Design Of Several Wideband Balanced Bandpass Filter With High Common-Mode Suppression

As an important kind of frequency selective devices,filters often have a direct influence on the entire communication system.In addition,there are also many problems in present wireless systems,such as noises in environment and lack of frequency bandwidth.Therefore,filters that have a wide operating bandwidth and anti-interference performance become hot spots in recent years.The proposed balanced filter not only has the performances of better anti-noise and compact structure,but also improve the signal-to-noise ratio and reduce the insertion loss in the communication systems.Some new design techniques are studied to get microstrip balanced filters in this thesis.The author's main works and contributions are summarized as follows.1.A wideband balanced bandpass filter loaded with the coupled line is proposed.The filter is designed based on the transmission characteristics of the coupled line.A virtual short or open circuit characteristic is exhibited at the center line when a differential-mode or common-mode is excited,respectively.As a result,the differential-mode band-pass characteristic is obtained,and common-mode suppression is achieved on the whole band.Based on stub-loaded technique on the coupled line,six transmission zeroes are generated in both differential-mode and common-mode,which further improve the characteristics of out-of-band selectivity for differential-mode and common-mode suppression.In order to achieve the out-of-band selection characteristic for differential-mode,four transmission zeroes is adjusted by the impedance of the loaded stub.Finally,a prototype is fabricated and measured.The measured bandwidth for differential-mode is 44%.The proposed filter reaches a good common-mode suppression performance in the working frequency below2f₀.Specifically,more than 15.5 dB and 25 dB common-mode suppression ratio is obtained in the passband and stopband for differential-mode,respectively.The experimental results are consistent with the simulated ones,which meet the design requirements of the balanced filter.2.A wideband balanced bandpass filter is proposed based on the half-wavelength path difference structure.Unlike the balanced filter whose ports are symmetrical about the centerline,the differential ports arrange at the same side,which significantly reduce the physical size of the filter.Based on the half-wave path difference structure,signals for differential-mode or common-mode are in phases or out of phases to realize a differential-mode passband or a common-mode suppression,respectively.The out-of-band characteristics for differential-mode are improved by cascading two quarter-wavelength coupled lines.In order to further improve the differential-mode out-of-band and common-mode suppression characteristics,two transmission zeroes are generated in both differential-mode and common-mode loaded by a quarter-wavelength short-circuit line and a half-wavelength open line.Finally,a prototype is fabricated and measured.The measured bandwidth for the differential-mode is 37%.More than 38.5 dB and 30 dB common-mode suppression ratio is obtained in the working frequency of 0.8-3.2 GHz and 0.5-4 GHz,respectively.The consistency of the measured and simulated results verify the theoretical analysis.3.A wideband balanced bandpass filter is proposed based on the microstrip-slot conversion structure.A pair of microstrip line input structures are used to obtain a good common-mode suppression characteristic in the whole band.When common-mode signal is excited,the microstrip-slot transition represents an ideal magnetic wall with no energy coupling between the feed line and slot line.As a result,the transmission of common-mode signals is blocked.When the differential-mode signal is excited,the microstrip-slot transition represents an ideal electrical wall,which realizes the coupling of the microstrip to the slot line to obtain the characteristic of differential-mode signal transmission.The filter performs a wideband characteristic through third-order coupling slot line,suppresses the second harmonic through a feed point in the middle of slot line,and improves the band selection characteristic for the differential-mode by generating two transmission zeroes at higher frequency.A prototype is fabricated and measured.The measured bandwidth of the filter is 33%,and the return loss is better than 20 dB.More than 60 dB common-mode suppression is obtained in the frequency below 2f₀.The measured and simulated results are basically consistent.