I would suggest that you not even use a header cable. Put the FIFO in the probe.
If you really have to use buffers and cables, LVDS is the way to go.
How variable? I'm guessing that you want to cover several decades (=
10 octaves). If just powers-of-two are OK, there are oscillator cans in the 166 MHz region and you can select off a tap from some ripple divide-by-twos.I've been having a lot of fun playing with the AD9954 series DDS's. The 9952/9954 have comparators just for clock generation. 166 MHz is just over the top of the official spec although I'm sure it'll work. Circa $25 for the chip from Digikey, then a crystal and some L's/C's.
Tim.