This article presents a novel finite-difference time-domain (FDTD) framework for the analysis and design of multiport nonreciprocal (NR) circuits based on spatiotemporally modulated (STM) resonators. Traditional frequency-domain methods, such as harmonic balance (HB) simulations, face significant challenges in modeling time-modulated elements due to harmonic truncation. In contrast, the proposed time-domain (TD) approach inherently supports arbitrary waveforms and adaptive components, enabling accurate and efficient analysis of space–time-modulated and tunable RF systems. Specifically, the proposed FDTD-based analysis framework introduces a generalized update scheme for interconnected transmission line (TL) junctions and lumped-element resonators using modified Telegrapher’s equations and custom boundary conditions (BCs), allowing to support complex circuit topologies, including NR-bandpass filters (NR-BPFs), NR-filtering power dividers (NR-FPDs), and NR-filtering couplers, and extract their key performance metrics such as TD waveforms, S-parameters, and constellation diagrams. Real-time simulations demonstrate the method’s ability to model direction-dependent behavior, frequency conversion, and broadband signal propagation with high accuracy. The method is exhaustively compared and validated with simulations and measurements through the manufacturing and testing of three experimental prototypes at the UHF band. These include: 1) a tunable 3rd-order in-line NR-BPF; 2) a tunable 4th-order single-band NR-BPF; and 3) a 3rd-order NR-FPD. © 1963-2012 IEEE.

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