The Research And Implementation Of Multiband Bandpass Filter

In recent years,a variety of wireless communication technologies have emerged,such as GPRS,WCDMA,TD-SCDMA,LTE-Advanced,WIFI,and Wi MAX.Currently,it is the trend in technology development to integrate more applications,have stronger performance,smaller size,and cost savings in the same RF communication system.In a multi-frequency communication system that integrates various wireless communication technologies,the multiband bandpass filter is one of the key components,responsible for frequency selection and noise suppression,and its performance has a direct impact on the system performance.Therefore,in-depth research on the design principles of multiband bandpass filter and the development of new filter topology structures have important academic research and engineering application value.This dissertation focuses on multiband bandpass filter,integrating network synthesis theory into physical implementation of filters,proposing new methods for multiband bandpass filter polynomials synthesis and coupling matrices synthesis.This method is then applied to the actual circuit design of specific multiband bandpass filter,revealing the physical mechanism and developing a fast design method.The work of this dissertation is as follows.1.A polynomial synthesis method of multiband bandpass filter,named Rough Prototype and Fine Tuning(RP-FT)method,is proposed in this dissertation.Existing methods have difficulty in obtaining corresponding polynomials of multiband bandpass filter with complex response.This dissertation constructs a characteristic function for each band,and multiplys all the characteristic functions to form an overall initial characteristic function,i.e.,the rough prototype.Furthermore,the initial characteristic function is optimized and fine-tuned,resulting in a multi-band filtering polynomial that meets the technical specifications.The advantages of the RP-FT method lie in its ability to flexibly set the number of bands,the position,bandwidth,the order,the return loss and the specified transmission zero point of each pass band,thus meeting the requirements of multiband bandpass filter with complex response.Additionally,the Direct Synthesis Technique and Optimization(DST-O)method is proposed,which directly derives the polynomial of the multiband bandpass filter in the actual multi-band frequency domain,avoiding frequency transformation and can be directly applied to specific scenarios such as the design of digital infinite impulse response filters.2.A method for synthesizing L-branch folded coupling matrix was proposed to derive a coupling matrix that is easy to be implemented by practical circuits.The order of the coupling matrix of multiband bandpass filter is generally high.If the design carried out is not reasonable,a large amount of coupling will accumulate at the same node(i.e.,source node,load node or resonator node),leading to difficulties in physical implementation.In this dissertation,the transverse coupling matrix of the multi-band filter was grouped and eliminated to obtain the L-branch folded coupling matrix.Meanwhile,based on the fact that different branche in the L-branch folded coupling matrix mainly act on different passband,this dissertation presents a method for finding different coefficient relative bandwidths for different branches,and uses the coefficient relative bandwidths to determine the circuit design parameters(i.e.resonant frequency,coupling coefficient,external quality factor)of each branch respectively.The obtained circuit design parameters are frequency-invariant and easy to implement.3.Three new topological structures of multiband bandpass microstrip filters have been proposed.The first structure is a quad-band bandpass filter,which is composed of two sets of step impedance resonators coupled with the source and load.The fundamental and second harmonic of each set of resonators are used to form two passbands,thereby forming a quad-band filter response.The four bands can be adjusted and have four specified transmission zeros to enhance frequency selectivity.The second topological structure introduces stub-loaded resonators and defect ground structures,further improving the performance of the quad-band bandpass filter response.The four bands can be adjusted over a wide frequency range while improving harmonic suppression within the stopband.The third topological structure,a tri-band bandpass filter with four specified transmission zeros,is based on a mesh type multimode resonator structure.By integrating physical mechanism analysis of the resonator and optimization algorithm,the initial dimentions of the filter structure can be quickly determined,thereby accelerating the design process.