diff --git a/Software/PC_Application/LibreVNA-GUI/Device/LibreVNA/librevnadriver.cpp b/Software/PC_Application/LibreVNA-GUI/Device/LibreVNA/librevnadriver.cpp
index a232edc..a51ba48 100644
--- a/Software/PC_Application/LibreVNA-GUI/Device/LibreVNA/librevnadriver.cpp
+++ b/Software/PC_Application/LibreVNA-GUI/Device/LibreVNA/librevnadriver.cpp
@@ -112,6 +112,7 @@ LibreVNADriver::LibreVNADriver()
 {
     connected = false;
     skipOwnPacketHandling = false;
+    isIdle = true;
     SApoints = 0;
     hardwareVersion = 0;
     protocolVersion = 0;
@@ -453,6 +454,9 @@ bool LibreVNADriver::setVNA(const DeviceDriver::VNASettings &s, std::function<vo
     p.settings.syncMode = (int) sync;
     p.settings.syncMaster = syncMaster ? 1 : 0;
 
+    isIdle = false;
+    lastNonIdlePacket = p;
+
     return SendPacket(p, [=](TransmissionResult r){
         if(cb) {
             cb(r == TransmissionResult::Ack);
@@ -507,6 +511,9 @@ bool LibreVNADriver::setSA(const DeviceDriver::SASettings &s, std::function<void
     p.spectrumSettings.syncMode = (int) sync;
     p.spectrumSettings.syncMaster = syncMaster ? 1 : 0;
 
+    isIdle = false;
+    lastNonIdlePacket = p;
+
     return SendPacket(p, [=](TransmissionResult r){
         if(cb) {
             cb(r == TransmissionResult::Ack);
@@ -531,11 +538,14 @@ bool LibreVNADriver::setSG(const DeviceDriver::SGSettings &s)
     p.generator.cdbm_level = s.dBm * 100;
     p.generator.activePort = s.port;
     p.generator.applyAmplitudeCorrection = true;
+    isIdle = false;
+    lastNonIdlePacket = p;
     return SendPacket(p);
 }
 
 bool LibreVNADriver::setIdle(std::function<void (bool)> cb)
 {
+    isIdle = true;
     Protocol::PacketInfo p;
     p.type = Protocol::PacketType::SetIdle;
     return SendPacket(p, [=](TransmissionResult res) {
@@ -608,7 +618,21 @@ bool LibreVNADriver::setExtRef(QString option_in, QString option_out)
     case Reference::OutFreq::MHZ100: p.reference.ExtRefOuputFreq = 100000000; break;
     }
 
-    return SendPacket(p);
+    bool ret;
+    if(isIdle) {
+        // can switch reference directly
+        ret = SendPacket(p);
+    } else {
+        // switching the reference while a sweep (or any frequency generation is active)
+        // can result in wrong frequencies when a frequency calibration is applied to
+        // the internal reference. Stop any activity before switching the reference and
+        // start it again afterwards
+        ret = sendWithoutPayload(Protocol::PacketType::SetIdle);
+        ret &= SendPacket(p);
+        ret &= SendPacket(lastNonIdlePacket);
+    }
+
+    return ret;
 }
 
 void LibreVNADriver::registerTypes()
diff --git a/Software/PC_Application/LibreVNA-GUI/Device/LibreVNA/librevnadriver.h b/Software/PC_Application/LibreVNA-GUI/Device/LibreVNA/librevnadriver.h
index ff9d9cd..36340c9 100644
--- a/Software/PC_Application/LibreVNA-GUI/Device/LibreVNA/librevnadriver.h
+++ b/Software/PC_Application/LibreVNA-GUI/Device/LibreVNA/librevnadriver.h
@@ -212,6 +212,9 @@ protected:
     Synchronization sync;
     bool syncMaster;
 
+    bool isIdle;
+    Protocol::PacketInfo lastNonIdlePacket;
+
     std::map<int, int> portStageMapping; // maps from excitedPort (count starts at one) to stage (count starts at zero)
 
     // Driver specific settings
diff --git a/Software/PC_Application/LibreVNA-GUI/VNA/vna.cpp b/Software/PC_Application/LibreVNA-GUI/VNA/vna.cpp
index 58eb4a7..48c43fa 100644
--- a/Software/PC_Application/LibreVNA-GUI/VNA/vna.cpp
+++ b/Software/PC_Application/LibreVNA-GUI/VNA/vna.cpp
@@ -1041,7 +1041,7 @@ void VNA::NewDatapoint(DeviceDriver::VNAMeasurement m)
         markerModel->updateMarkers();
     }
 
-    if(m_avg.pointNum > 0 && m_avg.pointNum != lastPoint + 1) {
+    if(m_avg.pointNum > 0 && m_avg.pointNum != (unsigned int) (lastPoint + 1)) {
         qWarning() << "Got point" << m_avg.pointNum << "but last received point was" << lastPoint << "("<<(m_avg.pointNum-lastPoint-1)<<"missed points)";
     }
     lastPoint = m_avg.pointNum;