What is the difference between a driven modal and a driven terminal simulation in HFSS?
HFSS Solution Type Selection
Understanding the difference between modal and terminal solution types is essential for correct S-parameter interpretation, especially for multi-conductor and differential structures commonly found in high-speed digital and RF designs.
Technical Considerations
In the modal approach: each wave port supports N propagating modes (N depends on the port geometry and frequency). For each mode, the solver calculates the modal field pattern (E and H distributions across the port cross-section), characteristic impedance (power-voltage or power-current definition), and propagation constant (phase velocity and attenuation). S-parameters are defined between modes: S_mn describes the coupling from mode n at port j to mode m at port i. For a two-port, single-mode structure: S-matrix is 2×2 (same as standard S-parameters). For a two-port, two-mode structure (coupled microstrip): S-matrix is 4×4 (even mode and odd mode at each port, creating a 4-port equivalent). Modal impedances: HFSS calculates modal impedance using the power-voltage definition (Z_PV = V^2/(2P)) or power-current definition (Z_PI = 2P/I^2). For TEM modes: Z_PV = Z_PI = Z_characteristic. For non-TEM modes (waveguide): Z_PV ≠ Z_PI, and the choice affects S-parameter normalization.
- 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
Performance Analysis
In the terminal approach: the user draws "integration lines" on each wave port, connecting two conductors (signal to ground). The terminal voltage is the integral of the E field along this line, and the terminal current is the integral of the H field around one conductor. S-parameters are defined in terms of terminal voltages and currents normalized to the terminal impedance. For a single-ended port: one integration line per port. For a differential port: two integration lines (positive and negative terminals), enabling direct extraction of mixed-mode (differential/common-mode) S-parameters. The terminal approach is intuitive for circuit designers. The S-parameters directly correspond to what a VNA measures when probing a structure. For multi-mode structures: the terminal S-parameters automatically combine the mode contributions into terminal voltage/current, which is the physically observable quantity.
Frequently Asked Questions
Do I get different answers from modal vs terminal?
For single-mode, single-conductor structures: no, the results are identical (just different mathematical representations of the same physics). For multi-mode structures: the S-parameter matrices are different representations but contain the same information: you can convert between modal and terminal S-parameters using a transformation matrix M (the mode-to-terminal mapping). In practice: the driven terminal result is more directly useful for circuit simulation (it gives the terminal voltages/currents that the circuit simulator needs), while the driven modal result is more useful for understanding the electromagnetic behavior (which modes are excited, how modes couple).
How do I draw integration lines for differential ports?
For a differential microstrip pair: (1) On the wave port face, draw a line from the positive trace to the ground plane (integration line 1, terminal T1). (2) Draw a second line from the negative trace to the ground plane (integration line 2, terminal T2). (3) Assign T1 and T2 to a common "differential port" in HFSS. (4) HFSS will automatically compute the differential S-parameters (Sdd, Scc, Sdc, Scd). The integration lines should be drawn from the center of each trace to the nearest ground plane point, following the shortest path. Incorrect integration line placement (e.g., from trace to a distant ground point) will give incorrect terminal impedance and S-parameter normalization.
Which mode should I excite for a rectangular waveguide?
For standard rectangular waveguide (WR-90 at X-band): the dominant mode is TE10, which is mode 1 in the modal solution. Higher-order modes (TE20, TE01, TM11) have higher cutoff frequencies and are evanescent below their cutoff. In simulation: set the port to solve for 1 mode (TE10 only) if operating well below the TE20 cutoff (for WR-90: TE10 cutoff = 6.56 GHz, TE20 cutoff = 13.12 GHz, so single-mode operation is valid from 6.56 to 13.12 GHz). If operating near the TE20 cutoff or with bends/discontinuities that may excite higher modes: solve for 2-3 modes to capture mode conversion.