test_coupled_stripline_offset.cpp

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#include <qa_utils/wx_utils/unit_test_utils.h>
 
#include <cmath>
#include <limits>
 
#include <transline_calculations/coupled_stripline.h>
#include <transline_calculations/units.h>
#include <transline_calculations/units_scales.h>
 
 
namespace TC = TRANSLINE_CALCULATIONS;
using TCP = TRANSLINE_PARAMETERS;
 
 
namespace
{
// Shared geometry spec.  b is the full plate spacing; W, S, T, eps_r, f match the Plan 4 task.
struct Geometry
{
    double b;
    double w;
    double s;
    double t;
    double er;
    double f;
    double L;
};
 
 
Geometry Geometry1()
{
    return { 20.0 * TC::UNIT_MIL, 8.0 * TC::UNIT_MIL, 8.0 * TC::UNIT_MIL, 0.7 * TC::UNIT_MIL,
             4.3, 1.0e9, 100.0 * TC::UNIT_MM };
}
 
 
// stripline_a < 0 leaves STRIPLINE_A unwritten so the backward-compat default-to-centred path is
// exercised; stripline_a >= 0 writes it explicitly.
COUPLED_STRIPLINE MakeCalc( const Geometry& g, double stripline_a )
{
    COUPLED_STRIPLINE calc;
    calc.SetParameter( TCP::EPSILONR, g.er );
    calc.SetParameter( TCP::TAND, 0.02 );
    calc.SetParameter( TCP::MUR, 1.0 );
    calc.SetParameter( TCP::MURC, 1.0 );
    calc.SetParameter( TCP::ROUGH, 0.0 );
    calc.SetParameter( TCP::SIGMA, 5.8e7 );
    calc.SetParameter( TCP::H, g.b );
    calc.SetParameter( TCP::PHYS_WIDTH, g.w );
    calc.SetParameter( TCP::PHYS_S, g.s );
    calc.SetParameter( TCP::T, g.t );
    calc.SetParameter( TCP::PHYS_LEN, g.L );
    calc.SetParameter( TCP::FREQUENCY, g.f );
 
    if( stripline_a >= 0.0 )
        calc.SetParameter( TCP::STRIPLINE_A, stripline_a );
 
    return calc;
}
 
 
struct Impedances
{
    double z0e;
    double z0o;
    double zdiff;
};
 
 
Impedances Run( COUPLED_STRIPLINE& calc )
{
    calc.Analyse();
    return { calc.GetParameter( TCP::Z0_E ), calc.GetParameter( TCP::Z0_O ),
             calc.GetParameter( TCP::Z_DIFF ) };
}
} // namespace
 
 
BOOST_AUTO_TEST_SUITE( CoupledStriplineOffset )
 
 
// Geometry 1 symmetric centred baseline.  Numbers come from the existing Cohn implementation with
// the finite-thickness fringe correction at t = 0.7 mil; they are not independent targets but a
// snapshot of the pre-offset code path so later tests can compare against it.
BOOST_AUTO_TEST_CASE( CenteredSymmetric )
{
    const Geometry g = Geometry1();
    COUPLED_STRIPLINE calc = MakeCalc( g, -1.0 );
    const Impedances z = Run( calc );
 
    BOOST_TEST( z.z0e == 55.34, boost::test_tools::tolerance( 0.05 ) );
    BOOST_TEST( z.z0o == 43.59, boost::test_tools::tolerance( 0.05 ) );
    BOOST_TEST( z.zdiff == 87.18, boost::test_tools::tolerance( 0.05 ) );
    BOOST_TEST( z.zdiff == 2.0 * z.z0o, boost::test_tools::tolerance( 1e-12 ) );
}
 
 
// Writing STRIPLINE_A = H/2 must be indistinguishable from leaving it unset.  Regression guard
// for the default-to-centred branch in Analyse().
BOOST_AUTO_TEST_CASE( OffsetWithExplicitCenter )
{
    const Geometry g = Geometry1();
 
    COUPLED_STRIPLINE baseline = MakeCalc( g, -1.0 );
    const Impedances zBaseline = Run( baseline );
 
    COUPLED_STRIPLINE explicitCenter = MakeCalc( g, g.b / 2.0 );
    const Impedances zExplicit = Run( explicitCenter );
 
