Compact Balanced Dual-Band BPFs Based on Short and Open Stub Loaded Resonators with Wide Common-Mode Suppression

Two balanced microstrip dual-band bandpass filters (BPFs) based on short stub loaded resonator (SSLR) and open stub loaded resonator (OSLR) are presented in this brief, respectively. The center frequencies of the two differential-mode (DM) passbands can be controlled quasi-independently by adjusting the lengths of resonators. Moreover, a stepped impedance microstrip line etched with an interdigital coupling line is utilized to introduce a coupling between the two resonators, which generates two extra controlled transmission zeros (TZs) to obtain better selectivities. Furthermore, balanced microstrip/ slotline (MS) transition structures are employed as feed networks, which can suppress common-mode (CM) inherently while the DM responses are not affected, thereby the design procedure can be simplified significantly. In order to demonstrate the theoretical design, two balanced dual-band BPFs were designed and fabricated. The simulated responses are compared with the measured ones and a good agreement is obtained.