Design of microstrip lowpass filter in combination with defected ground and defected microstrip structures

Abstract

An asymmetric defected ground structure (DGS) in combination with back to back connected two spiral Defected microstrip structure (DMS) cells is proposed here to design of a lowpass filter with elliptic roll-off. The unit cell of DGS unit consists of one rectangular slot and two square shaped slots connected by two thin slots underneath the microstrip line on the ground plane. The passband insertion loss has been improved to 0.2 dB using High-Low impedance line. Both IE3D simulated and measured S-parameter responses are found a very good agreement. Finally, a lowpass filter at 3 dB cutoff frequency at 2.5 GHz with a 20 dB rejection bandwidth of 40% and sharpness factor of 60 dB/GHz has been achieved.