Using the 100MHz clock

Has anyone used the 100Mhz LVDS clock?
It seems I can’t get the right UCF constraint for this. ISE won’t let me make the pin LVDS?
Does anyone have a working UCF file with all the pins?
At the moment I get the clock (using LVCMOS) but its has too much jitter.

The RAMTester sample uses the LVDS clock. Have you looked at the UCF and HDL for that sample?

The Counters sample also uses the LVDS clock and even has an alternative way of setting the constraints.