Here's the schematic.  It uses a CP2102N for USB connectivity, and ignores the serial data lines, instead just using DTR and RTS to control the switch.

A Mini-Circuits LRMS-2MH mixer (5 - 1000 MHz) is used to selectively invert the phase of the input signal.  When DTR is high and RTS low, the output has one phase, and with DTR low and RTS high, the output is shifted 180-degrees (pi!).  In a more typical use, the IF port is driven with a single-ended input matched to 50-Ohms.  In this case, I'm driving it balanced with 24.9 Ohms in each leg.  Same thing.  The LRMS-2MH is nice in this respect, since both sides of the IF are brought out to pins.  In many mixers, one side of the IF is grounded internally.