Design and Analysis of Bandpass Filter With Improved Out of Band Characteristics Using Coupled Lines

In this paper, the design and analysis of a bandpass filter with a steep transition and high rejection in the stopband are presented. The out-of-band characteristics are improved by introducing multiple transmission zeros (TZs). Eight transmission zeros have been introduced to achieve a highrejection stopband and compact size. The filter is implemented using three coupled microstrip lines. The structure is designed to operate at a center frequency of 2 GHz. The simulated return loss is greater than 30 dB within the passband from 1.85 to 2.14 GHz (with a 3-dB fractional bandwidth of 14.5%) and a center frequency of 1.99 GHz. Eight finite transmission zeros from 0 to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2 \mathrm{f}_{0}$</tex> are obtained at 0, 0.73, 1.36, 1.73, 2.27, 2.7, 3.27, and 4.01 GHz, respectively. Stopband rejection levels of over -33 dB have been achieved in the lower stopband (0-1.74 GHz) and the upper stopband (2.25-4 GHz). The structure was also simulated using an EM simulator with a substrate material of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\varepsilon r=2.65$</tex>, thickness <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{h}=\mathbf{1. 6 ~ m m}$</tex>, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{\operatorname { tan } \delta}=\mathbf{0. 0 0 9}$</tex>.