Sensitive Triple Modulation Method for Measuring Widths of Microwave Spectral Lines

A method has been devised for the measurement of microwave spectral line widths with high sensitivity. This method consists of a combined source modulation of the microwave power at 2.3 kc and Stark modulation at 100 kc in addition to the slow sawtooth sweep voltage. The use of two rf modulations eliminates the base line which arises from the variation of the klystron power with frequency. The base remains flat for a source modulation amplitude as high as 1 v. The sensitivity is greatly improved over the previous method in which only source modulation is used. It is suggested that the technique of triple modulation may also be used to resolve closely spaced lines.