Methods and devices for real-time monitoring of tunable filters

An evanescent-mode RF filter, comprising:

• an RF filter resonator having a first membrane enclosing a first cavity;
• a monitoring resonator having a second membrane enclosing a second cavity, the monitoring resonator mounted opposing the filter resonator such that the first and second membranes are facing one another;
• a planar actuator mounted between the first and second membranes, wherein the planar actuator comprises a piezoelectric element, wherein the piezoelectric element is electrically isolated from the first membrane and the second membrane by an electrically insulating material;
• and
• a power supply configured to apply a voltage bias signal to the actuator, the voltage bias signal causing the actuator to increase or decrease the operating frequency of the filter resonator;
• a pulse injection circuit operatively connected to an input of the monitoring resonator, the pulse injection circuit configured to supply a pulse signal to the monitoring resonator;
• a readout circuit connected to an output of the monitoring resonator, the readout circuit configured to determine a number of pulses from the output having a voltage greater than a predetermined threshold in a predetermined time period, the number of pulses corresponding to a natural response frequency of the filter resonator in response to the pulse signal, wherein the readout circuit outputs a value in a binary digital format, the value corresponding to said number of pulses;
• and
• a feedback control circuit, the feedback control circuit having an input configured to receive output from the readout circuit and an output operatively connected to the power supply, the feedback control circuit configured to tune the filter to a desired operating frequency.