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A passive 480-GHz frequency doubler based on accumulation-mode MOS varactor in CMOS process is reported. Using a compact partially-coupled ring structure, the doubler achieves a simultaneous broadband matching for fundamental and 2nd-harmonic signals. With optimized gate length and oxide thickness, the MOS varactor achieves a dynamic cutoff frequency of 870GHz while sustaining large voltage swing for high power generation. At 480-GHz output frequency, the doubler has a measured minimum conversion loss of 14.3dB and an unsaturated output power of 0.23mW. The simulated 3-dB output bandwidth is 70GHz (14.6%). The doubler is fabricated using 65-nm low power bulk CMOS technology and consumes zero DC power.