Dual-Band Filter Based on Commensurate-Line Network

A tenth-order dual-band commensurate-line filter with five transmission zeros (TZs) is proposed in this paper. The TZs are generated by the parallel structure of T - and $\Pi$-shaped networks in the middle while the frequency selectivity and out-of-band rejection are further improved by the remain structures connected to the two ports. By using the Richard's transformation, the equivalent circuit is derived. Then, the S-parameters can be calculated and optimized. Broadside-coupled striplines are used to realize strong coupling. Finally, a prototype of dual-band commensurate-line filter is designed with the size of $112.5\times 34.1\times 3.37\ \text{mm}^{3}$. The first and second passbands are centered at 0.68 and 1.3 GHz with the fractional bandwidth of 36.8% and 19.2%, respectively. The insertion losses of the two passbands are 0.4 and 0.74 dB, and the in-band return losses are better than 20 and 16.8 dB, respectively.