If you are using a fast processor you will probably get a better accuracy by measuring pulse width. I recommend a derivative with a PCA and the tricks for one such can be found in The PCA cookbook
http://www.intel.com/design/MC...085101.PDF

Erik

PS all derivatives (at least those I know) with PCA basically work the same way so a Philips or Atmel Rx2 or 66x would be my choice for a modern '51 with a PCA