Ultra Sharp Transition-Band LPF with Miniaturized Size for GSM Applications

Abstract

In this paper, a microstrip low-pass filter (LPF) using a bended transmission line is presented. The LPF has a simple topology, which is composed of a T-shaped stub as main resonator, one conventional rectangular resonator (as suppressing stub) and two T-shaped stubs. To approach a high operation and small dimensions, this structure is bent. The proposed LPF has a compact size (0.090 lambda_g × 0.119 lambdag) and presents an ultra sharp transition-band (ksi = 880.95 dB/GHz) from 0.945 to 0.987 GHz with –3 and –40 dB suppression levels, respectively. This filter, with 3- dB cut-off frequency (fc) of 0.945 GHz, is useful for GSM applications. The performance of each section is investigated based on LC equivalent circuit. The designed filter is fabricated and measured. Also, the results exhibit an excellent figure of merit (FOM) of 239756.