Research On Key Technology Of Microstrip Filter

With continuous development and progress of science and technology,wireless communication has been widely applied to people's daily life.Especially short distance wireless data services,represented by WiFi,WLAN,Bluetooth and UWB et al,are developing rapidly,greatly promoting flourishing progress of microwave/RF filter technoloy,and also raised higher requirement for filter performances.In recent years,techniques such as tunable bandwidth,high selectivity and notched band and so on have gradually become research focus from researchers all countries of the world.To study key technology points for performances of microstrip filters,the topic of this paper has important theoretical significance and broad application prospects.With tunable bandwidth,high selectivity and notched band as the themes,also giving consideration to dual-band impedance matching converter,this paper designs several high performance microstrip broadband filters working in the wireless communication band.The following innovative research results have been completed.1.To solve the inconvenient bandwidth regulation of the traditional broadband bandpass filter,this paper puts forward a scheme of tunable bandwidth microstrip filter with two different types of resonators coupled by inverted ?-type resonator.First of all,the resonant characteristics and transmmission zeros of the inverted ?-type resonator are analyzed in depth.The traditional filter is simulated by HFSS software,and the existed problems of insertion loss,bandwidth regulation and passband selectivity are pointed out.Then S-parameter simulation test of four related variables is done for the filter proposed.The results show that the new filter can effectively adjust the upper sideband of the passband,while keeping the lower sideband fixed,and the upper sideband selectivity of the passband,due to a new produced transmission zero,has been significantly improved.Experimental and simulation data show that compared with the traditional filter,the proposed filter structure has obvious performance improvement in terms of bandwidth adjustment and high selectivity.2.In order to realize better bandwidth adjustment ability,restrain superfluous frequency interference and improve selectivity,a microstrip filter based on a three mode resonator structure is proposed.First of all,resonant frequency characteristics of three mode resonator,formed by two back-to-back open loop resonators and connnected by two microstrip lines,are analyzed in detail,and three resonant modes are deduced.Then only bottom connecting line of three mode resonator is retained and tuned by position,frequency performances with adjustable upper sideband of passband and fixed lower one are easily realized.Finally,adding top connecting line in three mode resonator and tuning its location can suppress superfluous frequency interference without affecting already designed passband by bottom connecting line,and two transmission zeros can be generated on right side of passband under condition of relatively narrower bandwidth.Experimental and simulation results show that compared with traditional filters without connecting lines,proposed filter has obvious performance advantages in wide range bandwidth regulation,design flexibility and interference band suppression.3.Aiming at the problem that two transmission zeros of traditional broadband filters are far away from the upper and lower sides of the passband and poor passband selectivity is caused,a filter scheme with loaded microstrip line with two open branch lines is proposed.By separating two branches,the influences of two branch lines on the stopband zone near upper and lower sides of passband is independently analyzed.In addition,in order to reduce the size of the filter,the long open branch line in two branch lines can be replaced by a short terminal grounding branch line.Combining two sub branches,compared with the spectrum curve of the traditional filter,the results show that the new filter proposed can significantly improve the selectivity performance of both sides of the passband under the premise of keeping the traditional filter passband unchanged.4.To obtain tunability of notched band position,high selectivity and adjustable bandwidth,a filter scheme based on a five mode resonator is proposed.First,the S parameter simulation curve under weak coupling condition is used to analyze five resonant modes of proposed resonator.The results show that four modes of the five mode resonator can be independently tuned,and three resonant modes own respective accompanying transmission zeroe.The approximate expressions of five resonant modes are provided,and insertion loss simulation curves under four key parameters of the filter are given.The simulation and test results show that,compared with the general bandpass filter with notched band,The filter presented in this paper has obvious performance advantages from four aspects:tunable notched band location,adjustable upper sideband of passband,high selectivity and independent regulation ability of notched band and upper sideband of passband.5.In dual-band impedance matching converter,independent cascade design structure is with large size of loading line and complexity of calculation.To solve this problem,a new impedance matching converter scheme with relatively simple structure and calculation is presented.First,the structure of parallel open and short-circuited loaded microstrip lines with total length of 1/4 guided wavelength is systematically analyzed,and the order of two working bands in impedance matching is determined.Then dual-band impedance matching diagram is provided with step-by-step instructions,and relevant calculation formula are given.The simulation and test results show that compared with the traditional modified T-type loaded microstrip line,The proposed parallel open-and short-circuited line loading structure owns obvious advantages of small size and simple calculation.Finally,the research work of the full text is summarized,and the key technology and performance of the microstrip filter are prospected.