[time-nuts] What is a Time-Nut grade Zero Crossing Circuit?

[Bob Paddock]

Can you point me to a Time-Nut grade Zero Crossing
circuit that I can feed a Actel Igloo FPGA (It doesn't
like sine waves)?

For the sake of discussion the source signal
is a ThunderBolt at 10 MHz.

The FPGA is rated to 350 MHz, so no need to have
a 5.0000000000000000 GHz Zero Crossing circuit. ;-)

The FPGA has several interface styles,
so we are not limited to just TTL or CMOS.

Suggestions?