L-band quadriphase phase shift keyed modulator

The design of a phase shift keyed modulator, which is a part of an L-band transmitter developed for the space related portion of a detection and digital communication network, is discussed. The important aspect of the design is that, unlike a conventional 2-bit phase shifter, this device utilizes 180-degree bit phase shifters which are identical and achieve a 90-degree offset between them by virtue of the inherent 90-degree phase difference of the backward wave coupler. In this case, the backward wave coupler is realized as a Lange, interdigital microstrip device. An inherent 3 dB power loss with this configuration is of no consequence since amplitude leveling by an AGC circuit preceding the phase shift keyed modulator is used and is an acceptable tradeoff for phase stability purposes.