Broad-band direct QPSK modulator/demodulator for wireless gigabit communication

The design and measured results of a broad-band direct quadrature phase shift keying (QPSK) modulator and demodulator are described in this paper. The circuits are fabricated using 1-μm GaAs HBT technology. To suppress the local oscillator (LO) leakage, the double-balanced mixer is selected as the core unit in the modulator/demodulator. An embedded four-way quadrature divider which includes a Lange coupler and two Baluns is utilized in the system to generate quadrature-phase LO signals. As results of a back-to-back test, the system can operate at data rates in excess of 2 Gb/s (1 Gb/s per I and Q channels) at 30 GHz. The supplies of the modulator and demodulator are 5.0 V and 4.5 V with size of 1.35 mm×3.5 mm and 1.36 mm×3.4 mm, respectively.