| name | em-modeling |
| description | Solve the electromagnetics of an on-chip passive or antenna (inductor, transformer, transmission line, balun, antenna), extract a converged passive S-parameter model, and fit a lumped equivalent for circuit-level RF simulation. Use when building the passive model rf-design consumes, or to service an rf-design fix_request (route_to: em-modeling) by re-solving the passive. Loop-backs are stage-local (passivity/fit → meshing/geometry_definition); passivity is a hard gate; a fundamental geometry/stackup gap escalates.
|
| version | 1.0.0 |
| author | chuanseng-ng |
| license | MIT |
| allowed-tools | Read, Write, Bash |
Skill: EM Modeling
Invocation
- If invoked by a user presenting an EM-modeling task: immediately spawn the
analog-chip-design-agents:em-modeling-orchestrator agent and pass the full user request and any
available context. Do not execute stages directly.
- If invoked by the
em-modeling-orchestrator mid-flow (including fix_request re-solves): do not
spawn a new agent. Treat this file as read-only — return the requested stage rules, sign-off
criteria, or loop-back guidance.
Spawning the orchestrator from within an active orchestrator run causes recursive delegation and
must never happen.
Pre-run Context
Before executing or advising on any stage, read the following if they exist:
memory/em/knowledge.md — known meshing recipes, passivity/fit fixes, de-embedding patterns,
solver-selection rules, and PDK/tool quirks. Incorporate its guidance into every stage.memory/em/run_state.md — current run identity for resume-after-interruption.
Purpose
Solve the electromagnetics of an on-chip passive or antenna, extract a converged, passive
S-parameter model, and fit a lumped equivalent for circuit-level RF simulation. Seven stages with
explicit QoR gates. EM modeling is a data-dependency producer and a cross-domain servicer: it
writes a Touchstone S-parameter model + fitted lumped model into design_state.em that rf-design
reads as a passive input. Its loop-backs are stage-local (passivity/fit fail → meshing /
geometry_definition, max 2×); a fundamental geometry/stack-up gap escalates to the user. EM
modeling does not open fix_requests, but it services rf-design-raised ones
(route_to: em-modeling): when dispatched with a fix_request.id it re-solves the passive toward a
higher-Q / higher-SRF target and closes the entry with a circuit_response so the
pipeline-orchestrator re-validates RF.
Supported EDA Tools
Open-Source
- openEMS (
openEMS) — FDTD full-wave solver (distributed passives, antennas, mmWave)
- FastHenry (
fasthenry) / FastCap (fastcap) — quasi-static RL / C field solvers
- gmsh (
gmsh) — mesh generation
- scikit-rf (Python
skrf) — passivity / causality checks, fitting, de-embedding
Proprietary (detect-only — never installed)
- Ansys HFSS / SIwave / RaptorX (
hfss) — 3D / planar EM
- Keysight Momentum / RFPro / EMPro (
momentum)
- Cadence EMX (
emx) & AWR AXIEM / Analyst (axiem)
- Sonnet (
sonnet) — planar EM
Stage: em_setup
Domain Rules
- Read the passive intent (device type, target inductance/coupling/impedance, operating band) and
the geometry source — a GDS/layout cell (
design_state.layout.gds), a parametric-generator
spec, or explicit dimensions — plus the metal stack-up / substrate from constraints.pdk.
- Select the solver class by problem: full-wave FDTD (openEMS) for distributed / high-frequency
passives and antennas; quasi-static (FastHenry/FastCap) for lumped RL/C below SRF. Record the
choice.
- Define the extraction frequency grid (DC/near-DC through past the expected SRF) and the port
definitions / de-embedding reference planes.
