Research Of Transmission Zeros And Distributed Circuits For The LTCC Bandpass Filter

With the coming age of the wireless communication, people chase the communication handheld device that being simple, portable and stable. Simultaneously, in order to process the signal, it is more important for the filter which locates on the front-end of the communication device. In the traditional designing process of filter, PCB technology is of the great maturity, however, in the terms of volume, it is difficult to fulfill the feature of miniaturization for the filters. For the reason of miniaturization, a new technology which named Low Temperature Co-fired Ceramic (LTCC) comes into being.Based on both the EM simulation and its circuit schematic analysis, this paper is aimed at designing a bandpass filter whose frequency band locates on ISM band, miniaturization and relative good suppression of outband are two features that need to be realized.With regard to the lumped circuit, which comes from Chebyshev lowpass prototype and curcuit transformation that presents a worse outband suppression. Therefore, in order to increase the outband suppression, it can be fulfilled by introducing the transmission zeros. Based on the traditional researches of the transmission zeros, this paper proposes a method to introduce two transmission zeros on both side of the passband to increase the outband suppression, that is to summate the Y parameters in circuit network between two ports.For the designing of the LTCC bandpass filter multilayer structure, we transfer the aforementioned circuit with transmission zeroes into LTCC multilayer structure, then discover the relationship of coupling between metal plates and the transmission zeroes, eventually, adjust the multilayer structure of the filter.Finally, during the production of the LTCC multilayer structure, due to the error of processing, the misplacement between two metal plates takes place. This would lead to inconformity between practical productions and specifications. Therefore, in this paper, we take advantage of Schwarz-Christoffel transformation to analyze the electric charge distribution on the metal plates, then according to the electric charge distribution law to optimize the multilayer structure, thereby, minimize the effect which stems from the product process.