Synthesis and Design of Miniaturized Wideband Bandpass Filters With Scaled Transmission Line for Spurious-Response Suppression

A scaled transmission line method has been proposed to realize a class of size miniaturized and spurious-response suppressed wideband bandpass filters (BPFs). The mechanism is based on a dual-band BPF with its lower passband assigned as the primary passband and far away from the upper one. Therefore, the primary passband is always at a relatively small electrical length frequency, which is a prerequisite to scale the transmission line network. Three types of line scaling/approximating are discussed as stub loaded transmission lines, short-, and open-circuited stubs. Comparing before and after scaling, these circuit components preserve an almost exact characteristic in the primary passband frequency range but introduce adjustable transmission nulls in the spurious passband. For suppressing different spurious responses, line scaling factors are varied for different circuit components to locate transmission nulls at desired places. Two circuit examples in the circuit topology as distributed and mixed circuit networks have been experimentally demonstrated, which has approved the miniaturization and spurious suppression benefits of the proposed technique.