Design And Implementation Of High Performance Microstrip Bandpass Filter

As the rapid development of wireless communication technology,there is an increasing demand for wireless communication in human daily life.Filter,an indispensable part of communication systems,is one of the most important components in wireless communication systems.The quality of communication systems is closely related to the performance of filters.For making full use of the limited spectrum resources,the trend of filters is towards multi-function and high performance.There are higher and higher requirements for portability in all kinds of electronic communication equipments.And the trend of filters is towards minimization.The microstrip filters are widely studied for their low cost,simple design,easy integration and easy processing.The parallel coupled line filter,which is also the basis of other microstrip structure filters,is a filter with simple structure.So the study of the parallel coupled line filters is very important.For improving the performance of the filters,this paper designs four filters based on the transmission line theory and filter design theory,they are a seventh-order parallel coupled line bandpass filter;a piezoelectric transducer electrically tunable band-pass filter;a high selective microstrip bandpass filter;a high performance microstrip bandpass filter for suppressing first harmonic.1.In order to solve the problems of time-wasting,low efficiency and poor reflection during the process of the design and optimization of filters,a method based on momentum was proposed to optimize the filters.An asymmetric structure is adopted to improve filter's reflection performance.This method means that the model is built in layout,then simulated and optimized in advanced design system(ADS),finally,verified and corrected only in high frequency structure simulator(HFSS).The results indicated that the method greatly improves the design efficiency.A seventh-order parallel coupled microstrip line bandpass filter with asymmetric structure was designed and optimized.2.In order to solve the problem of high insertion loss of varactor tuned filters,a piezoelectric transducer(PET)electrically tuned seventh-order parallel-coupled line filter is designed with a piezoelectric transducer used as a tuning element and a dielectric substrate used as a perturber.The variations of coupling coefficient and external quality factor against the gap between the perturber and the microstrip lines were analyzed.A piece of alumina ceramic substrate,a PET and the microstrip filter were assembled together to construct a PET electrically tuned filter,and the S parameters of the proposed filter are measured with a network analyzer under various dc voltages.Measurement results show that the center frequency is tuned from 4.38 GHz to 4.78 GHz,the reflection loss in the tuning range is more than 10 d B,and the insertion loss is less than 1.5 d B,which meets the design requirements.3.In order to reduce the size of the parallel-coupled microstrip filters,a miniaturized microstrip bandpass filter with high-selectivity was designed.An energetic method is used to analyze the coupling coefficient of a pair of parallel-coupled microstrip lines.The results show that controlling the electrical length of the coupling region can not only reduce the size of the filter,but also produce transmission zeros in the stopband4.In order to suppress the first harmonic of the filter,a cross capacitive coupling is introduced between resonator 1and 3,a stub is loaded in input and output.The effects of the stubs' length on the 1st and 2nd harmonics of the filter are analyzed.The simulation results show that the stub can effectively suppress the first harmonic and hardly influence the passband.The proposed filter was fabricated on a substrate with a dielectric constant of 2.7.The measurements agree well with the simulations,which verifies the rationality of the proposed method.