Need to measure frequency/period/frequency with high resolution to 6GHz?

 Up to 10 significant digits?

AIM-TTi precision handheld and bench-top frequency counters up to 6GHz. Universal counters with VLF capability and pulse measurement and totalizing in addition to frequency/period, USB control interface.

http://www.saelig.com/category/aim-tti-frequency-counters.htm