V1.6.1 has the ability to be configured for oscillators other than the FE-5680A. In particular, you can configure the oscillator output to be either on pin 7 of the DB9 or on a U.FL footprint adjacent to the DB9 connector. You can also configure pins 8 and 9 of the DB9 for TTL or RS-232 level signaling.

To switch to the external oscillator input, look for the two-way solder jumper labeled "DB9" and "U.FL". Unsolder the shorting blob from the DB9 side and replace it with one on the U.FL side. Make sure that you don't short all three pads together when you do this.

To switch from RS-232 levels to TTL, find the three solder jumpers between the divide-by-ten and MAX-232 chips. The two two-way jumpers need to be switched from the 232 side to the TTL side. That will change the signal routing to go directly to the DB9, bypassing the MAX-232. The other jumper must be opened. That will disconnect the MAX-232's input side from the DB9 pin (this must be done because the MAX-232 has a 5kΩ pull-down on its RS-232 inputs).

Unless you make firmware changes, your oscillator must be configured for 9600 bps 8N1 and must respond to the FE-5680A tuning commands.