Abstract
This paper describes a measuring system which automatically displays the phase variation of two different electromagnetic waves radiated together in the measuring channel. The two waves are an 8‐mm carrier wave and its second harmonic 4‐mm wave. The latter is produced by distortion of the carrier signal before its entrance into the measuring channel. The carrier wave is sawtooth frequency modulated. By using a long reference path a low frequency sinusoidal interference signal is produced for both waves. Tuned amplification does not affect the phase information of the interference signal and the high sensitivity needed for the second harmonic detection is achieved by biasing of the detector crystals. The drive power may be as low as 10 mW and the phase measurement is to large extent independent of amplitude variation for both waves.

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