Fix impedance renormalization + add unit tests

Author: jankae

Software/PC_Application/LibreVNA-GUI/Util/util.cpp

@@ -62,7 +62,19 @@ double Util::distanceToLine(QPointF point, QPointF l1, QPointF l2, QPointF *clos
 }
 
 std::complex<double> Util::SparamToImpedance(std::complex<double> d, std::complex<double> Z0) {
-    return Z0 * (1.0 + d) / (1.0 - d);
+    if (d != 1.0) {
+        return Z0 * (1.0 + d) / (1.0 - d);
+    } else {
+        return std::complex<double>(std::numeric_limits<double>::infinity(), 0.0);
+    }
+}
+
+std::complex<double> Util::ImpedanceToSparam(std::complex<double> Z, std::complex<double> Z0) {
+    if(std::isinf(Z.real())) {
+        return 1.0;
+    } else {
+        return (Z-Z0)/(Z+Z0);
+    }
 }
 
 double Util::dBmTodBuV(double dBm)

Software/PC_Application/LibreVNA-GUI/Util/util.h

@@ -52,6 +52,7 @@ namespace Util {
         return SparamToVSWR(abs(d));
     }
     std::complex<double> SparamToImpedance(std::complex<double> d, std::complex<double> Z0 = 50.0);
+    std::complex<double> ImpedanceToSparam(std::complex<double> Z, std::complex<double> Z0 = 50.0);
     // all these conversions assume series connection of real and imag part
     static inline double SparamToResistance(std::complex<double> d, std::complex<double> Z0 = 50.0) {
         return SparamToImpedance(d, Z0).real();

Software/PC_Application/LibreVNA-GUI/VNA/Deembedding/impedancerenormalization.cpp

@@ -3,6 +3,7 @@
 #include "ui_impedancenormalizationdialog.h"
 #include "Tools/parameters.h"
 #include "appwindow.h"
+#include "Util/util.h"
 
 #include <complex>
 
@@ -44,9 +45,11 @@ void ImpedanceRenormalization::transformDatapoint(DeviceDriver::VNAMeasurement &
         name = "S"+QString::number(ports+1)+QString::number(ports+1);
     }
     for(auto i=1;i<=ports;i++) {
+        // handle reflection parameters
         auto S11name = "S"+QString::number(i)+QString::number(i);
         auto S11 = p.measurements[S11name];
-        transformed[S11name] = Sparam(ABCDparam(Sparam(S11, 0.1, 0.1, 1.0), p.Z0), impedance).m11;
+        transformed[S11name] = Util::ImpedanceToSparam(Util::SparamToImpedance(S11, p.Z0), impedance);
+        // handle transmission parameters
         for(auto j=i+1;j<=ports;j++) {
                 auto S12name = "S"+QString::number(i)+QString::number(j);
                 auto S21name = "S"+QString::number(j)+QString::number(i);

Software/PC_Application/LibreVNA-Test/LibreVNA-Test.pro

@@ -152,6 +152,7 @@ SOURCES +=  \
     ../LibreVNA-GUI/touchstone.cpp \
     ../LibreVNA-GUI/unit.cpp \
     ffttests.cpp \
+    impedancerenormalizationtests.cpp \
     main.cpp \
     parametertests.cpp \
     portextensiontests.cpp \
@@ -346,6 +347,7 @@ HEADERS += \
     ../LibreVNA-GUI/touchstone.h \
     ../LibreVNA-GUI/unit.h \
     ffttests.h \
+    impedancerenormalizationtests.h \
     parametertests.h \
     portextensiontests.h \
     utiltests.h

