Compact closed‐loop resonator filters with wide spurious free band and extended common‐mode noise suppression

Abstract

A theoretical analysis is presented for miniaturised filters with an extended stopband and extended common-mode (CM) rejection, using loaded closed-loop resonators. A numerical method is presented to solve the equations extracted from the ideal transmission line model. Selection of critical resonator parameters to minimise filter area while simultaneously maximising the differential-mode (DM) stopband or the CM noise suppression is explored. The trade-off between size, stopband and CM rejection is presented and used to design three filters at 1 GHz, which are designed to verify the proposed concept. First, a differential second-order filter with an extended DM stopband ( dB) up to and extended CM noise suppression ( dB) up to 6.42 is realised to show the parameter trade-offs. Then optimal resonators are used to develop a compact singled-ended filter () with an extended stopband ( dB) up to and the smallest size. Lastly, a second order filter with an optimal extended CM rejection ( dB) up to is presented. Measured results are presented. All three filters stand out compared to other works by exhibiting the smallest footprint relative to the operating frequency.