whi ch contains the salient information of the analyte gas, is carried by the high-frequency signal f2, the former being extracted and amplified by a series of filters, demodulators and tuned amplifiers.