function setModelType(f,option_no) {
  var i=0;
  
  // clear selection options	 
  while (i < f.category.length) {
    f.category.options[i] = null;
    i++;
  }

    f.category.options[0] = new Option("Choose Phase Detector Type:","phase_detectors.html");

  if (option_no == "coax") {
    f.category.options[1] = new Option("High DC Output","phase_detectors_coax.html");
  }
  else if (option_no == "pi") {
    f.category.options[1] = new Option("High DC Output","phase_detectors_pic.html");
  }
  else if (option_no == "sm") {
    f.category.options[1] = new Option("High DC Output","phase_detectors.html");
  }

}