From b6f26eb6dce15e20bf8ddc72e796f129f5c46919 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20K=C3=A4berich?= <j.kaeberich@gmx.de>
Date: Wed, 26 Feb 2025 08:54:45 +0100
Subject: [PATCH] Interpolate calibration with magnitude/phase instead of
 real/imag

---
 .../LibreVNA-GUI/Calibration/calibration.cpp   | 12 ++++++------
 .../PC_Application/LibreVNA-GUI/Util/util.cpp  | 18 ++++++++++++++++++
 .../PC_Application/LibreVNA-GUI/Util/util.h    |  3 +++
 3 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/Software/PC_Application/LibreVNA-GUI/Calibration/calibration.cpp b/Software/PC_Application/LibreVNA-GUI/Calibration/calibration.cpp
index 7872e07..b922c16 100644
--- a/Software/PC_Application/LibreVNA-GUI/Calibration/calibration.cpp
+++ b/Software/PC_Application/LibreVNA-GUI/Calibration/calibration.cpp
@@ -1971,35 +1971,35 @@ Calibration::Point Calibration::Point::interpolate(const Calibration::Point &to,
     ret.frequency = frequency * (1.0-alpha) + to.frequency * alpha;
     ret.D.resize(D.size(), 0.0);
     for(unsigned int i=0;i<D.size();i++) {
-        ret.D[i] = D[i] * (1.0-alpha) + to.D[i] * alpha;
+        ret.D[i] = Util::interpolateMagPhase(D[i], to.D[i], alpha);
     }
     ret.R.resize(R.size(), 0.0);
     for(unsigned int i=0;i<R.size();i++) {
-        ret.R[i] = R[i] * (1.0-alpha) + to.R[i] * alpha;
+        ret.R[i] = Util::interpolateMagPhase(R[i], to.R[i], alpha);
     }
     ret.S.resize(S.size(), 0.0);
     for(unsigned int i=0;i<S.size();i++) {
-        ret.S[i] = S[i] * (1.0-alpha) + to.S[i] * alpha;
+        ret.S[i] = Util::interpolateMagPhase(S[i], to.S[i], alpha);
     }
     ret.T.resize(T.size());
     for(unsigned int i=0;i<T.size();i++) {
         ret.T[i].resize(T[i].size(), 0.0);
         for(unsigned int j=0;j<T[i].size();j++) {
-            ret.T[i][j] = T[i][j] * (1.0 - alpha) + to.T[i][j] * alpha;
+            ret.T[i][j] = Util::interpolateMagPhase(T[i][j], to.T[i][j], alpha);
         }
     }
     ret.L.resize(L.size());
     for(unsigned int i=0;i<L.size();i++) {
         ret.L[i].resize(L[i].size(), 0.0);
         for(unsigned int j=0;j<L[i].size();j++) {
-            ret.L[i][j] = L[i][j] * (1.0 - alpha) + to.L[i][j] * alpha;
+            ret.L[i][j] = Util::interpolateMagPhase(L[i][j], to.L[i][j], alpha);
         }
     }
     ret.I.resize(I.size());
     for(unsigned int i=0;i<I.size();i++) {
         ret.I[i].resize(I[i].size(), 0.0);
         for(unsigned int j=0;j<I[i].size();j++) {
-            ret.I[i][j] = I[i][j] * (1.0 - alpha) + to.I[i][j] * alpha;
+            ret.I[i][j] = Util::interpolateMagPhase(I[i][j], to.I[i][j], alpha);
         }
     }
     return ret;
diff --git a/Software/PC_Application/LibreVNA-GUI/Util/util.cpp b/Software/PC_Application/LibreVNA-GUI/Util/util.cpp
index 2c0fc6e..0f3fd76 100644
--- a/Software/PC_Application/LibreVNA-GUI/Util/util.cpp
+++ b/Software/PC_Application/LibreVNA-GUI/Util/util.cpp
@@ -237,3 +237,21 @@ bool Util::firmwareEqualOrHigher(QString firmware, QString compare)
         return true;
     }
 }
+
+std::complex<double> Util::interpolateMagPhase(const std::complex<double> &from, const std::complex<double> &to, double alpha)
+{
+    auto magFrom = abs(from);
+    auto magTo = abs(to);
+    auto phaseFrom = arg(from);
+    auto phaseTo = arg(to);
+    // unwrap phase
+    if(phaseTo - phaseFrom > M_PI) {
+        phaseTo -= 2*M_PI;
+    } else if(phaseTo - phaseFrom < -M_PI) {
+        phaseTo += 2*M_PI;
+    }
+    auto magInterp = magFrom * (1.0 - alpha) + magTo * alpha;
+    auto phaseInterp = phaseFrom * (1.0 - alpha) + phaseTo * alpha;
+
+    return std::polar<double>(magInterp, phaseInterp);
+}
diff --git a/Software/PC_Application/LibreVNA-GUI/Util/util.h b/Software/PC_Application/LibreVNA-GUI/Util/util.h
index de83dc9..6b5b32c 100644
--- a/Software/PC_Application/LibreVNA-GUI/Util/util.h
+++ b/Software/PC_Application/LibreVNA-GUI/Util/util.h
@@ -117,6 +117,9 @@ namespace Util {
         }
         return ret;
     }
+
+    std::complex<double> interpolateMagPhase(const std::complex<double> &from, const std::complex<double> &to, double alpha);
+
     void unwrapPhase(std::vector<double> &phase, unsigned int start_index = 0);
 
     // input values are Y coordinates, assumes evenly spaced linear X values from 0 to input.size() - 1