Impedance and loss

For each net class and routing layer it finds the trace geometry from the stackup (outer layers are microstrip, inner layers are stripline between the nearest copper above and below), then the impedance (Hammerstad-Jensen microstrip, Wheeler stripline, conformal-mapping grounded coplanar with the copper's thickness and side walls, all uncoated; the coplanar form lands within 4% of the field solver from a 0.08 mm to a 0.21 mm core), the width that meets the target, and the IPC-2221 current capacity. agentee show <board> prints all of it as JSON.

With solver = "field" the class is checked by the 2D field solver instead: a node-based finite difference Laplace solve on a graded mesh, run on the GPU through wgpu, with copper thickness, solder mask, coplanar grounds and pairs included. On exact references it lands within 0.5% of Cohn's zero-thickness stripline and 0.4% of Hammerstad-Jensen microstrip. When the class is off target, check suggests the track width that meets it. agentee calc field --netclass RF prints the full result (Z0, eeff, C and L per metre, delay, grid).

--sweep 10MHz,20GHz,21 (MCP sweep) adds a loss table per frequency: R, L, G, C, Z0 and dB per metre and per inch, split into conductor and dielectric loss. Conductor loss comes from Wheeler's incremental inductance, solved by receding every copper surface in the field solver (within 1% of the Wheeler stripline formula in Pozar), with skin depth from annealed copper and the DC resistance blended in as sqrt(Rdc^2 + Rac^2). Dielectrics follow the causal Djordjevic-Sarkar model fitted to each layer's er and loss_tangent, taken as 1 GHz values. Copper roughness is set on the stackup:

[stackup]
roughness = "0.5um"            # rms, Hammerstad-Jensen
# huray = { radius = "0.5um", ratio = 2.0 }   # or the Huray snowball model
finish = "ENIG"
nickel = "4.5um"               # ENIG nickel thickness, default 4.5 um
gold = "0.075um"               # ENIG immersion gold thickness, default 0.075 um

The 2D solver applies the roughness as a loss factor on the resistance at each frequency. The FDTD uses the roughness and the ENIG finish as described under "Losses in the FDTD" below; nickel and gold only matter when finish = "ENIG". The defaults are the middle of the IPC-4552 windows (3 to 6 um nickel, gold 0.05 um minimum with fabs aiming for 0.05 to 0.1 um).

A class with diff_gap is a pair: its impedance is the differential impedance, and the solver runs both the odd and even modes. calc field then adds a pair block with Zdiff, Zcommon, the odd and even mode impedances and delays, the coupling coefficient (Ze - Zo)/(Ze + Zo), and the saturated near-end crosstalk of a long line (half the coupling). The two modes land within 0.5% of Cohn's exact edge-coupled stripline. A difference between the odd and even delays is what drives far-end crosstalk on microstrip.