What is the co-simulation workflow between a circuit simulator and an electromagnetic solver?

Co-simulation combines the accuracy of 3D electromagnetic simulation for passive structures with the speed and nonlinear modeling capability of circuit simulation for active devices. The workflow: (1) Partition the design: identify which parts need EM simulation (custom passive structures: filters, couplers, transitions, matching networks, interconnects) and which use circuit models (active devices, standard lumped components with manufacturer models). (2) EM simulation: simulate each passive structure in the 3D EM solver (HFSS, CST, Momentum), extracting S-parameters or equivalent circuit models. Ensure port definitions match the circuit connectivity (port impedance, reference planes, number of ports). (3) Import to circuit simulator: bring the EM-simulated S-parameter data into the circuit simulator (ADS, Microwave Office) as an N-port S-parameter block. Connect the S-parameter block to the active device models and other circuit elements. (4) Circuit simulation: run the combined circuit using the desired analysis type (linear S-parameter, harmonic balance, transient). The S-parameter block from the EM simulation is treated as a passive sub-network with frequency-dependent behavior. (5) Optimize: if the combined circuit performance needs improvement, adjust the EM geometry, re-simulate, and re-import. Modern tools support automated co-simulation where the EM solver is called from within the circuit simulator during optimization, enabling geometry parameters to be included in the optimization variable set. Keysight ADS + Momentum and AWR + AXIEM both support this automated workflow. Full 3D co-simulation (ADS + HFSS) is available through the FEM Element in ADS, which launches HFSS simulations on-demand during circuit optimization.