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In the present work, a method for tracking the center frequency of a widely tunable evanescent-mode cavity filter in-situ is introduced. The goal is to be able to monitor the performance of a filter without disturbing the fields or degrading the quality. The proposed method is to monitor the resonant frequency of each resonator of the filter independently by inducing higher order differential modes. This method enables a continuous feedback loop to lock in the filter center frequency and shape. An example filter is fabricated to demonstrate the concept and tuned from 1.4 to 3 GHz while monitoring the differential mode at 4 to 6.5 GHz. The ability to independently monitor and control the individual resonators in-situ without disturbing the main mode is demonstrated and is a crucial step towards a robust fielded widely tunable filter.