Single-ended and differential-mode bandpass filters for UWB applications

Abstract

The objective of this PhD thesis is to design and implement a class of differential–mode ultra–wideband (UWB) bandpass filters on microstrip line with good common–mode suppression. As far as I know, this is the first reported work that exhibits such a wide differential–mode passband where good common–mode rejection is well achieved. Differential–mode filter on microstrip line is selected in this research because the microstrip line is the most popular planar transmission line and it can be easily integrated to a lot of existing balanced devices. The proposed four–port balanced filter is ideally symmetric with respect to the central plane in horizon. Therefore, this horizontal symmetrical plane can be considered as a perfectly electric or magnetic wall if one of the paired ports are driven by differential– or common–mode signals respectively. By adding a pair of open–ended stubs along this symmetrical plane, the introduced stubs are electrically short–circuited in differential–mode and thus giving no influence on the differential–mode frequency response. However, under the common–mode excitation, these introduced stubs actually lead to extension and reconstruction of the vertical arms, thereby providing us with an expected capacity on re–shaping the common–mode frequency response. Next, instead of introducing an open–ended stub along the symmetrical plane as discussed earlier, a connecting line is used to join the shunt stubs together. With this connecting line, three transmission zeros are produced in the common–mode frequency response while bandpass frequency response in the status of differential–mode remains intact.