A novel 180° microstrip phase shifter based on cascaded branch-line couplers for direct QPSK modulation

This paper presents a design and construction of a microstrip QPSK modulator using a novel 180° phase shifter. This phase shifter consists of two cascaded branch-line couplers, thus having two input ports. By feeding an RF carrier to different input ports controlled by digital bits, 0 and 1, the phase of the RF carrier is shifted by 180°. The microstrip prototype of this modulator yields a good result. The insertion loss is lower than 11.7 dB. The phase and magnitude error is lower than ±0.4° and ±1.49% respectively. The image ratio is lower than -31 dBc and the carrier leakage ratio is lower than -21 dBc. The prototype was fabricated onto a hydrocarbon ceramic, RO4350B, with the dimension of 8.6×7.8 cm.