Research On Balanced Bandpass Filter Based On CBCPW And Microstrip Patch

With the rapid development of wireless communication technology,the balanced circuit has attracted more and more scholars' attention.The performance of the balanced filter directly affects the performance of the whole communication system.Compared with classical single-ended filters,balanced filters have the advantage of high degree of immunity to environmental noise and lower electromagnetic interference.At present,the proposed balanced filters are mostly microstrip line structure.This paper mainly studies the bandpass balanced filter based on CBCPW(Conductor Backed Coplanar Waveguide)and microstrip patch structure.The main contents and the innovation points are as follows.Firstly,a CBCPW balanced bandpass filter with multiple transmission zeros is proposed.The half-wavelength resonator has different resonance characteristics under the excitation of differential and common modes,so it can be used to realize balanced filter.The resonance conditions with differential-mode signals and common-mode signals are analyzed by the analysis method of odd-even model.Four transmission zeros are achieved by using separate electric and magnetic coupling(SEMC),source-load coupling and quarter-wavelength shorted stubs.Adjusting the physical parameters of the structure,we can control three transmission zeros in the design process.And the influence law is analyzed.Secondly,on the basis of the original CBCPW balance bandpass filter,by using the step-model half-wavelength resonator to replace bent-model half-wavelength resonator,a modified CBCPW balanced bandpass filter is proposed.And the resonance characteristics of these two resonators are compared.Under the condition that the performance of the balanced filter is not affected,the size of the device can be effectively reduced.So this structure is conducive to miniaturization design.Thirdly,an isosceles right-triangled patch balanced bandpass filters is proposed.For triangular patch resonators,different TM modes can be excited under DM or CM excitation,and the resonance frequencies are different.So a balanced filter is set up by using two isosceles right-triangled patch resonators.The distribution of current and electromagnetic field of different TM modes are studied.By etching rectangular slotline, the distribution of electric field or magnetic field of TM mode is changed,which can affect the resonant characteristic of TM mode.The common mode suppression of the balanced filter is optimized by adjusting the feed position of the wave port feeders.The influence of the rectangular slotline on the transmission zero produced by electromagnetic coupling is analyzed.Finally,by analyzing the high order modes of the triangle resonator,a balanced dual-band bandpass filter based on isosceles right-triangled patch resonator is designed.And matching with balanced single-band bandpass filter,the balanced dual-band filter is no increased in size.This method that using the high order modes of patch resonator to realize more passband is meaningful for the miniaturization of the filter design.