Using a VCO and a standard programmable PLL Synthesizer it is possible to generate different LO frequencys to downconvert every frequency within the 13cm band. The PLL can be programmed with a PC via the LPT1 port. Since there is a backup battery connected to the PLL the programmed bits are stored in the PLL even if the supply voltage is disconnected for a longer period. (Forget about replacing the battery, the PLL draws only 10uA in sleep mode).
Have a look a some photos first:
If you want to know how it works, have a look at the circuit diagrams.