diff --git a/Software/PC_Application/LibreVNA-GUI/Calibration/CalStandardThroughEditDialog.ui b/Software/PC_Application/LibreVNA-GUI/Calibration/CalStandardThroughEditDialog.ui
index 7dfd09ce..4746bd0b 100644
--- a/Software/PC_Application/LibreVNA-GUI/Calibration/CalStandardThroughEditDialog.ui
+++ b/Software/PC_Application/LibreVNA-GUI/Calibration/CalStandardThroughEditDialog.ui
@@ -72,7 +72,7 @@
        <item row="0" column="1">
         <widget class="SIUnitEdit" name="Z0">
          <property name="enabled">
-          <bool>false</bool>
+          <bool>true</bool>
          </property>
         </widget>
        </item>
diff --git a/Software/PC_Application/LibreVNA-GUI/Calibration/calstandard.cpp b/Software/PC_Application/LibreVNA-GUI/Calibration/calstandard.cpp
index ea0f26ca..942a0233 100644
--- a/Software/PC_Application/LibreVNA-GUI/Calibration/calstandard.cpp
+++ b/Software/PC_Application/LibreVNA-GUI/Calibration/calstandard.cpp
@@ -579,7 +579,13 @@ Sparam Through::toSparam(double freq)
         double through_att = pow(10.0, -through_att_db / 10.0);
         auto through = polar<double>(through_att, through_phaseshift);
         // Assume symmetric and perfectly matched through for other parameters
-        return Sparam(0.0, through, through, 0.0);
+        auto S = Sparam(0.0, through, through, 0.0);
+        // update S parameters if Z0 does not match system impedance exactly
+        if(Z0 != 50.0) {
+            auto abcd = ABCDparam(S, Z0);
+            S = Sparam(abcd, 50.0);
+        }
+        return S;
     }
 }
 
diff --git a/Software/PC_Application/LibreVNA-Test/LibreVNA-Test.pro b/Software/PC_Application/LibreVNA-Test/LibreVNA-Test.pro
index 6f7f79ed..08634f39 100644
--- a/Software/PC_Application/LibreVNA-Test/LibreVNA-Test.pro
+++ b/Software/PC_Application/LibreVNA-Test/LibreVNA-Test.pro
@@ -150,6 +150,7 @@ SOURCES +=  \
     ../LibreVNA-GUI/touchstone.cpp \
     ../LibreVNA-GUI/unit.cpp \
     main.cpp \
+    parametertests.cpp \
     portextensiontests.cpp \
     utiltests.cpp
 
@@ -339,6 +340,7 @@ HEADERS += \
     ../LibreVNA-GUI/tcpserver.h \
     ../LibreVNA-GUI/touchstone.h \
     ../LibreVNA-GUI/unit.h \
+    parametertests.h \
     portextensiontests.h \
     utiltests.h
 
diff --git a/Software/PC_Application/LibreVNA-Test/main.cpp b/Software/PC_Application/LibreVNA-Test/main.cpp
index 0dff291d..610ad4ca 100644
--- a/Software/PC_Application/LibreVNA-Test/main.cpp
+++ b/Software/PC_Application/LibreVNA-Test/main.cpp
@@ -1,5 +1,6 @@
 #include "utiltests.h"
 #include "portextensiontests.h"
+#include "parametertests.h"
 
 #include <QtTest>
 
@@ -10,6 +11,7 @@ int main(int argc, char *argv[])
     int status = 0;
     status |= QTest::qExec(new UtilTests, argc, argv);
     status |= QTest::qExec(new PortExtensionTests, argc, argv);
+    status |= QTest::qExec(new ParameterTests, argc, argv);
 
