Miniaturization of Broadband Stub Bandpass Filters Using Bridged-T Coils

In this paper, broadband stub bandpass filters (BPFs) with very compact circuit sizes are proposed. Specifically, the circuit size of the conventional broadband stub BPF is largely reduced by replacing all transmission line sections with bridged-T coils. In addition, the design consideration regarding the use of cascaded bridged-T coils is investigated in detail. The proposed broadband BPF designs are based on lumped elements only, and they can be implemented using the integrated passive device process to achieve very compact circuit sizes. As an example, a miniature 7th-order broadband BPF with a passband from 4 to 12 GHz, which is to be used at the mixer's IF output of a radio astronomical receiver, is presented. The in-band insertion loss is within 2.46 +/- 0.46 dB and the circuit size is only 2.30 mm x 2.03 mm. The long-lasting problem of large circuit size for conventional stub BPFs is thus solved.