Design of Compact Planar Lowpass Filters by Using Fragment-Type Structure With Multi-Bit Scheme

A novel design method for compact planar lowpass filter (LPF) with prominent roll-off rate and wide stopband suppression is proposed by using fragment-type structures (FTS) with multi-bit scheme. The proposed multi-bit FTS design has the potential to achieve higher LPF performance at higher optimization efficiency, without any prerequisite knowledge about the pivotal LPF structures needed in conventional LPF designs. The definition of multi-bit FTS and the implementation of LPF design with multi-bit FTS scheme are presented in detail. Compared to the LPF design with 1-bit FTS, the 2-bit FTS design may use smaller decision space, take fewer iterations in multi-objective optimization searching, and yet achieve sharper roll-off rate, higher stopband suppression and a wider stopband. For demonstration, planar LPFs on PCB using the 1-bit FTS and 2-bit FTS are designed and tested. The 2-bit FTS design yields a sharp roll-off rate of 123.3 dB/GHz, a relative stopband bandwidth of 165% referred to stopband suppression of 20 dB, and a high figure-of-merit of 21060. Effects of the geometry parameters of the 2-bit elements are further discussed to give some guide rule on using the 2-bit FTS design.