     return status;
 }
diff --git a/Software/PC_Application/LibreVNA-Test/parametertests.cpp b/Software/PC_Application/LibreVNA-Test/parametertests.cpp
new file mode 100644
index 00000000..c41b06b4
--- /dev/null
+++ b/Software/PC_Application/LibreVNA-Test/parametertests.cpp
@@ -0,0 +1,63 @@
+#include "parametertests.h"
+
+#include "Tools/parameters.h"
+
+#include <QDebug>
+
+ParameterTests::ParameterTests()
+{
+
+}
+
+void ParameterTests::S2ABCD()
+{
+    using namespace std::complex_literals;
+
+    std::complex<double> S11 = 0.0038 + 0.0248i;
+    std::complex<double> S12 = 0.9961 - 0.0250i;
+    std::complex<double> S21 = 0.9964 - 0.0254i;
+    std::complex<double> S22 = 0.0037 + 0.0249i;
+    auto S = Sparam(S11, S12, S21, S22);
+    auto abcd = ABCDparam(S, 50.0);
+
+    std::complex<double> A = 0.99988705861501226 + 0.00014900771660107621i;
+    std::complex<double> B = 0.31256891513454765 + 2.5194422425174801i;
+    std::complex<double> C = -2.7673838933081734e-09 + 6.9983236736743502e-06i;
+    std::complex<double> D = 0.99978420576400329 + 0.00024674711602337137i;
+
+    QVERIFY(qFuzzyCompare(abcd.m11.real(), A.real()));
+    QVERIFY(qFuzzyCompare(abcd.m11.imag(), A.imag()));
+    QVERIFY(qFuzzyCompare(abcd.m12.real(), B.real()));
+    QVERIFY(qFuzzyCompare(abcd.m12.imag(), B.imag()));
+    QVERIFY(qFuzzyCompare(abcd.m21.real(), C.real()));
+    QVERIFY(qFuzzyCompare(abcd.m21.imag(), C.imag()));
+    QVERIFY(qFuzzyCompare(abcd.m22.real(), D.real()));
+    QVERIFY(qFuzzyCompare(abcd.m22.imag(), D.imag()));
+}
+
+void ParameterTests::ABCD2S()
+{
+    using namespace std::complex_literals;
+
+    std::complex<double> A = 0.99988705861501226 + 0.00014900771660107621i;
+    std::complex<double> B = 0.31256891513454765 + 2.5194422425174801i;
+    std::complex<double> C = -2.7673838933081734e-09 + 6.9983236736743502e-06i;
+    std::complex<double> D = 0.99978420576400329 + 0.00024674711602337137i;
+    auto abcd = ABCDparam(A, B, C, D);
+
+    auto s = Sparam(abcd, 50.0);
+
+    std::complex<double> S11 = 0.0038 + 0.0248i;
+    std::complex<double> S12 = 0.9961 - 0.0250i;
+    std::complex<double> S21 = 0.9964 - 0.0254i;
+    std::complex<double> S22 = 0.0037 + 0.0249i;
+
+    QVERIFY(qFuzzyCompare(s.m11.real(), S11.real()));
+    QVERIFY(qFuzzyCompare(s.m11.imag(), S11.imag()));
+    QVERIFY(qFuzzyCompare(s.m12.real(), S12.real()));
+    QVERIFY(qFuzzyCompare(s.m12.imag(), S12.imag()));
+    QVERIFY(qFuzzyCompare(s.m21.real(), S21.real()));
+    QVERIFY(qFuzzyCompare(s.m21.imag(), S21.imag()));
+    QVERIFY(qFuzzyCompare(s.m22.real(), S22.real()));
+    QVERIFY(qFuzzyCompare(s.m22.imag(), S22.imag()));
+}
diff --git a/Software/PC_Application/LibreVNA-Test/parametertests.h b/Software/PC_Application/LibreVNA-Test/parametertests.h
new file mode 100644
index 00000000..743187d8
--- /dev/null
+++ b/Software/PC_Application/LibreVNA-Test/parametertests.h
@@ -0,0 +1,17 @@
+#ifndef PARAMETERTESTS_H
+#define PARAMETERTESTS_H
+
+#include <QtTest>
+
+class ParameterTests : public QObject
+{
+    Q_OBJECT
+public:
+    ParameterTests();
+
+private slots:
+    void S2ABCD();
+    void ABCD2S();
+};
+
+#endif // PARAMETERTESTS_H