Studies On The Synthesizing Theory And Miniaturization Of Ultra-wideband Bandpass Filters

Under the background of rapid development of modern wireless communication, study on the ultra-wideband (UWB) systems has wide application prospects and huge market values. In this dissertation, some research works are implemented on UWB bandpass filter, which is one of the key passive components in UWB systems. After comprehensive investigations on the UWB bandpass filters currently available, we proposed five new types of high-performance UWB filters for the first time, and developed efficient and accurate synthesis theory and design methods for them. The designed filters are compact, low-loss, exhibit sharp attenuations and wide-stopbands, and are easy to fabricate under moderate fabrication precision. The measured characteristics of the filters agree well with the theoretical predictions, and satisfy well the Federal Communications Commission's indoor limit. The main contents of the dissertation include the following five parts:Firstly, by using z-transform technology and optimization algorithm, a novel synthesis method based on distributed parameters is developed to design the UWB filters consisting ofλ₀/4 (λ₀ is the wavelength at central frequency of the filter) short-circuited lines andλ₀/2 connected lines precisely. On the other hand, shunt two-section open-circuited stubs are employed to produce transmission zeros in the lower and upper stopbands of the filter in order to get miniaturized designs.Secondly, a wideband equivalent circuit model of the parallel-coupled lines is analyzed in detail, and the ideal Chebyshev transfer functions which can be realized by the dual-mode resonant parallel-coupled lines UWB filters are obtained. Moreover, a synthesized method to design this kind of UWB filters is proposed by using the z-transform optimization algorithm. The experimental results indicate that a three-order dual-mode resonant parallel-coupled lines UWB filter exhibits good performance within the passband and stopband of the filter. In addition, the insertion loss of the filter can meet the FCC indoor limit excellently.Thirdly, a complete analysis of the square ring resonator with twoλ₀/4 (or 3λ₀/4) open-circuited tuning stubs is accomplished by employing the even- and odd-mode analysis theory, and explicit accurate closed-form formulas are obtained which facilitate greatly the design of transmission zeros in the stopband of the square ring resonator. A design process is also proposed to design dual-mode UWB square ring filters with a fractional bandwidth of about 60% or 30%.Fourthly, several methods for widening the stopband of the dual-mode square ring UWB filters are proposed according to the characteristics of the frequency responses of the filters. Based on the methods, UWB filters with passband within 3.1-5.1GHz and 22-29GHz are designed, which have widened stopbands.Finally, a novel dual-mode UWB ring BPF is developed. The factors are analyzed in detail, which influence the bandwidth, return loss and insertion loss of the filter. In order to design a filter with good performance, a novel coupled structure is proposed and the process to design this kind of filter is also presented.