Impedance control for critically coupled cavities

Cavity resonators are often used in low phase noise oscillators that utilize carrier suppression. These oscillators are employed in systems that measure phase noise in frequencies as high as 100 GHz. In order to achieve the maximum suppression needed in these low noise designs, the cavity must be operated at critical coupling. However, this large coupling level loads the cavity, which significantly lowers its quality factor and shifts its resonant frequency. In this paper, we present a matching technique that can be used to minimize these loading effects by controlling the input impedance of the cavity and optimizing its response at resonance, thus producing the maximum amount of carrier suppression.