SUMMARYThis paper deals with wave digital modeling of passive state-space models. The set of differential equations must be of linear state-space form, but all parameters can be time-variant and/or nonlinear. For such statespace models, a canonical internally passive reference circuit is presented and used for deriving wave digital structures. In order to show the usability, special solutions for important basic linear time-variant models are compared with wave digital simulation results. Moreover, the wave digital modeling of a nonlinear and time-variant oscillator is discussed. Especially for a lossless oscillator an implementation is proposed, which preserves energy under finite-arithmetic conditions. This is verified by comparing simulation results with the analytical solution of a gravity pendulum.