A 180?GHz differential Colpitts VCO in 65?nm CMOS is presented. The choice and placement of the varactors are analyzed with the Y parameters looking into the oscillation loop. Based on this analysis, the varactors are placed close to the source terminal of the transistors to minimize their load effect on the oscillator. The VCO occupies 0.097?mm2?core die area and consumes 21.8?mW DC power from a 1.2?V power supply. The measured tuning range is from 174.9? to 178.9?GHz. The measured output power after calibrating out the measurement conversion loss is ?13.15?dBm and the measured phase noise at 10?MHz offset is ?94?dBc/Hz.