This paper presents a 143.4-151.5 GHz high DC-RF efficiency signal source. Using the proposed mutual inductance cancelling technique, 1) the fundamental signal generated by the V-band oscillator is fed to the varactors, 2) the second harmonic signal generated by the varactors is prevented from flowing back to the V-band oscillator, and is all fed to the output buffer instead. With post-simulated results using 65 nm CMOS technology, the signal source shows an output power of -0.65 dBm, a DC-RF efficiency of 4.0% and a tuning range of 5.5%. The signal source consumes 21.53 mW using a 1.2 V voltage supply. The whole die area including the pads is 0.075 mm2.