All-Pass-Filter-Based PLL Systems: Linear Modeling, Analysis, and Comparative Evaluation

All-pass filter (APF) passes all frequency components of a signal without altering their amplitude, but changes their phase. This feature has made the APF a versatile building block in different signal processing applications. The focus of this article is on APF-based phase-locked loops (PLLs), where the APF is required for creating a 90° phase shift at the fundamental frequency. Such a phase shift is needed for generating a fictitious orthogonal signal in single-phase applications and rejecting the grid voltage imbalance in three-phase systems. To the best of authors' knowledge, none of the APF-based PLLs have an accurate model yet. This gap in knowledge makes the analysis of these synchronization systems and identifying their advantages/disadvantages compared to state-of-the-art structures complicated. The main objective of this article is to bridge this knowledge gap.