This design can be used to build a highly accurate frequency counter. It requires a 10Mhz rubidium oscillator (FE-5680A) which you can pickup on ebay second hand for $50-200. These oscillators typically have a stability of 5x10^-12 (5x10^-11 over temperature) and a drift of 2x10^-10 per year (datasheet). Pretty accurate!
This design uses an Atmel XMEGAD4 to do most of the heavy lifting such as
The design files (included in the zip) contain most of the parts needed to assemble the design however it would be wise to check over the parts as the design was done some time ago.
The firmware was written in Atmel Studio in C. It can be compiled with and uploaded to the MCU through Atmel Studio 6.2.
Anyway, I hope this design is useful to some people out there. If you have any problems feel free to drop me a line at email@example.com.