An X-Band Low Phase Noise Free-Running Oscillator Using Substrate Integrated Waveguide Dual-Mode Bandpass Filter With Circular Cavity

In this letter, a low phase noise free-running oscillator employing a substrate integrated waveguide (SIW) dual-mode bandpass filter (BPF) is presented. As a frequency stabilization element in the feedback loop, the dual-mode SIW BPF is designed at the oscillation frequency. Due to the transmission zero near the upper passband of the filter, a large group delay peak at the upper passband edge can be achieved to improve the phase noise performance significantly. An X-band oscillator using a SiGe HBT has been designed and experimentally demonstrated at 11.571 GHz. Measured results show that, the phase noise of this proposed oscillator is -135.5 dBc/Hz at 1 MHz offset frequency with a figure of merit (FOM) of -206.2 dBc/Hz.