Miniaturized reverse modulation loop for a CQPSK 120-Mbit/s modem

The design and performance of a miniaturized reserve modulation loop (RML) for carrier recovery in a 120-Mb/s coherent quadrature phase shift keying (CQPSK) modem for onboard satellite applications are discussed. The RML circuit, consisting of modulator, demodulator, and comparator circuits, has been fabricated using quasi-monolithic techniques with dimensions of 1.65*4 cm. The relative phase for all four states of the modulator is in close agreement with design values of 90 degrees +or-1 degrees over a 200-MHz bandwidth at 3.95 GHz. The demodulator and comparator circuits of the RML have successfully recovered a 120-Mb/s bit stream.<>