On the J2 connector, there is :

* Frequency output : Where the divided input frequency goes.
 - Pin 2 on counter - Pin 6 on EXT_PCB

* CH3CODE0 & CH3CODE1 : Used to detect the type of prescaler 
 - Pin 9&10 on counter - Pin 5&10 on EXT_PCB
 - 1/1 on 12.4 GHz board - 0/0 on 3 GHz board (0 = GND / 1 = +5V)

* CH3_DET_EN : Presence detection. If grounded, CH3(2) is enabled.
 - Pin 4 on counter - Pin 7 on EXT_PCB

* 5V line  - Pin 7 on counter - Pin 4 on EXT_PCB

* 12V line - Pin 5&6 on counter - Pin 3&8 on EXT_PCB

* Ground - Pin 1&3 on counter - Pin 1&2 on EXT_PCB

The 3 GHz board is built with 4-stage of Avago MSA-0986 (7 dB gain each, rated 0.1 to 3 Ghz) and a fujitsu MB510-PF divider, rated up to 2.7 GHz. Pin 6 & 3 of the MB510-PF are high, so the divide ratio is set to 128.