Electrically Tunable Capacitor for EMC Band-Stop Filters

Band-Stop filters can be used to replace bulky low-pass filters in specific applications, particularly with frequencies up to 150 kHz. The use of a tunable capacitor could greatly improve the performance of such band-stop filters by tuning its resonant frequency. Tunable capacitors are widely utilized in radio-communication systems but are therefore designed specifically for high-frequency and low-power applications. However, the use of these specific components for EMC applications below 150 kHz is not possible since it focuses on low-frequency and high-power systems. Multilayer ceramic capacitors could be well suited for such applications because they can provide high capacitance values, high voltage ratings, and exhibit significant reduction in capacitance when a DC bias voltage is applied. This generally unwanted capacitor property will be explored in this study for its potency to influence the resonance frequency of a tunable band-stop filter. The capacitors showed a reduction in capacitance of up to 70% using a bias voltage ranging from 0 V to 40 V. This reduction in capacitance allowed the band-stop filter to achieve a resonance frequency ranging from 87 kHz to 134 kHz with insertion loss of 17 dB to 23 dB.