pdnkit: CavityModel uses HJ roughness

pdnkit/pi/CavityModel.cpp

@@ -1,4 +1,5 @@
 #include "pi/CavityModel.h"
+#include "pi/Roughness.h"
 
 #include <Eigen/Dense>
 
@@ -21,7 +22,16 @@ std::complex<double> cavity_impedance(
 
     const double mu  = cfg.mu_r * kMu0;
     // Complex permittivity: eps = eps_r eps_0 (1 - j tan_delta).
-    const cd eps = cfg.eps_r * kEps0 * cd(1.0, -cfg.tan_delta);
+    cd eps = cfg.eps_r * kEps0 * cd(1.0, -cfg.tan_delta);
+    // Optional conductor surface loss (HJ roughness x delta_s / d).
+    if (cfg.conductor_roughness_rq_m > 0.0 && cfg.d > 0.0) {
+        const double f_hz = omega / (2.0 * std::numbers::pi);
+        const double delta_s = skin_depth_copper(omega);
+        const double k_hj = hj_roughness_multiplier(
+            cfg.conductor_roughness_rq_m, f_hz);
+        const double tan_delta_cond = k_hj * delta_s / cfg.d;
+        eps -= cd(0.0, 1.0) * cfg.eps_r * kEps0 * tan_delta_cond;
+    }
     const cd k_squared = omega * omega * mu * eps;
 
     cd sum(0.0, 0.0);

pdnkit/pi/CavityModel.h

@@ -34,6 +34,15 @@ struct CavityConfig {
     double mu_r = 1.0;         // relative permeability (~1 for PCB dielectrics)
     double tan_delta = 0.020;  // dielectric loss tangent (FR-4 default)
     int max_modes = 30;        // m, n each summed 0..max_modes inclusive
+
+    // Optional conductor surface roughness contribution. When > 0 the
+    // effective dielectric loss tangent is augmented per-frequency with
+    // K_HJ * (delta_s / d) -- the surface-loss term for a parallel-plate
+    // cavity, multiplied by the Hammerstad-Jensen roughness factor.
+    // Default 0 = smooth, preserves existing behavior.
+    // Typical values: 0.4e-6 (smooth ED foil), 1.0e-6 (standard rolled),
+    // 2.0-5.0e-6 (black-oxide treated).
+    double conductor_roughness_rq_m = 0.0;
 };
 
 // Self/transfer impedance at angular frequency omega (rad/s) between ports

pdnkit/tests/CMakeLists.txt

@@ -10,6 +10,7 @@ add_executable(pdnkit_tests
     ir_solver_test.cpp
     ir_result_mesh_test.cpp
     cavity_model_test.cpp
+    cavity_hj_test.cpp
     cavity_mount_l_test.cpp
     decap_optimizer_test.cpp
     transient_test.cpp

pdnkit/tests/cavity_hj_test.cpp

@@ -0,0 +1,64 @@
+#include <catch2/catch_approx.hpp>
+#include <catch2/catch_test_macros.hpp>
+
+#include <cmath>
+#include <complex>
+#include <numbers>
+
+#include "pi/CavityModel.h"
+
+using namespace pdnkit::pi;
+using Catch::Approx;
+
+namespace {
+CavityConfig fr4() {
+    CavityConfig cfg;
+    cfg.a = 0.100;
+    cfg.b = 0.080;
+    cfg.d = 1.6e-3;
+    cfg.eps_r = 4.3;
+    cfg.tan_delta = 0.020;
+    cfg.max_modes = 15;
+    return cfg;
+}
+}  // namespace
+
+TEST_CASE("cavity-hj: zero roughness is identical to default",
+          "[cavity-hj][validation]") {
+    CavityConfig a = fr4();
+    a.conductor_roughness_rq_m = 0.0;
+    CavityConfig b = fr4();
+    const double f = 1.0e8;
+    const double w = 2.0 * std::numbers::pi * f;
+    auto za = cavity_impedance(a, 0.020, 0.020, 0.080, 0.060, w);
+    auto zb = cavity_impedance(b, 0.020, 0.020, 0.080, 0.060, w);
+    REQUIRE(za.real() == Approx(zb.real()));
+    REQUIRE(za.imag() == Approx(zb.imag()));
+}
+
+TEST_CASE("cavity-hj: roughness damps the resonance peak",
+          "[cavity-hj][validation]") {
+    const double f = 723.0e6;
+    const double w = 2.0 * std::numbers::pi * f;
+    CavityConfig smooth = fr4();
+    CavityConfig rough  = fr4();
+    rough.conductor_roughness_rq_m = 5.0e-6;
+    auto z_smooth = cavity_impedance(smooth, 0.020, 0.020, 0.080, 0.060, w);
+    auto z_rough  = cavity_impedance(rough,  0.020, 0.020, 0.080, 0.060, w);
+    INFO("|Z_smooth| = " << std::abs(z_smooth)
+         << "  |Z_rough| = " << std::abs(z_rough));
+    REQUIRE(std::abs(z_rough) < std::abs(z_smooth));
+}
+
+TEST_CASE("cavity-hj: off-resonance change is small", "[cavity-hj]") {
+    const double f = 50.0e6;
+    const double w = 2.0 * std::numbers::pi * f;
+    CavityConfig smooth = fr4();
+    CavityConfig rough  = fr4();
+    rough.conductor_roughness_rq_m = 5.0e-6;
+    auto z_smooth = cavity_impedance(smooth, 0.020, 0.020, 0.080, 0.060, w);
+    auto z_rough  = cavity_impedance(rough,  0.020, 0.020, 0.080, 0.060, w);
+    const double rel = std::abs(std::abs(z_rough) - std::abs(z_smooth))
+                       / std::abs(z_smooth);
+    REQUIRE(rel < 0.05);
+}