AWR Application Notes

Design of a BAW Quadplexer Module Using AWR Software 4 www.cadence.com/go/awr Filter Diplexers are often required for dual-band operation; therefore, the diplexer is a key component in the transceiver module. Since diplexers are intended for use in handheld devices and personal communication systems, they need to be as compact and planar as possible, and the common procedure is to combine two bandpass filters (BPFs) via an optimized T-junction. 3 It is important to design the BPFs for low insertion loss, the appropriate bandwidth, selectivity, and out-of-band rejection. In addition, the isolation of the overall system is an important parameter in order to avoid unnecessary loading at the front-end devices. BPFs present controllable transmission zeros, which is the frequency at which the transfer function of a linear two-port network has zero transmission. This ensures an acceptable stopband rejection is available to maximize isolation between crossbands (TX/RX frequencies). Figure 6 shows the structure of the proposed diplexer, which consists of two BPFs, a stepped-impedance resonator (SIR), and a matching circuit between filters combined at the antenna port with a T-junction/matching structure. The T-junction is typically used in a combining circuit and requires careful design of the width and length of the microstrip transmission lines, which was accomplished using AWR Microwave Office software. 4 Figure 6: Structure of the proposed diplexer The center frequencies of designed BPFs were 1.8GHz and 2.6GHz for the B3 and B7 LTE bands, respectively. There was a tradeoff between the order and dimensions of the filters. In order to constrain the size, a first-order Chebyshev filter was preferred due to the fewer number of reactive components, low ripple at the passband, and steep roll-off at the skirt of the transition bands. 4 In order to design the BPF, one transmission line and two stepped-impedance open stubs were used because they generated two transmission zeros near the passband frequency 5 and the bandwidth of the filters could be easily controlled by relocating those transmission zeros. The two stepped-impedance stubs provided low loss, good selec- tivity, and a steep transition band. Figure 7 shows the configuration of the stepped-impedance model, where each step equals a λ/4 electrical length at the center frequency of the filter and Z1 and Z2 are the impedance of each segment. Figure 7: Stepped-impedance open stub

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