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A compact double-balanced frequency doubler with simple harmonic suppression and gain compensation functions is proposed. In order to enhance the suppression of fundamental and third harmonics of the doubler, the band-reject filter (BRF) is integrated into the Marchand balun, which also provides the ground for the coupler line of the balun. In addition, the simple bias circuit is applied to solve the gain variation, which improves the conversion efficiency at low input power. This doubler exhibits 11.7-16 dB conversion loss, while the fundamental suppression is better than 22.5 dB from 25 to 37 GHz. By controlling the voltage from 0 to 0.6 V, the doubler can increase the gain and provide better operational bandwidth.