Research And Design Of The Microwave Balanced Bandpass Filter

Balanced circuit is widely used in modern communication system due to its higher immunity toenvironmental noise compared with conventional single-ended structure, i.e., unbalanced structure.Since differential active devices are been increasingly used in modern communication system, if somepassive devices such as antennas and filters are also the balanced structure, it will of greatconvenience to connect them with active devices so as to constitute full-balanced system which canprovide better performance in suppressing common-mode interference. The main task of thedissertation is the study and the design of full-balanced microstrip microwave filters.The main contribution and innovation of this dissertation are as follows.1. In order to analysis and measure differential-mode response, common-mode response, andmode conversion response of the balanced (i.e., differential) network, we firstly study mixed-modeS-parameters, then build relationship between conventional single-ended and mixed-modeS-parameters. We researched the influence of reference plane translation to mixed-mode S-parametersand come to the conclusion that reference plane translation doesn’t influence the magnitude ofmixed-mode S-parameters which is experimentally validated.2. Applying the design method of balanced the single-ended bandpass filter, we design a3rdorder balanced bandpass filter with fractional bandwidth of15%centered at2.45GHz based on λg/4short-circuited branch line structure. Aiming at solving the problem of parasitic passband indifferential-mode operation, we introduce the concept of the equivalent T-shaped lines to restrict theparasitic passband without influencing its main passband transmission characteristics. The proposedfilter is then fabricated and measured, great agreement between the simulated and measured resultsare achieved, verifying the validity of the design.3. Applying the design method mentioned above, we design a5th order ultra-widebandbalanced bandpass filter. Aiming at solving the problem of its large size, we propose fractals forminiaturization to achieve some50%size reduction. Since that the common-mode suppressionbandwidth is not wide enough, we apply the open-circuited stub attached to the stub symmetry planeto produce transmission zeros for common-mode response, also some responding coupling structureare added. The simulated and measured results are in good agreement with each other, verifying thevalidity of the design.4. During the design of the aforementioned ultra-wideband filter, we find that the frequencyselectivity in differential-mode response is not good enough and the common-mode suppressionbandwidth is not wide enough, therefore, we simultaneously extend the branch line and transmission line between branch lines to optimize its frequency selectivity and add open-circuited line at thesymmetry plane of the stubs to extend common-mode suppression bandwidth. In final, predictedresults are well validated by measuring the fabricated filter over a wide frequency range.