    BOOST_TEST( zExplicit.z0e == zBaseline.z0e, boost::test_tools::tolerance( 1e-6 ) );
    BOOST_TEST( zExplicit.z0o == zBaseline.z0o, boost::test_tools::tolerance( 1e-6 ) );
    BOOST_TEST( zExplicit.zdiff == zBaseline.zdiff, boost::test_tools::tolerance( 1e-6 ) );
}
 
 
// Geometry 2: plate spacing 20 mil, strip plane offset to a = 6 mil (b - a = 14 mil).  Reference
// values come from the Shelton admittance-sum construction applied analytically to the Cohn
// zero-thickness formula (Z0e = 53.98, Z0o = 45.10 zero-thickness) with the same finite-thickness
// fringe correction applied to each virtual stripline.  Tolerances widened to 8 percent to cover
// residual approximation error from applying symmetric fringe correction to asymmetric geometry.
BOOST_AUTO_TEST_CASE( OffsetAt6Mil )
{
    const Geometry g = Geometry1();
    COUPLED_STRIPLINE calc = MakeCalc( g, 6.0 * TC::UNIT_MIL );
    const Impedances z = Run( calc );
 
    BOOST_TEST( z.z0e == 50.0, boost::test_tools::tolerance( 0.08 ) );
    BOOST_TEST( z.z0o == 41.5, boost::test_tools::tolerance( 0.08 ) );
    BOOST_TEST( z.zdiff == 83.0, boost::test_tools::tolerance( 0.08 ) );
 
    COUPLED_STRIPLINE centered = MakeCalc( g, -1.0 );
    const Impedances zCentered = Run( centered );
 
    // Moving a from b/2 = 10 mil toward one plane brings both mode impedances down because the
    // nearer plane capacitance increases.  Z0e drops more than Z0o in absolute terms since the
    // even-mode field extends further vertically and is more perturbed by plane proximity.
    const double dropE = zCentered.z0e - z.z0e;
    const double dropO = zCentered.z0o - z.z0o;
    BOOST_TEST( dropE > 0.0 );
    BOOST_TEST( dropO > 0.0 );
    BOOST_TEST( dropE > dropO );
}
 
 
// Offset path at a = b/2 must collapse to the centred result to machine precision.  The tight
// tolerance verifies the admittance-sum implementation correctly short-circuits (or arithmetically
// recovers) the symmetric case.
BOOST_AUTO_TEST_CASE( OffsetReproducesCenteredWhenCentered )
{
    const Geometry g = Geometry1();
 
    COUPLED_STRIPLINE centered = MakeCalc( g, -1.0 );
    const Impedances zCentered = Run( centered );
 
    COUPLED_STRIPLINE offsetCentered = MakeCalc( g, g.b / 2.0 );
    const Impedances zOffset = Run( offsetCentered );
 
    BOOST_TEST( zOffset.z0e == zCentered.z0e, boost::test_tools::tolerance( 1e-6 ) );
    BOOST_TEST( zOffset.z0o == zCentered.z0o, boost::test_tools::tolerance( 1e-6 ) );
}
 
 
// Physical-consistency smoke test.  Moving the strip plane toward one ground plane must drive
// Z0e monotonically downward because the nearer plane adds parallel capacitance.
BOOST_AUTO_TEST_CASE( OffsetMonotonicInA )
{
    const Geometry g = Geometry1();
 
    const double aValues[] = { g.b / 2.0, 9.0 * TC::UNIT_MIL, 7.0 * TC::UNIT_MIL,
                               5.0 * TC::UNIT_MIL, 3.0 * TC::UNIT_MIL };
 
    double prevZ0e = std::numeric_limits<double>::infinity();
 
    for( double a : aValues )
    {
        COUPLED_STRIPLINE calc = MakeCalc( g, a );
        const Impedances z = Run( calc );
        BOOST_TEST( z.z0e < prevZ0e );
        prevZ0e = z.z0e;
    }
}
 
