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 Oscillator Type:","vco_coax_dual.html");

  if (option_no == "coax") {

    f.category.options[1] = new Option("Dual Output","vco_coax_dual.html");
    f.category.options[2] = new Option("Linear Tuning,Wideband","vco_coax_linear.html");
    f.category.options[3] = new Option("5V Tuning for PLL","vco_coax_5v.html");

  }
  else if (option_no == "pi") {

    f.category.options[1] = new Option("Linear Tuning,Wideband","vco_pic_linear.html");
    f.category.options[2] = new Option("5V Tuning for PLL","vco_pic_5v.html");

  }
  else if (option_no == "sm") {

    f.category.options[1] = new Option("Linear Tuning,Wideband","vco_sm_linear.html");
    f.category.options[2] = new Option("5V Tuning for PLL","vco_sm_5v.html");
  }

}