Wideband differential-mode bandpass filters for UWB wireless communication

The objective of this report is to study and simulate a class of differential–mode ultra– wideband (UWB) bandpass filters on microstrip line with good common–mode suppression. Differential–mode filter on microstrip line is selected in this FYP topic 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 is 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. The microstrip-line filter is fabricated on the RT/Duroid 6010 with a substrate thickness of 0.635 mm and dielectric constant of 10.8. After its initial dimensions are determined via simple transmission-line model, its final layout is then optimally designed using the fullwave ADS simulator. A wide differential-mode passband is achieved in the frequency range of 2.7-5.3 GHz with a fractional bandwidth of about 65 % centre at 4.0 GHz. Over this frequency range, the common-mode attenuation with three transmission zeros is higher than 20.0 dB in simulation.Bachelor of Engineerin