QoR Metrics to Evaluate
- Geometry source resolved; metal stack-up bound from
pdk
- Frequency grid spans the band → past the expected SRF
- Ports / de-embedding reference planes defined
Common Issues & Fixes
| Issue | Fix |
|---|
| No metal stack-up for the PDK | escalate constraint_gap — require pdk with a layer stack |
| Frequency grid stops below SRF | extend the grid past the expected self-resonance |
Output Required
- EM setup (device type, geometry source, stack-up, solver class, port/de-embed plan, frequency grid)
Stage: geometry_definition
Domain Rules
- Build the 3D / 2.5D geometry from the source (import GDS layers to the stack-up, or instantiate
the parametric generator); assign conductors, dielectrics, and ports with correct layer
thicknesses and conductivity.
- Validate the geometry is physically closed / manifold and ports are on valid reference planes;
this is a loop-back target for fit/passivity failures traced to geometry.
- Confirm the modeled region includes enough substrate / ground and air margin for the boundary
conditions.
QoR Metrics to Evaluate
- Geometry closed / manifold; layers mapped to the stack-up
- Ports correctly placed on valid reference planes
- Adequate ground / air margin for the boundaries
Common Issues & Fixes
| Issue | Fix |
|---|
| Open / non-manifold geometry | repair the geometry; re-map GDS layers to the stack-up |
| Port reference plane misplaced | re-define ports on a valid plane and re-de-embed |
Output Required
- Solver-ready geometry (conductors / dielectrics / ports mapped to the stack-up)
Stage: meshing
Domain Rules
- Generate the mesh (gmsh for FE; openEMS rectilinear grid) with cell size ≤ λ/20 at the top
frequency and refinement at conductor edges / coupled gaps (skin-depth-aware).
- This is the primary loop-back target: a passivity or fit failure traced to under-resolution
re-enters here to refine (max 2×).
- Record the mesh cell count and the limiting feature size; ensure the mesh resolves the smallest
critical gap.
QoR Metrics to Evaluate
- Cell size ≤ λ/20 at f_max
- Conductor edges / coupled gaps refined
- Mesh resolves the smallest critical gap
Common Issues & Fixes
| Issue | Fix |
|---|
| Coarse mesh → non-passive / poor fit | refine the mesh at edges/gaps (loop-back target, max 2×) |
| Mesh misses a thin coupled gap | add local refinement to resolve the gap |
Output Required
- Mesh summary (cell count, min cell size, refinement regions)
Stage: em_solve
Domain Rules
- Run the solver (openEMS FDTD / FastHenry-FastCap / proprietary if detected) across the frequency
grid; confirm energy / residual convergence (FDTD: energy decay below threshold; FE: residual
below tolerance).
- On non-convergence (energy not decayed) or a runtime/resource limit, treat as
convergence/resource_limit: extend timesteps or coarsen sensibly (within λ/20), else
escalate.
- Read the convergence summary from the solver summary file — never the raw field dump.
QoR Metrics to Evaluate
- Solver converged at every frequency point
- Energy decay / residual below threshold
- No resource-limit abort
Common Issues & Fixes
| Issue | Fix |
|---|
| FDTD energy not decayed (no convergence) | extend timesteps / check excitation & boundaries; re-solve |
| Resource / runtime limit hit | reduce the domain / coarsen the mesh within λ/20, or escalate resource_limit |
Output Required
- Solver convergence report (per-frequency energy decay / residual, runtime)
Stage: sparameter_extraction
Domain Rules
- Extract the N-port S-matrix to a Touchstone (
.sNp) file, de-embedding to the defined reference
planes; compute the passivity check (all eigenvalues of I − SᴴS ≥ 0 across frequency) and a
causality / reciprocity sanity check with scikit-rf.
- A non-passive S-matrix is a hard fail → loop back to
meshing (then geometry_definition)
— max 2×.
- Derive the physical figures:
q_factor vs frequency, srf_ghz (first self-resonance), and
coupling (k) for transformers.
QoR Metrics to Evaluate
- S-matrix passive across the full band (
passivity_pass)
- Reciprocal where expected
q_factor / srf_ghz / coupling extracted
Common Issues & Fixes
| Issue | Fix |
|---|
| Non-passive S-matrix | loop back → meshing (refine), then geometry_definition (max 2×) |
| SRF / Q implausible vs expectation | check ports / de-embedding; re-extract |
Output Required
- Touchstone (
.sNp) file + passivity report + q_factor / srf_ghz / coupling table
Stage: model_fitting
Domain Rules
- Fit a lumped equivalent (π/T or broadband pole-residue / vector-fit) to the extracted S-params;
compute
fit_error_pct = max |fit − EM| / EM × 100 across the band (S-param or Z/Y error).
