A direct pi /4 DQPSK modulator with memory based prefiltering and predistortion

A direct pi /4 differential quadrature phase shift keying (DQPSK) 48.6-kb/s 824-849-MHz modulator for North American digital cellular applications is described. Square root raised cosine roll-off prefiltering with a roll-off factor 0.35 is implemented using digital memory circuits. The same memory circuits are used to incorporate predistortion for the nonlinearity of the biphase attenuators. Although predistortion for the modulator only has been incorporated, the same circuit can be used to predistort for power amplifier nonlinearities. The modulator is suitable for silicon monolithic microwave integrated circuit (MMIC) implementation using digital and memory circuits on the same chip. The measured results are presented.