Research And Design Of Compact DBR Filter With High Spurious Responses Suppression

Planar filter with high selectivity and excellent spurious responses suppression isan essential component of microwave and wireless communication systems, which hasbeen widely used in both transmitters and receivers. To satisfy the stringent systemrequirements, filters are often needed to have some capabilities of low insertion losses,high miniaturization, high isolation, low cost and independent control of the centralfrequency and transmission zeros (TZs). The Dual Behavior Resonator (DBR) filterinspired a great deal of interest owing to its independent control of the centralfrequency and transmission zeros, low insertion loss, narrow band and simplicity infabrication. However, its main drawbacks are also evident, not compact size andspurious responses on both sides of the central frequency. Thus, the research anddesign on the compactness and spurious response suppression have importantacademic value and application prospect.Firstly, the paper presents a new design structure for compact (72%) DualBehavior Resonator (DBR) filter using Π-Network and dual-line equivalent circuits.The proposed Π-Network replaces the J-inverter in DBR filter, which consists of ahigh impedance transmission line and parallel capacitances loaded on both sides. Basedon half angle formula, the dual-line structure is proposed creatively to replaceopen-circuited transmission line segment. The whole structures have the advantages ofbetter spurious responses suppression and easy fabrication. Then, the equivalenttopology capacitive inverter is utilized to achieve further compact (83%) DBR filter,with high spurious responses suppression at low frequencies (LF).In addition, the paper reaches a conclusion that lower total resonator reactancecorresponding to higher suppression through the analysis of the relationship betweentotal resonator reactance and spurious response suppression. A lower total reactance isderived by replacing LF (low frequency) stubs with short-circuited stubs. With thecombination of capacitive inverter and Π-Network, respectively, the former realizes60dB spurious responses suppression at LF band, with88%compactness, the latter, forthe first time, realizes high spurious responses suppression (40dB suppression at lowfrequencies(0~0.6f0)，40dB suppression up to4f0) on both sides of the centralfrequency, with91%compactness. Moreover, the new structure always has highsuppression better than40dB up to at different central frequency. Finally, acompact DBR diplexer with high output isolation is presented in this paper based on the compact DBR filter mentioned above. It is summarized that this presented diplexeris better than the referenced diplexers in terms of the size (0.23×0.77λ20), stopband suppression (60dB), insertion losses (0.9dB/1.7dB) and isolation(62dB).