Software/PC_Application/LibreVNA-Test/impedancerenormalizationtests.cpp

@@ -0,0 +1,42 @@
+#include "impedancerenormalizationtests.h"
+
+ImpedanceRenormalizationTests::ImpedanceRenormalizationTests()
+{
+    renorm = new ImpedanceRenormalization();
+
+    // set it up to normalize to 75 Ohm
+    nlohmann::json j;
+    j["impedance"] = 75;
+    renorm->fromJSON(j);
+}
+
+ImpedanceRenormalizationTests::~ImpedanceRenormalizationTests()
+{
+    delete renorm;
+}
+
+void ImpedanceRenormalizationTests::OnePortTests()
+{
+    // Create dummy measurements
+    DeviceDriver::VNAMeasurement measShort;
+    measShort.pointNum = 0;
+    measShort.Z0 = 50.0;
+    measShort.measurements["S11"] = -1.0;
+
+    auto measLoad = measShort;
+    measLoad.measurements["S11"] = 0.0;
+
+    auto measOpen = measShort;
+    measOpen.measurements["S11"] = 1.0;
+
+    // perform renormalization
+    renorm->transformDatapoint(measShort);
+    renorm->transformDatapoint(measLoad);
+    renorm->transformDatapoint(measOpen);
+
+    QVERIFY(measShort.measurements["S11"] == -1.0);
+    // a Ohm load renormalized to 75 Ohm impedance has a reflection coefficient of -0.2
+    QVERIFY(measLoad.measurements["S11"] == -0.2);
+    QVERIFY(measOpen.measurements["S11"] == 1.0);
+}
+

Software/PC_Application/LibreVNA-Test/impedancerenormalizationtests.h

@@ -0,0 +1,21 @@
+#ifndef IMPEDANCERENORMALIZATIONTESTS_H
+#define IMPEDANCERENORMALIZATIONTESTS_H
+
+#include <QtTest>
+
+#include "impedancerenormalization.h"
+
+class ImpedanceRenormalizationTests : public QObject
+{
+    Q_OBJECT
+public:
+    ImpedanceRenormalizationTests();
+    ~ImpedanceRenormalizationTests();
+private slots:
+    void OnePortTests();
+
+private:
+    ImpedanceRenormalization *renorm;
+};
+
+#endif // IMPEDANCERENORMALIZATIONTESTS_H

Software/PC_Application/LibreVNA-Test/main.cpp

@@ -2,6 +2,7 @@
 #include "portextensiontests.h"
 #include "parametertests.h"
 #include "ffttests.h"
+#include "impedancerenormalizationtests.h"
 
 #include <QtTest>
 
@@ -14,6 +15,7 @@ int main(int argc, char *argv[])
     status |= QTest::qExec(new PortExtensionTests, argc, argv);
     status |= QTest::qExec(new ParameterTests, argc, argv);
     status |= QTest::qExec(new fftTests, argc, argv);
+    status |= QTest::qExec(new ImpedanceRenormalizationTests, argc, argv);
 
     return status;
 }

Software/PC_Application/LibreVNA-Test/utiltests.cpp

@@ -78,3 +78,31 @@ void UtilTests::FirmwareComparison()
     QVERIFY(Util::firmwareEqualOrHigher("2.2.2", "2.3") == false);
     QVERIFY(Util::firmwareEqualOrHigher("2.2", "2.3.1") == false);
 }
+
+void UtilTests::ImpedanceSparameterCalculation()
+{
+    // Check impedance to S parameter conversion and back for different values
+    auto Z = std::complex<double>(50.0);
+    auto S = Util::ImpedanceToSparam(Z);
+    QVERIFY(S == 0.0);
+    QVERIFY(Util::SparamToImpedance(S) == Z);
+
+    Z = std::complex<double>(0.0);
+    S = Util::ImpedanceToSparam(Z);
+    QVERIFY(S == -1.0);
+    QVERIFY(Util::SparamToImpedance(S) == Z);
+
+    Z = std::complex<double>(100.0);
+    S = Util::ImpedanceToSparam(Z);
+    QVERIFY(qFuzzyCompare(S.real(), 1.0/3));
+    QVERIFY(S.imag() == 0.0);
+    QVERIFY(qFuzzyCompare(Util::SparamToImpedance(S).real(), Z.real()));
+    QVERIFY(qFuzzyCompare(Util::SparamToImpedance(S).imag(), Z.imag()));
+
+    // Edge case: convert S parameter to impedance at 1.0 (which will give a +inf impedance)
+    S = 1.0;
+    Z = Util::SparamToImpedance(S);
+    // convert back, we must land back at 1.0
+    auto S_from_Z = Util::ImpedanceToSparam(Z);
+    QVERIFY(S_from_Z == S);
+}

Software/PC_Application/LibreVNA-Test/utiltests.h

@@ -14,6 +14,7 @@ private slots:
     void IdealArcApproximation();
     void NoisyCircleApproximation();
     void FirmwareComparison();
+    void ImpedanceSparameterCalculation();
 };
 
 #endif // UTILTESTS_H