An isolating circuit for an electrosurgical resection system comprises a combining circuit (308) having a first input connectable to an RF channel (306), a second input connectable to a microwave channel (304), and an output in communication with the first and second inputs for conveying RF and microwave energy to a common signal pathway. A waveguide isolator (312) is connected to isolate the microwave energy from the RF energy. The waveguide isolator comprises a conductive input section that mates with a conductive output section to define a waveguide cavity within a volume enclosed by the input and output sections. A DC isolation barrier is arranged between the input and output sections. The combining circuit output includes a signal conductor and a ground conductor. A capacitive structure is provided between the ground conductor and the conductive input section to inhibit coupling of the RF energy and leakage of the microwave energy.