(a) Schematic of the FSF using two BLCs and two HPFs. The green lines (from port 1 to port 4) show the transmission of signals that can pass the HPFs. The red line (from port 1 to port 2) shows the transmission of signals that cannot pass the HPFs. (b) Schematic of LO leakage transmission on the FSF. The red lines show the transmission of the LO leakage passed by the HPFs from the LSB mixer’s LO coupler. The green line shows the transmission of LO leakage that cannot pass the HPFs from the USB mixer’s LO coupler. In both cases, the leakages reaching port 3 would be terminated by the absorption load. (Color online)