Add shunt resistor attenuator

The shunt resistor attenuator is unmatched at both ports. The input and output impedance is shown in a box.
The derivation of the design equations will be provided separately in the pull request

qucs-attenuator/attenuatorfunc.cpp

@@ -168,9 +168,20 @@ int QUCS_Att::Calc(tagATT *ATT)
        ATT->R3 = ATT->Zin + ATT->R1;//Output impedance
        // Power dissipation
        ATT->PR1 = ATT->Pin*(1-L);// Shunt resistor
-
        break;
       }
+    case R_SHUNT:
+      {
+      L = pow(10, -ATT->Attenuation / 10);
+      // Design equations
+      ATT->R1 = (2*sqrt(ATT->Zout*ATT->Zin*L)*ATT->Zout*ATT->Zin + (ATT->Zout*ATT->Zout*ATT->Zin + ATT->Zout*ATT->Zin*ATT->Zin)*L)/(4*ATT->Zout*ATT->Zin - (ATT->Zout*ATT->Zout + 2*ATT->Zout*ATT->Zin + ATT->Zin*ATT->Zin)*L);// Shunt resistor
+      ATT->R2 = (ATT->Zout*ATT->R1)/(ATT->Zout+ATT->R1); // Input impedance
+      ATT->R3 = (ATT->Zin*ATT->R1)/(ATT->Zin+ATT->R1);//Output impedance
+      // Power dissipation
+      ATT->PR1 = ATT->Pin*(1-L);// Shunt resistor
+      break;
+      }
+
 	}
    return 0;
 
@@ -750,6 +761,58 @@ QString* QUCS_Att::createSchematic(tagATT *ATT, bool SP_box)
          }
       *s += "</Paintings>\n";
       break;
+
+      case R_SHUNT:
+
+        *s += QString("<R R1 1 250 200 -15 60 0 1 \"%1 Ohm\" 1 \"26.85\" 0 \"0.0\" 0 \"0.0\" 0 \"26.85\" 0 \"US\" 0>\n").arg(QString::number(ATT->R1, 'f', 1));
+        *s += "<GND * 1 250 230 0 0 0 0>\n";
+        if (SP_box)
+         {
+           // S-parameter simulation block
+           //-----------------------------
+           // Resistor attenuators are broadband ckts, so it's pointless to ask the user to input the analysis freq sweep. Let's do a wideband
+           // sweep and then the user can modify that in the schematic
+           *s += "<.SP SP1 1 140 350 0 83 0 0 \"lin\" 1 \"50 MHz\" 1 \"3 GHz\" 1 \"200\" 1 \"no\" 0 \"1\" 0 \"2\" 0 \"no\" 0 \"no\" 0>\n";
+
+           // Equations
+           *s += "<Eqn Eqn1 1 360 350 -32 19 0 0 \"S21_dB=dB(S[2,1])\" 1 \"S11_dB=dB(S[1,1])\" 1 \"S22_dB=dB(S[2,2])\" 1 \"yes\" 0>\n";
+
+           // Input term
+           *s += QString("<Pac P1 1 50 200 18 -26 0 1 \"1\" 1 \"%1 Ohm\" 1 \"0 dBm\" 0 \"1 GHz\" 0 \"26.85\" 0>\n").arg(ATT->Zin);
+           *s += "<GND * 1 50 230 0 0 0 0>\n";
+
+           // Output term
+           *s += QString("<Pac P1 1 460 200 18 -26 0 1 \"1\" 1 \"%1 Ohm\" 1 \"0 dBm\" 0 \"1 GHz\" 0 \"26.85\" 0>\n").arg(ATT->Zout);
+           *s += "<GND * 1 460 230 0 0 0 0>\n";
+         }
+        *s += "</Components>\n";
+        *s += "<Wires>\n";
+        *s += "<140 130 350 130 \"\" 0 0 0 \"\">\n";
+        *s += "<250 130 250 170 \"\" 0 0 0 \"\">\n";
+        if (SP_box)
+             {   // Additional wiring because of the input/output ports
+
+                 // Input port
+                 *s += "<50 130 50 170 \"\" 0 0 0 \"\">\n";
+                 *s += "<50 130 140 130 \"\" 0 0 0 \"\">\n";
+
+                 // Output port
+                 *s += "<460 130 460 170 \"\" 0 0 0 \"\">\n";
+                 *s += "<460 130 350 130 \"\" 0 0 0 \"\">\n";
+             }
+        *s += "</Wires>\n";
+        *s += "<Diagrams>\n";
+        *s += "</Diagrams>\n";
+        *s += "<Paintings>\n";
+        *s += QString("<Text 160 60 12 #000000 0 \"%1 dB Shunt R Attenuator\">\n").arg(ATT->Attenuation);
+        if (!SP_box)
+           {// If the SP simulation box option is activated, then the input and output ports are attached.
+            // Thus, it doesn't make sense to have a text field indicating the input/output impedance
+              *s += QString("<Text 50 122 10 #000000 0 \"Z1: %1 Ohm\">\n").arg(ATT->Zin);
+              *s += QString("<Text 360 122 10 #000000 0 \"Z2: %1 Ohm\">\n").arg(ATT->Zout);
+           }
+        *s += "</Paintings>\n";
+        break;
     }
 
   return s;

qucs-attenuator/attenuatorfunc.h

@@ -22,6 +22,7 @@
 #define L_PAD_1ST_SERIES 6
 #define L_PAD_1ST_SHUNT 7
 #define R_SERIES 8
+#define R_SHUNT 9
 
 #define C0 299792458
 #define PI 3.141592653589793238462643383279502884197169399375105820974944592307816406286

qucs-attenuator/qucsattenuator.cpp

@@ -92,6 +92,7 @@ QucsAttenuator::QucsAttenuator()
   ComboTopology->insertItem(7, "L-pad 1st series");
   ComboTopology->insertItem(8, "L-pad 1st shunt");
   ComboTopology->insertItem(9, "Rseries");
+  ComboTopology->insertItem(10, "Rshunt");
   connect(ComboTopology, SIGNAL(activated(int)), SLOT(slotTopologyChanged()));
   topoGrid->addWidget(ComboTopology, 1,0,1,2);
 
@@ -625,15 +626,18 @@ void QucsAttenuator::slotTopologyChanged()
       lineEdit_R2_Pdiss->show();
       ComboR2_PowerUnits->show();
       break;
+
       case R_SERIES:
-        pixTopology->setPixmap(QPixmap((":/bitmaps/Rseries.png")));
+      case R_SHUNT:
+       (ComboTopology->currentIndex() == R_SERIES) ? pixTopology->setPixmap(QPixmap((":/bitmaps/Rseries.png")))
+                                                          : pixTopology->setPixmap(QPixmap((":/bitmaps/Rshunt.png")));
         LabelImp1->setText("Zin:");
         LabelImp2->show();
         QSpinBox_Zout->show();
         LabelImp2_Ohm->show();
-        LabelR2->setText("Zin:");
+        LabelR2->setText("Z1:");
         LabelR3->show();
-        LabelR3->setText("Zout:");
+        LabelR3->setText("Z2:");
         LabelR4->hide();
         lineEdit_R3->show();
         lineEdit_R4->hide();

qucs-attenuator/qucsattenuator.qrc

@@ -12,5 +12,6 @@
         <file>bitmaps/L_pad_1st_series.png</file>
         <file>bitmaps/L_pad_1st_shunt.png</file>
         <file>bitmaps/Rseries.png</file>
+        <file>bitmaps/Rshunt.png</file>
     </qresource>
 </RCC>