var PreviewWindow = null;

function openpreview(doc)
{
  if (PreviewWindow != null && PreviewWindow.closed == false)
  {
    PreviewWindow.location.href = doc;
    PreviewWindow.focus();
  }
  else
  {
    PreviewWindow = window.open(doc,"preview","scrollbars=yes,width=700,height=550,location=no,menubar=no,status=no,toolbar=no,resizable=yes");
  }
}