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 effect 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