High frequency rejection using L shaped defected microstrip structure in ultra wideband bandpass filter

Abstract

The frequency band from 3.1 to 10.6 GHz with a bandwidth of 7.5 GHz is defined as Ultra wideband (UWB). Federal Communication Commission (FCC) of United States permitted the use of UWB for commercial applications in February 2002. UWB provide very high data rate of the order of 100 Mbps to 1 Gbps. UWB signals are capable of penetrating through obstacles, they are immune to multi path and their power requirement is low. This paper presents a simple UWB bandpass filter (BPF) based on stepped impedance resonator (SIR) in folded form. SIR consists of two unequal width microstrip lines cascaded together. It has the ability to operate as a multimode resonator (MMR). When SIR is folded and the folded lines are kept in close proximity, due to multipath two transmission zeros are introduced in the frequency response. The frequency range between the two transmission zeros determines the passband of the BPF. This simple BPF has the disadvantage of small values of lower and upper rejection band. A simple L-shaped defected microstrip structure (DMS) is added to improve the upper rejection band. DMS is realized by etching slots or gaps in the microstrip structure. It has similar properties as that of defected ground structure (DGS) but does not have any leakage through ground. A simple LDMS behaves like a band notch structure. Length and width are the two parameters which determine the spectral behavior of LDMS. For optimized length of 2.6 mm and width 1.1 mm, the LDMS clearly reject high frequency components above 13 GHz. An insertion loss of less than −10 dB is obtained up to 25 GHz. Substrate used for the filter is FR4. Filter simulation is carried out using ANSYS HFSS software.