Baseband compensation of an offset phase locked loop

Abstract

A phase modulator faithfully reproduces higher frequency modulation using an offset phase-locked loop (OPLL) without passing excessive noise through an increased bandwidth of the OPLL. A quadrature modulator modulates information from a baseband signal onto a passband IF signal and, after a limiter strips away amplitude variations, the OPLL reproduces the phase modulation on an RF signal. The OPLL introduces a group delay that does not vary linearly with the modulation frequency and that consequently causes distortion when uncompensated. A baseband filter filters the amplitude of the baseband signal and introduces a complementary group delay that compensates for the OPLL group delay and results in a combined group delay of the baseband filter, quadrature modulator, limiter and OPLL that remains substantially constant as modulation frequency varies. Compensating for the OPLL group delay reduces distortion and the spectral energy at offset frequencies from the carrier frequency of the RF signal.