Compact High-Selectivity Wide Stopband Microstrip Cross-Coupled Bandpass Filter With Spurline

Abstract

The article presents the design of a compact narrowband microstrip cross-coupled bandpass filter with improved selectivity and a wide stopband. The proposed fourth-order quasi-elliptic filter is designed at 2.5 GHz with a fractional bandwidth of 4% suitable for WLAN applications. At first, doublyfolded half-wavelength hairpin lines have been arranged symmetrically in a cross-coupled configuration combining the electric, magnetic, and mixed coupling. Accordingly, a size reduction of 17% over the folded inline hairpin-line filter with the same specifications has been achieved. Moreover, the selectivity has been improved greatly by the introduction of two deep transmission zeros with an attenuation level of 48 dB at the edges of the passband. However, the presence of the spurious harmonics with an attenuation level of 10 dB limits the performance of the filter related to the stopband rejection. As a remedy, conventional and meander spurlines have been incorporated in each hybrid coupled section of adjacent cross-coupled cells for achieving the modal phase velocity compensation. Accordingly, an extended stopband with a rejection level of 38 dB up to 4f 0 has been recorded by using a meander spurline and an overall size reduction of 33% has been achieved.INDEX TERMS Bandpass filter, high selectivity, harmonics suppression, spurline, wide stopband I. INTRODUCTION

A New Class of High-Selectivity Bandpass Filters With Constant Bandwidth and 5:1 Bandwidth Tuning Ratio

“…INTRODUCTIONCompact planar microwave passive bandpass filters (BPFs) with superior selectivity and high-performance characteristics such as passband flatness, low insertion loss, sharp roll-off skirt, and high out-of-band rejection are some of the main filter performance requirements for modern RF communication systems. A variety of design approaches and structures has been published in literature [1][2][3][4][5][6][7][8][9][10][11][12]. In particular, [1] reports the implementation of different pairs of coupled lines in a BPF with multiple transmission zeros (TZs) and transmission poles (TPs).…”

“…To achieve a large stopband interference at the cost of weak in-band selectivity, open stubs are linked to the input/output stages in [7], and in [8] the upper stopband rejection is improved by using shunt-shorted stubs. A new method to achieve a wide stopband with high selectivity using crosscoupled spur lines is proposed in [9] whereas in [10] high selectivity with controllable transmission zeros is achieved using substrate integrated waveguide (SIW) technology. Recently, in order to minimize the cross talk while preserving high stop band rejection and low insertion loss low-temperature co-fired ceramic (LTCC) based BPFs [11][12] were proposed using stepped impedance resonators (SIRs) and quasi-lumped and distributed elements respectively.…”