The present disclosure provides systems and methods for wirelessly charging an implantable medical device. An external charging device includes a coil, signal generating circuitry to drive current through the coil at a charging frequency to induce current in a second coil in the implantable medical device, monitoring circuitry to generate an output signal to monitor charging operations, and a comb filter. The comb filter is configured to apply filtering to the output signal to remove noise from the output signal, wherein the filtering is applied based on the charging frequency. The external charging device is configured to process the filtered output signal to detect a circuit state of charging circuitry of the implantable medical device during charging operations, and the external charging device is configured to vary the charging frequency based, in part, on detection of the circuit state of the charging circuitry of the implantable medical device.