What is the co-simulation workflow between a circuit simulator and an electromagnetic solver?
EM-Circuit Co-Simulation
Co-simulation is the standard professional workflow for RF design above 3 GHz, where the parasitic behavior of passive structures becomes significant and cannot be captured by simple circuit models. Below 3 GHz, circuit simulation alone may suffice for designs using standard components on well-characterized substrates.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
When is co-simulation necessary vs circuit-only?
Co-simulation is necessary when: (1) Operating above 6 GHz (parasitic effects dominate). (2) Designing custom passive structures on PCB or MMIC (filters, couplers, transitions). (3) Via transitions between layers. (4) Connector interfaces. (5) Understanding coupling between adjacent structures. Circuit-only is adequate when: (1) All passive structures are standard (vendor-characterized components with S-parameter files). (2) Operating below 3 GHz with standard RF layout practices. (3) The system has large design margin (>3 dB) that absorbs modeling errors. Between 3-6 GHz: co-simulation is recommended for precision designs but may be optional if the design uses well-characterized components and established layout practices.
How long does a co-simulation optimization take?
Depends on the EM complexity: Simple planar structure (single-layer microstrip matching network, 5 optimization variables): Momentum 2.5D solve: 30-60 seconds per iteration. 100 iterations: 1-2 hours. Complex 3D structure (via transition, multi-layer, 8 optimization variables): HFSS 3D solve: 5-30 minutes per iteration. 50 iterations: 4-25 hours. Strategies to reduce time: (1) Use 2.5D (Momentum, AXIEM) instead of 3D when possible (10-50× faster). (2) Use surrogate models: run 50-100 EM simulations at strategic parameter combinations, fit a response surface model, then optimize on the fast surrogate. (3) Reduce the optimization variable count by using analytical pre-optimization. (4) Use gradient-based optimization (fewer iterations than genetic algorithms) when the response surface is smooth.
Can I co-simulate with different vendor tools?
Yes, using file-based co-simulation: (1) Run the EM simulation in any tool (HFSS, CST, COMSOL). (2) Export S-parameters in Touchstone format. (3) Import into any circuit simulator (ADS, Microwave Office, Cadence Spectre RF). This is the universal approach. For integrated co-simulation: vendor pairing matters: ADS + Momentum (both Keysight): seamless integrated co-simulation. ADS + HFSS (Keysight + Ansys): supported through the FEM Element in ADS. Microwave Office + AXIEM (both Cadence/AWR): seamless. Microwave Office + Analyst (both Cadence): 3D FEM integration. CST + other circuit simulators: file-based or Simulia CST-ADS link. Cross-vendor integrated co-simulation is possible but may have workflow friction compared to same-vendor solutions.