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A very low phase-noise planar X -band oscillator employing an active resonator has been demonstrated. The high frequency selectivity of the active filter, used as the resonator, is the key factor in phase-noise reduction. A design procedure to achieve the resonator's optimum performance for low phase-noise applications is presented. In particular, the effect of the excess noise introduced by the active filter is addressed and its impact on the oscillator's phase noise is minimized. The oscillator, operating at 8 GHz, shows a measured phase noise of -150 dBc/Hz at 1-MHz frequency offset with 10-dBm output power. To the best of our knowledge, this oscillator demonstrates the lowest phase noise among published X -band planar oscillators to date.