Wideband differential-mode bandpass filters for UWB wireless communication

Abstract

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.