{
  ifstream data("S-parameter.s2p");
  string buf;
  for(int j=0; j<9; j++) {
    getline(data, buf); 
    cout << buf << endl;
  }
  double f, Re_S11, Im_S11;
  double Re_S21, Im_S21;
  double Re_S12, Im_S12;
  double Re_S22, Im_S22;
  TGraph *g_Re_S11_vs_f = new TGraph(); 
  g_Re_S11_vs_f->SetMarkerColor(2);
  g_Re_S11_vs_f->SetLineColor(2);
  g_Re_S11_vs_f->SetFillColor(0);

  TGraph *g_Im_S11_vs_f = new TGraph(); 
  g_Im_S11_vs_f->SetMarkerColor(4);
  g_Im_S11_vs_f->SetLineColor(4);
  g_Im_S11_vs_f->SetFillColor(0);

  TGraph *g_S11_vs_f = new TGraph(); 
  g_S11_vs_f->SetMarkerColor(1);
  g_S11_vs_f->SetLineColor(1);
  g_S11_vs_f->SetFillColor(0);

  int num=0;
  cout <<" loading the data..." << endl;
  while(data>>f>>Re_S11>>Im_S11>>Re_S21>>Im_S21>>Re_S12>>Im_S12>>Re_S22>>Im_S22) {
    g_Re_S11_vs_f->SetPoint(num, f/1e9, Re_S11);
    g_Im_S11_vs_f->SetPoint(num, f/1e9, Im_S11);
    g_S11_vs_f->SetPoint(num, f/1e9, 10.*log10(Re_S11*Re_S11 + Im_S11*Im_S11));
    num++;
  }
  num = g_Re_S11_vs_f->GetN();
  cout << "# points = " << num << endl;

  /////////////////
  // FFT
  ////////////////

  double f_min = g_Re_S11_vs_f->GetX()[0];
  double f_max = g_Re_S11_vs_f->GetX()[num-1];
  double t_min = 0.;
  double t_max = ((double)num)/(f_max-f_min);
  cout << "f_min = " << f_min << ",   f_max = " << f_max << endl;
  cout << "t_min = " << t_min << ",   t_max = " << t_max << endl;

  Double_t *list_Re_S11 = new Double_t[num];
  Double_t *list_Im_S11 = new Double_t[num];
  for(int i=0; i<num; i++) {
    list_Re_S11[i] = g_Re_S11_vs_f->GetY()[i];
    list_Im_S11[i] = g_Im_S11_vs_f->GetY()[i];
  }

  cout <<" FFT the data..." << endl;
  TVirtualFFT *fft = TVirtualFFT::FFT(1, &num, "C2C");
  fft->SetPointsComplex(list_Re_S11, list_Im_S11);
  fft->Transform();

  TGraph *g_Re_S11_vs_t = new TGraph(); 
  g_Re_S11_vs_t->SetMarkerColor(2);
  g_Re_S11_vs_t->SetLineColor(2);
  g_Re_S11_vs_t->SetFillColor(0);

  TGraph *g_Im_S11_vs_t = new TGraph(); 
  g_Im_S11_vs_t->SetMarkerColor(4);
  g_Im_S11_vs_t->SetLineColor(4);
  g_Im_S11_vs_t->SetFillColor(0);

  TGraph *g_S11_vs_t = new TGraph(); 
  g_S11_vs_t->SetMarkerColor(1);
  g_S11_vs_t->SetLineColor(1);
  g_S11_vs_t->SetFillColor(0);

  for (int i=0; i<num; i++) {
     double t = t_min + (t_max-t_min) * ((double)i)/((double)num);
     double re, im;
     fft->GetPointComplex(i, re, im);
     g_Re_S11_vs_t->SetPoint(i, t, re/((double)num));
     g_Im_S11_vs_t->SetPoint(i, t, im/((double)num));
     g_S11_vs_t->SetPoint(i, t, 10.*log10(sqrt(re*re + im*im)/((double)num)));
  }

  TCanvas *c = new TCanvas("c", "c", 1200, 800);  
  c->Divide(2, 2);

  c->cd(1);
  g_Re_S11_vs_f->Draw("AP");
  g_Im_S11_vs_f->Draw("P");
  g_Re_S11_vs_f->GetXaxis()->SetTitle("f [GHz]");
  g_Re_S11_vs_f->GetYaxis()->SetTitle("linear S_{11}");
  TLegend *leg_freq = new TLegend(0.65, 0.65, 0.8, 0.8);
  leg_freq->SetFillColor(0);
  leg_freq->AddEntry(g_Re_S11_vs_f, "Re");
  leg_freq->AddEntry(g_Im_S11_vs_f, "Im");
  leg_freq->Draw();

  c->cd(2);
  g_S11_vs_f->Draw("AP");
  g_S11_vs_f->GetXaxis()->SetTitle("f [GHz]");
  g_S11_vs_f->GetYaxis()->SetTitle("S_{11} [dB]");

  c->cd(3);
  g_Re_S11_vs_t->Draw("AL");
  g_Im_S11_vs_t->Draw("P");
  g_Re_S11_vs_t->GetXaxis()->SetTitle("t [ns]");
  g_Re_S11_vs_t->GetYaxis()->SetTitle("linear S_{11}");
  TLegend *leg_time = new TLegend(0.65, 0.65, 0.8, 0.8);
  leg_time->SetFillColor(0);
  leg_time->AddEntry(g_Re_S11_vs_t, "Re");
  leg_time->AddEntry(g_Im_S11_vs_t, "Im");
  leg_time->Draw();

  c->cd(4);
  g_S11_vs_t->Draw("AL");
  g_S11_vs_t->GetXaxis()->SetTitle("t [ns]");
  g_S11_vs_t->GetYaxis()->SetTitle("S_{11} [dB]");


}