An on-chip Ruthroff-type balun fabricated in tsmcTM 0.18 μm CMOS technology has been realised and used in a single balanced mixer. By using the phase compensation line and appropriately choosing the characteristic impedance of transmission lines, the balun was realised with a minimum insertion loss of 0.71 dB and the 1 dB bandwidth over a frequency range from 3.2 to 8.4 GHz. The compact size of 0.088 mm2 is achieved owing to the short length of the transmission line winding, approximately 1/18 λ g, and the adoption of broadsidecoupled and spiral-shape structures. A simple single balanced mixer with the Ruthroff-type balun was then designed to demonstrate the capability of the broadband and low-loss balun. A minimum conversion loss of 12.48 dB with a 3 dB bandwidth from 2.5 to 7 GHz was measured at the LO drive of only 6 dBm. The chip area, including pads, is 0.49 mm 2.