What is the role of electromagnetic simulation in millimeter wave circuit design?
EM Simulation for mmWave
The choice of EM simulator and the simulation methodology are as important as the design itself at mmWave frequencies.
- 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
Frequently Asked Questions
When can I skip EM simulation?
Almost never at mmWave. The only cases: (1) A single, short transmission line with no discontinuities (straight line, controlled impedance, no bends). The analytical model is adequate. (2) An exact copy of a previously fabricated and measured design (same layout, same substrate, same process). The measurement data is more accurate than any simulation. (3) A design at < 5 GHz where the dimensions are much smaller than the wavelength (lumped-element regime). Even then: EM simulation provides useful insight and verification. Rule: if you are designing anything at > 10 GHz and you are not using EM simulation: your design will not work as expected on the first prototype. Budget for EM simulation time in the project schedule.
How accurate are EM simulators at mmWave?
Properly set up EM simulations at 28 GHz: insertion loss: ±0.3-0.5 dB accuracy. Return loss: ±3-5 dB accuracy (the return loss is very sensitive to small dimensional errors). Center frequency: ±1-3% accuracy. Phase: ±5-15° accuracy. At 77 GHz: accuracy degrades slightly (the physical dimensions are smaller, so the mesh resolution must be finer, and manufacturing tolerance effects are larger). Source of error: (1) Material properties: the simulation uses the Dk and loss tangent from the laminate datasheet. These values have tolerances (±2-5% Dk). (2) Mesh resolution: insufficient mesh density near sharp features (via edges, trace corners) causes numerical error. (3) Boundary conditions: the simulation boundary (radiation boundary, waveguide port definition) must be properly configured. (4) Manufacturing tolerance: the fabricated dimensions differ from the simulation model. To improve accuracy: use measured material properties (not datasheet), refine the mesh near critical features, and correlate with measurement data from a test coupon.
Which solver should I choose: HFSS or CST?
Both are excellent full-wave 3D solvers: HFSS (FEM): best for resonant structures (filters, cavities, antennas with complex geometry). The adaptive meshing concentrates mesh elements where the fields change rapidly. Slower for electrically large structures (many wavelengths). CST MWS (FDTD): best for wideband simulations (one FDTD run covers the entire frequency range). Better for electrically large structures (phased arrays, housing effects). Faster for time-domain problems (pulse propagation, transient analysis). Momentum (2.5D MoM): best for planar structures. Much faster than 3D solvers. Use for initial design exploration before committing to 3D simulation. Recommendation: use Momentum or a 2.5D solver for initial layout optimization (fast iterations). Then verify the final design in HFSS or CST (complete 3D model). Both should give similar results for the same model; any significant discrepancy indicates a setup error.