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In this paper, a low phase noise planar oscillator employing an elliptic bandpass filter as a frequency stabilization element within its feedback loop is presented. The oscillator phase noise is significantly reduced by taking advantage of the group delay peaks formed at the passband edges of an elliptic filter. An X-band oscillator using a SiGe HBT packaged transistor has been designed and tested. At the oscillation frequency of 9 GHz, the measured phase noise is -140 dBc/Hz at 1 MHz offset frequency. The oscillator exhibits an output power of 3.5 dBm with a figure of merit of -204.4 dBc/Hz.