 
// Z_comm is by definition Z0e / 2 and the coupling coefficient k_c must equal the algebraic
// (Z0e - Z0o) / (Z0e + Z0o).  Both get dropped into the result map alongside Z0e / Z0o so the
// UI can read them back without re-deriving; this test pins the identity so future refactors
// cannot silently desync the cached values from Z0e / Z0o.
BOOST_AUTO_TEST_CASE( ZcommAndKcDerivedFromZ0Modes )
{
    const Geometry g = Geometry1();
    COUPLED_STRIPLINE calc = MakeCalc( g, -1.0 );
    calc.Analyse();
 
    const double z0e = calc.GetParameter( TCP::Z0_E );
    const double z0o = calc.GetParameter( TCP::Z0_O );
    const double zcomm = calc.GetParameter( TCP::Z_COMM );
    const double kc = calc.GetParameter( TCP::COUPLING_K );
 
    BOOST_TEST( zcomm == 0.5 * z0e, boost::test_tools::tolerance( 1e-12 ) );
    BOOST_TEST( kc == ( z0e - z0o ) / ( z0e + z0o ), boost::test_tools::tolerance( 1e-12 ) );
    BOOST_TEST( kc > 0.0 );
    BOOST_TEST( kc < 1.0 );
 
    BOOST_TEST_MESSAGE( "Geometry1 centred: Z0e=" << z0e << " Z0o=" << z0o
                        << " Zcomm=" << zcomm << " k_c=" << kc );
}
 
 
// Synthesis must keep working against the centred model.  Synthesize(FIX_SPACING) drives Z0_O to
// target, so we pick Z0_O = Zdiff_target/2, solve for W, re-Analyse, and check Zdiff.
BOOST_AUTO_TEST_CASE( SynthesisRoundTripCentered )
{
    const Geometry g = Geometry1();
    COUPLED_STRIPLINE calc = MakeCalc( g, -1.0 );
 
    const double zdiffTarget = 87.18;
    calc.SetParameter( TCP::Z0_O, zdiffTarget / 2.0 );
 
    const bool ok = calc.Synthesize( SYNTHESIZE_OPTS::FIX_SPACING );
    BOOST_REQUIRE( ok );
 
    calc.Analyse();
    const double zdiff = calc.GetParameter( TCP::Z_DIFF );
    BOOST_TEST( zdiff == zdiffTarget, boost::test_tools::tolerance( 0.01 ) );
}
 
 
// Offset must lie strictly inside (t/2, h - t/2) for the Shelton finite-thickness path.
// Outside that band the Cohn fringe formula divides by zero.  Drive a small offset against a
// 0.7 mil strip in a 20 mil cavity and verify the analysed impedances surface as TS_ERROR
// via NaN rather than silently returning bogus numbers.
BOOST_AUTO_TEST_CASE( OffsetBelowFiniteThicknessLimitYieldsError )
{
    const Geometry g = Geometry1();
    COUPLED_STRIPLINE calc = MakeCalc( g, 0.2 * TC::UNIT_MIL );
    calc.Analyse();
 
    const double z0e = calc.GetParameter( TCP::Z0_E );
    const double z0o = calc.GetParameter( TCP::Z0_O );
    BOOST_TEST( !std::isfinite( z0e ) );
    BOOST_TEST( !std::isfinite( z0o ) );
 
    auto& results = calc.GetAnalysisResults();
    BOOST_TEST( ( results[TCP::Z0_E].second == TRANSLINE_STATUS::TS_ERROR ) );
    BOOST_TEST( ( results[TCP::Z0_O].second == TRANSLINE_STATUS::TS_ERROR ) );
}
 
 
// Synthesize from a differential/common-mode target pair.  With FROM_ZDIFF_ZCOMM the math
// layer translates (Z_DIFF, Z_COMM) into (Z0_E, Z0_O) and runs the joint 2-D solver, so both
// the analysed Z_DIFF and Z_COMM must land on the requested targets.
BOOST_AUTO_TEST_CASE( SynthesisFromZdiffZcomm )
{
    const Geometry g = Geometry1();
    COUPLED_STRIPLINE calc = MakeCalc( g, -1.0 );
 
    const double zdiffTarget = 92.0;
    const double zcommTarget = 34.0;
    calc.SetParameter( TCP::Z_DIFF, zdiffTarget );
    calc.SetParameter( TCP::Z_COMM, zcommTarget );
 
    const bool ok = calc.Synthesize( SYNTHESIZE_OPTS::FROM_ZDIFF_ZCOMM );
    BOOST_REQUIRE( ok );
 
    calc.Analyse();
    const double zdiff = calc.GetParameter( TCP::Z_DIFF );
    const double zcomm = calc.GetParameter( TCP::Z_COMM );
    BOOST_TEST( zdiff == zdiffTarget, boost::test_tools::tolerance( 0.01 ) );
    BOOST_TEST( zcomm == zcommTarget, boost::test_tools::tolerance( 0.01 ) );
}
 
