Hamtronics 220 repeater
I've been working with these exciters as part of a 220 simulcast system I'm building. They lend themselves to making a nice building block as they use a 10.240MHz TXCO from the factory. It's quite easy to remove the uP and put in a PIC 16F88 to reprogram it for a divide by 2000 reference value vs 2048 to get 5 KHz.
Add a SMA on board and 2uF filter chip (it's not critical, .01 to 4uF have been used) replacing the TXCO and you can use a Thunderbolt GPSDO unit to lock the exciter frequency to under .00002 Hz at 220 MHz.
I'll be putting together some diagrams on this, but for now here's the code needed to program the 16f88. It's GPLv2 licensed. It should compile XC8, and will use the internal oscillator on the PIC.