A low-power frequency tripler is designed by using the sub-harmonic mixer configuration for K-band applications. The proposed circuit features quadrature signal generation, applicable to LO signal synthesis in millimeter-wave wireless transceivers. It achieves conversion gain of -5.7 dB at the output frequency of 21 GHz. Implemented in a 0.18 μm CMOS technology, the circuit consumes power of 7.5 mW with 1.5 V supply voltage. The entire die occupies an area of 1000 × 1050 μm2.