 
// 1-D synthesis modes (FIX_WIDTH / FIX_SPACING) only optimise Z0_O so they cannot honour a
// Z_COMM target.  Verify that requesting FROM_ZDIFF_ZCOMM via those modes is rejected, and
// that supplying Z_DIFF / Z_COMM under a 1-D mode does NOT silently override Z0_E / Z0_O.
BOOST_AUTO_TEST_CASE( ZdiffZcommIgnoredOutsideOptIn )
{
    const Geometry g = Geometry1();
    COUPLED_STRIPLINE calc = MakeCalc( g, -1.0 );
 
    // Stale Z_DIFF / Z_COMM in the parameter map (e.g. left over from a prior Analyse).
    calc.SetParameter( TCP::Z_DIFF, 200.0 );
    calc.SetParameter( TCP::Z_COMM, 5.0 );
 
    // Caller-supplied Z0_O target via FIX_SPACING must be respected, not overwritten.
    const double z0oTarget = 43.59;
    calc.SetParameter( TCP::Z0_O, z0oTarget );
 
    BOOST_REQUIRE( calc.Synthesize( SYNTHESIZE_OPTS::FIX_SPACING ) );
 
    calc.Analyse();
    const double z0o = calc.GetParameter( TCP::Z0_O );
    BOOST_TEST( z0o == z0oTarget, boost::test_tools::tolerance( 0.01 ) );
}
 
 
// Stale-target guard.  Analyse() populates Z_DIFF and Z_COMM as outputs; a subsequent
// Synthesize() call with new Z0_E / Z0_O targets must use the new targets, not silently
// re-translate the stale Analyse outputs back into Z0_E / Z0_O.
BOOST_AUTO_TEST_CASE( SynthesisDoesNotReuseStaleAnalyseOutputs )
{
    const Geometry g = Geometry1();
    COUPLED_STRIPLINE calc = MakeCalc( g, -1.0 );
 
    calc.Analyse();
    const double staleZdiff = calc.GetParameter( TCP::Z_DIFF );
    const double staleZcomm = calc.GetParameter( TCP::Z_COMM );
    BOOST_TEST( staleZdiff > 0.0 );
    BOOST_TEST( staleZcomm > 0.0 );
 
    // Pick fresh mode targets that are clearly distinct from the analysed snapshot.
    const double z0eTarget = 70.0;
    const double z0oTarget = 35.0;
    calc.SetParameter( TCP::Z0_E, z0eTarget );
    calc.SetParameter( TCP::Z0_O, z0oTarget );
 
    // Default mode means the joint 2-D solver; Z_DIFF / Z_COMM in the parameter map are
    // residual outputs from Analyse() and must not be consumed as targets.
    BOOST_REQUIRE( calc.Synthesize( SYNTHESIZE_OPTS::DEFAULT ) );
 
    calc.Analyse();
    const double z0e = calc.GetParameter( TCP::Z0_E );
    const double z0o = calc.GetParameter( TCP::Z0_O );
    BOOST_TEST( z0e == z0eTarget, boost::test_tools::tolerance( 0.02 ) );
    BOOST_TEST( z0o == z0oTarget, boost::test_tools::tolerance( 0.02 ) );
}
 
 
 
// Matches the geometry the tuning profile panel forwards to COUPLED_STRIPLINE: total plate
// spacing H = Top + Signal + Bottom, strip midplane at A = Top + Signal/2.  Asserts the
// midplane convention by pinning the solver against its two physical invariants: (1) a
// symmetric stack (Top == Bottom) must reproduce the centred result, and (2) swapping Top
// and Bottom must leave the impedances unchanged because the geometry is mirror-identical.
namespace
{
Impedances AnalyseStack( double topDielectric, double signal, double bottomDielectric )
{
    Geometry g = Geometry1();
    g.b = topDielectric + signal + bottomDielectric;
    g.t = signal;
 
    const double striplineA = topDielectric + 0.5 * signal;
 
