Design of a Microstrip Reconfigurable Balanced Filter With Enhanced Common‐Mode Rejection and an Expanded Ultra‐Wide Stopband

In this brief, a reconfigurable balanced filter is designed with high common‐mode (CM) suppression and ultra‐wide stopband characteristics. This filter consists of two types of resonators, λ/2 resonator causes the common‐mode and differential‐mode (DM) passbands to stagger, significantly increasing CM suppression. The cross‐shaped triple‐mode resonator introduces a pair of transmission zeros (TZs), greatly improving the selectivity of the DM channel. By analyzing the electromagnetic field distribution on the resonators and selecting the optimal coupling position P between the resonators, not only can CM suppression be improved again, but also an ultra‐wide upper stopband can be obtained. Loading varactors on the fixed filter upgrades it to a reconfigurable filter, which can simultaneously adjust the center frequency ( f 0 ) and bandwidth (BM). The f 0 can be tuned from 1.1 GHz to 2.1 GHz (62.5%). The 3‐dB bandwidth can be adjusted from 100 MHz to 320 MHz, and the relative bandwidth tuning range is 4.8% to 15.2%. The measured results showed a stopband suppression range of more than 22 dB and a CM suppression range better than 18 dB over a frequency range of 20 GHz.