- Enforce that the fitted model is itself passive (real-part / eigenvalue check) so
circuit-level RF sim stays stable; a non-passive fit, or a passive but high-error fit traced to
the EM data, → loop back to
meshing / geometry_definition (max 2×); otherwise refine the fit
order locally.
- Confirm the fitted SRF / Q match the EM extraction within tolerance.
QoR Metrics to Evaluate
fit_error_pct ≤ budget (default ≤ 5%)- Fitted model passive
- Fitted SRF / Q match the EM extraction within tolerance
Common Issues & Fixes
| Issue | Fix |
|---|
fit_error_pct over budget | raise fit order / broadband (vector) fit; if EM data is the limiter, loop back → meshing |
| Fitted model non-passive | enforce passivity in the fit; if unachievable, loop back → meshing (max 2×) |
Output Required
- Fitted lumped model (netlist / params) +
fit_error_pct + passivity-of-fit report
Stage: em_signoff
Sign-off Pass Criteria (all must pass)
| Check | Criterion |
|---|
| Passivity | extracted S-matrix and fitted model both passive across the band (passivity_pass) |
| Convergence | solver converged at every frequency point |
| Fit accuracy | fit_error_pct ≤ budget (default 5%) |
| Physical sanity | q_factor / srf_ghz / coupling extracted and plausible vs intent |
Domain Rules
- Confirm passivity, convergence, fit accuracy, and physical sanity all pass.
- Publish the Touchstone + fitted lumped model into
design_state.em for rf-design to consume.
- In fix-servicing mode, close the serviced
fix_request (status: fixed) with a
circuit_response and report PASS so the pipeline-orchestrator re-validates RF.
Failure Handling
- Passivity / fit fail after the retry cap → escalate to the user with the failing
frequency/metric and a recommendation (re-mesh, change the geometry, or relax the band). EM
modeling does not open
fix_requests; in fix-servicing mode leave the entry claimed on an
unresolved re-solve.
Output Required
- EM sign-off report (passivity, convergence, fit accuracy, physical sanity)
- Published Touchstone + fitted lumped model
Constraint Validation
See plugins/meta/skills/pipeline-orchestration/SKILL.md §Constraints Schema for the authoritative schema and stage-entry validation rule.
Required at entry (em_setup) — hard-fail if missing:
constraints.pdk — the metal stack-up / substrate (without it there is no geometry to solve)- a geometry source —
design_state.layout.gds (preferred) or an explicit
parametric-generator / dimension spec passed in the prompt/constraints
EM modeling does not require supply.vdd_v or rf_specs — passive extraction is
bias-independent (a deliberate divergence from the rf-design required set). Skip constraint
validation entirely when invoked in re-solve / fix-servicing mode.
Memory
Run state (write before first stage, update after each stage)
Write memory/em/run_state.md as the first action:
run_id: em_<YYYYMMDD>_<HHMMSS>
design_name: <design>
pdk: <pdk or unknown>
tool: <primary tool>
start_time: <ISO-8601>
last_stage: null
Update last_stage to the completed stage name only after each stage finishes successfully.
Write on stage completion
After each stage completes, upsert one JSON record in memory/em/experiences.jsonl keyed by
run_id. key_metrics fields: q_factor, srf_ghz, fit_error_pct. Set
signoff_achieved: false until em_signoff passes; then true. Create the file and parent
directories if they do not exist.
Optional: claude-mem index
If mcp__plugin_ecc_memory__add_observations is available, emit each meshing / passivity / fit fix
as an observation to entity analog-design-em-fixes after writing to experiences.jsonl. Skip
silently if absent — the JSONL file is the canonical record.
