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

Most conventional analog-input digital phase-locked loops (ADPLL's) suffer from the effects of in-loop quantization and from nonlinear behavior caused by the approximations inherent in practical digitally controlled 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 deleterious 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 be used for phase demodulation because their quantization error is well behaved even after discrete-time integration. The paper establishes an analogy between the ADPLL's and delta-sigma modulators, and applies existing delta-sigma modulator results to predict the frequency and phase demodulation accuracy of the ADPLL's. A mechanization of the general architecture consisting of easily implemented components such as analog integrators, digital flip-flops, and digital counters is then presented and analyzed.