diff --git a/Detector.cpp b/Detector.cpp
index fe078a880..8faf99354 100644
--- a/Detector.cpp
+++ b/Detector.cpp
@@ -7,7 +7,7 @@
 #include "moc_Detector.cpp"
 
 extern "C" {
-  void   fil4_(qint16*, qint32*, qint16*, qint32*);
+  void   fil4_(qint16*, qint32*, qint16*, qint32*, qint32*);
 }
 
 Detector::Detector (unsigned frameRate, unsigned periodLengthInSeconds,
@@ -85,8 +85,10 @@ qint64 Detector::writeData (char const * data, qint64 maxSize)
           qint32 framesAfterDownSample (m_samplesPerFFT);
           if(framesToProcess==13824 and dec_data.params.kin>=0 and
              dec_data.params.kin < (NTMAX*12000 - framesAfterDownSample)) {
+            int nrefspec=0;
+            if(g_bRefSpec) nrefspec=1;
             fil4_(&m_buffer[0], &framesToProcess, &dec_data.d2[dec_data.params.kin],
-                  &framesAfterDownSample);
+                  &framesAfterDownSample, &nrefspec);
             dec_data.params.kin += framesAfterDownSample;
           } else {
             // qDebug() << "framesToProcess     = " << framesToProcess;
diff --git a/Detector.hpp b/Detector.hpp
index 9855be9a2..a8daa0285 100644
--- a/Detector.hpp
+++ b/Detector.hpp
@@ -1,10 +1,10 @@
 #ifndef DETECTOR_HPP__
 #define DETECTOR_HPP__
-
 #include "AudioDevice.hpp"
-
 #include <QScopedArrayPointer>
 
+extern bool g_bRefSpec;
+
 //
 // output device that distributes data in predefined chunks via a signal
 //
diff --git a/lib/fil4.f90 b/lib/fil4.f90
index 4946da2fe..2a72b397e 100644
--- a/lib/fil4.f90
+++ b/lib/fil4.f90
@@ -1,4 +1,4 @@
-subroutine fil4(id1,n1,id2,n2)
+subroutine fil4(id1,n1,id2,n2,nrefspec)
 
 ! FIR lowpass filter designed using ScopeFIR
 
@@ -12,10 +12,15 @@ subroutine fil4(id1,n1,id2,n2)
 
   parameter (NTAPS=49)
   parameter (NDOWN=4)             !Downsample ratio
+  parameter (NFFT=13824,NH=NFFT/2)
   integer*2 id1(n1)
   integer*2 id2(*)
   real t(NTAPS)
-  data t/NTAPS*0.0/
+  real x(NFFT)
+  real s(0:NH)
+  complex cx(0:NH)
+  equivalence(x,cx)
+  data t/NTAPS*0.0/,nrefspec0/-999/
 
 ! Filter coefficients:
   real w(NTAPS)
@@ -33,7 +38,13 @@ subroutine fil4(id1,n1,id2,n2)
        -0.013752163325,-0.011720748164,-0.005202883094, 0.002613872664,   &
         0.008706594219, 0.011363155076, 0.010161983649, 0.010051920210,   &
         0.000861074040/
-  save w,t
+  save w,t,nsave,s,nrefspec0
+
+  if(nrefspec.ne.nrefspec0) then
+     s=0.
+     nsave=0
+     nrefspec0=nrefspec
+  endif
 
   n2=n1/NDOWN
   if(n2*NDOWN.ne.n1) stop 'Error in fil4'
@@ -45,5 +56,34 @@ subroutine fil4(id1,n1,id2,n2)
      id2(i)=nint(dot_product(w,t))
   enddo
 
+  if(nrefspec.eq.1) then
+     nsave=nsave+1
+     x=0.001*id1
+     call four2a(x,NFFT,1,-1,0)                 !r2c FFT
+
+     do i=1,NH
+        s(i)=s(i) + real(cx(i))**2 + aimag(cx(i))**2
+     enddo
+
+     if(mod(nsave,34).eq.0) then                   !About 9.8 sec 
+        df=48000.0/NFFT
+        ia=nint(500.0/df)
+        ib=nint(2500.0/df)
+        call pctile(s(ia),ib-ia+1,50,xmed)
+        db0=db(xmed)
+        nhadd=10
+        open(16,file='refspec.dat',status='unknown')
+        do i=1,NH
+           freq=i*df
+           ia=max(1,i-nhadd)
+           ib=min(NH,i+nhadd)
+           smo=sum(s(ia:ib))/(ib-ia+1)
+           write(16,1000) freq,db(smo)-db0
+1000       format(2f10.3)
+        enddo
+        close(16)
+     endif
+  endif
+
   return
 end subroutine fil4
diff --git a/mainwindow.cpp b/mainwindow.cpp
index 05e60eb48..48d1c7e7e 100644
--- a/mainwindow.cpp
+++ b/mainwindow.cpp
@@ -118,6 +118,7 @@ int   fast_jh2;
 int narg[15];
 QVector<QColor> g_ColorTbl;
 bool g_single_decode;
+bool g_bRefSpec;
 
 namespace
 {
@@ -676,6 +677,7 @@ MainWindow::MainWindow(bool multiple, QSettings * settings, QSharedMemory *shdme
   }
   VHF_features_enabled(m_config.enable_VHF_features());
   g_single_decode=m_config.single_decode();
+  g_bRefSpec=false;
 
   progressBar->setMaximum(m_TRperiod);
   m_modulator->setPeriod(m_TRperiod); // TODO - not thread safe
@@ -1472,7 +1474,11 @@ void MainWindow::closeEvent(QCloseEvent * e)
 void MainWindow::on_stopButton_clicked()                       //stopButton
 {
   monitor (false);
-  m_loopall=false;  
+  m_loopall=false;
+  if(g_bRefSpec) {
+//    msgBox("Reference spectrum saved.");
+    g_bRefSpec=false;
+  }
 }
 
 void MainWindow::msgBox(QString t)                             //msgBox
@@ -5336,7 +5342,7 @@ void MainWindow::fastPick(int x0, int x1, int y)
 
 void MainWindow::on_actionSave_reference_spectrum_triggered()
 {
-  msgBox("Reference spectrum not presently implemented");
+  g_bRefSpec=true;
 }
 
 void MainWindow::on_sbCQRxFreq_valueChanged(int n)
diff --git a/mainwindow.ui b/mainwindow.ui
index c456045e9..07dc1a3ee 100644
--- a/mainwindow.ui
+++ b/mainwindow.ui
@@ -2810,7 +2810,7 @@ QPushButton[state=&quot;ok&quot;] {
   </action>
   <action name="actionSave_reference_spectrum">
    <property name="text">
-    <string>Save reference spectrum</string>
+    <string>Measure reference spectrum</string>
    </property>
   </action>
   <action name="actionJTMSK">