Direct Synthesis for High Selectivity Lowpass/Bandpass Co-Designed Filters with Independent Sub-Band Responses

This paper presents a direct synthesis approach for lowpass/bandpass co-designed filters, allowing independent control of sub-band responses. The synthesis is based on a group of characteristic polynomials that provide combined lowpass and bandpass responses in the non-normalized frequency domain. A novel transversal array network is sequentially constructed to match the responses, which can be converted into practical topologies through matrix transformations. Synthesis and experimental results using a ladder-type model are proposed to validate the approach. The proposed technique offers significant advantages in sub-band performance control and out-of-band selectivity compared to traditional methods.