A 4-GHz QPSK modulator for high bit rates

A 4-GHz microwave integrated circuit, quadrature phase-shift keying (QPSK) modulator has been developed for 120-Mbit/s data rates. The measured phase and amplitude balance are within + or - 1 deg and + or - 0.15 dB, respectively, over a 200-MHz bandwidth. The measured rise time of a typical phase-state transition is less than 1.5 ns. These features guarantee good performance at bit rates even higher than 120 Mbit/s. The QPSK modulation is achieved by driving two balanced binary phase-shift keying (BPSK) modulators in parallel and using a 90 deg, 3-dB hybrid to combine their outputs. The BPSK modulators have been designed using a different approach, and each exhibits an excellent amplitude balance, + or - 0.1 dB, and a phase balance of + or - 0.5 deg over the 200-MHz bandwidth. The QPSK modulator is laid out on a 1.8 x 2.3-in. alumina substrate. It is directly compatible with standard emitter-coupled logic gate drivers. Its size, power consumption, and weight make it suitable for use in a spacecraft regenerative modem.