Design Of Compact Microwave Balanced Bandpass Filters

High integration and complex environment are the main features of modern microwave communication systems. Therefore, a high-level of electromagnetic interaction between circuit nodes and crosstalk from substrate coupling between components, as well as the interference from space will have an effect on the performance of these communication systems. As a result, balanced circuits have been more and more important in modern microwave communication systems. As to traditional single-ended communication systems, the noises are only suppressed by filters. However, single-ended filters can only suppress the out-of-band noises and are disabled for the in-band noises. Compared with single-ended filters, balanced filters can suppress not only the out-of-band noises but also the in-band noises by rejecting the common mode signals. In this way, balanced circuits will greatly improve the sensitivity of modern microwave communication systems.In this thesis, the importance of balanced bandpass filters and domestic and overseas research status of balanced bandpass filters are illustrated first. Then mixed-mode S-parameters for balanced circuits and filter design method are analyzed.In order to solve the problem that the balanced filters are usually bulky sizes, some compact balanced bandpass filters are researched and designed and additional loaded elements are not employed for common mode suppression. Firstly, single-ended microstrip-to-slotline transition is introduced and further study of single-ended microstrip-to-slotline transition is carried out. Then multilayer balanced microstrip-to- slotline transition(MBMST) is proposed and a compact wideband balanced bandpass filter is designed by utilizing the MBMST. The proposed wideband balanced filter is with compact size, low insertion loss and wideband common mode suppression. Secondly, based on the wideband filter, an independently controllable dual-band balanced bandpass filter is presented. In addition, the size of the dual-band keeps almost the same with that of the wideband balanced filter. Finally, to reduce the filter's size further, a compact dual-mode balanced bandpass filter has been designed by combining dual-mode technology and stepped impedance resonators. As the differential mode equivalent circuit performs as a bandpass filter while common mode equivalent circuit operates as a band-stop filter. The proposed dual-mode balanced bandpass filter shows good differential mode performance and common mode suppression. To conclude, the proposed compact balanced bandpass filters in this thesis are applicable in modern microwave communication systems.