ANALOG-INPUT DIGITAL PHASE-LOCKED LOOPS FOR PRECISE FREQUENCY AND PHASE DEMODULATION

Most conventional analog-input digital phase-locked loops (ADPLL's) su ffer from the effects of in-loop quantization and from nonlinear behav ior caused by the approximations inherent in practical digitally contr olled oscillators (DCO's). The resulting errors limit the accuracy of ADPLL-based frequency demodulation and usually make ADPLL-based phase demodulation impractical because of severe phase-drift problems. This paper presents a new class of ADPLL's that are insensitive to the dele terious effects of quantization, and do not exhibit nonlinear behavior when implemented with practical DCO's. The ADPLL's are well suited to applications requiring precise frequency demodulation, and can also b e used for phase demodulation because their quantization error is well behaved even after discrete-time integration. The paper establishes a n analogy between the ADPLL's and delta-sigma modulators, and applies existing delta-sigma modulator results to predict the frequency and ph ase demodulation accuracy of the ADPLL's. A mechanization of the gener al architecture consisting of easily implemented components such as an alog integrators, digital flip-flops, and digital counters is then pre sented and analyzed.