If you're not using the UHF options, then just leave out the UHF portions of the circuits
shown.

The code examples just assume you're using the UHF options, but there are sections
of code in the PKE transponder code example that show how to configure the AFE you
will want to look at.

The 16F639 is actually a 16F636 with an MCP2030 AFE built-in. So if you're only interested
in using the LF Talk-Back, you might want to start with the MCP2030 data sheet, and work
your way up from there. The AFE section of the 16F639 data sheet tells you how the LF
Talk-Back works. Look at 11.2 Modulation Circuit.

Once you understand how this works, you'll know exactly which parts of the circuits you'll
need.