A wide-band QPSK modulator using branch-line coupler and MESFET switches

This paper reports the design and implementation of microstrip based quadrature phase shift keying (QPSK) modulator using quadrature hybrid (Branchline) coupler and MESFET switches. The modulator is designed to operate between 7.5 GHz to 9.5 GHz. Fabricated modulator has carrier suppression of 29 dB across the frequency range and has an amplitude imbalance of less than 0.1 dB with phase imbalance of 0.4deg around its center frequency. The modulation is checked upto 160 Mbps and spurious suppression is found to be greater then 40 dB and has zero DC power consumption.