    COUPLED_STRIPLINE calc = MakeCalc( g, striplineA );
    return Run( calc );
}
} // namespace
 
 
// Symmetric-stackup invariant.  Top == Bottom with a finite-thickness signal layer must land
// back on the centred path.  Before the midplane fix this used A = Top (numerator) against a
// denominator Top + Bottom (missing Signal), so the centred short-circuit never fired and a
// finite-thickness-layer stack got routed through the asymmetric branch — wrong even when
// physically centred.  With A = Top + Signal/2 against full H the equality is exact.
BOOST_AUTO_TEST_CASE( TuningProfileSymmetricStackMatchesCentered )
{
    const double dielectric = 5.0 * TC::UNIT_MIL;
    const double signal = 0.7 * TC::UNIT_MIL;
 
    const Impedances zStack = AnalyseStack( dielectric, signal, dielectric );
 
    Geometry g = Geometry1();
    g.b = 2.0 * dielectric + signal;
    g.t = signal;
    COUPLED_STRIPLINE centered = MakeCalc( g, -1.0 );
    const Impedances zCentered = Run( centered );
 
    BOOST_TEST( zStack.z0e == zCentered.z0e, boost::test_tools::tolerance( 1e-9 ) );
    BOOST_TEST( zStack.z0o == zCentered.z0o, boost::test_tools::tolerance( 1e-9 ) );
    BOOST_TEST( zStack.zdiff == zCentered.zdiff, boost::test_tools::tolerance( 1e-9 ) );
}
 
 
// Mirror symmetry.  Swapping Top and Bottom dielectric thicknesses is a geometric reflection
// about the strip midplane and must leave Z0e / Z0o / Zdiff bit-identical.  Before the fix
// the panel's IsCenteredOffset denominator excluded Signal, so the centred fast path could
// trigger for Top == Bottom but never for the swapped pair under some numerical conditions;
// here we exercise asymmetric geometry where both paths route through the image method.
BOOST_AUTO_TEST_CASE( TuningProfileTopBottomSwapSymmetry )
{
    const double top = 4.0 * TC::UNIT_MIL;
    const double bottom = 10.0 * TC::UNIT_MIL;
    const double signal = 0.7 * TC::UNIT_MIL;
 
    const Impedances zOriginal = AnalyseStack( top, signal, bottom );
    const Impedances zSwapped = AnalyseStack( bottom, signal, top );
 
    BOOST_TEST( zOriginal.z0e == zSwapped.z0e, boost::test_tools::tolerance( 1e-9 ) );
    BOOST_TEST( zOriginal.z0o == zSwapped.z0o, boost::test_tools::tolerance( 1e-9 ) );
    BOOST_TEST( zOriginal.zdiff == zSwapped.zdiff, boost::test_tools::tolerance( 1e-9 ) );
}
 
 
// Zero-thickness signal layer.  When T = 0 the midplane collapses onto Top so A = Top exactly,
// matching the pre-fix formula.  This pins that the new expression reduces to the old one in
// the thin-strip limit and regressions that break either path will surface here.
BOOST_AUTO_TEST_CASE( TuningProfileZeroThicknessSignalMatchesTopDielectric )
{
    const double top = 6.0 * TC::UNIT_MIL;
    const double bottom = 14.0 * TC::UNIT_MIL;
 
    Geometry g = Geometry1();
    g.b = top + bottom;
    g.t = 0.0;
 
    COUPLED_STRIPLINE calcMidplane = MakeCalc( g, top + 0.5 * 0.0 );
    const Impedances zMidplane = Run( calcMidplane );
 
    COUPLED_STRIPLINE calcTop = MakeCalc( g, top );
    const Impedances zTop = Run( calcTop );
 
    BOOST_TEST( zMidplane.z0e == zTop.z0e, boost::test_tools::tolerance( 1e-12 ) );
    BOOST_TEST( zMidplane.z0o == zTop.z0o, boost::test_tools::tolerance( 1e-12 ) );
}
 
 
BOOST_AUTO_TEST_SUITE_END()