Practical Implementation of Frequency Monitoring for Widely Tunable Bandpass Filters

In the present work, a practical method to integrate sensing mechanisms into widely tunable evanescent-mode cavity resonators for tracking the center frequency is introduced. This mechanism allows for in-situ monitoring and outputs a signal that can be used to generate a closed loop feedback that can be used to lock in the center frequency of the resonator. The major benefit of this mechanism is that the performance of a resonator is not sacrificed since the higher order differential mode used for monitoring is orthogonal to the fundamental mode of the resonator. The resonator is created inside a standard printed circuit board using 3-dimensional laser patterning to allow the existence of the differential mode. An example resonator is fabricated to demonstrate the concept and tuned from 3.62 to 6.85 GHz. The differential mode was monitored to be at a frequency 1.8 times higher than the common mode. The unloaded quality factor of the resonator is extracted from measurements to verify that the sensing mechanism does not induce any additional losses. Continuous feedback is a crucial step towards a robust fielded widely tunable filter.