Wideband bandpass filters with suppressed harmonics using vertically installed planar structures

Abstract

This article presents a closed-form synthesis method for wideband bandpass filters with suppressed harmonics, where stepped-impedance resonators (SIRs), scaled lines, and vertically installed planar (VIP) structures are introduced to improve the in-band and outof-band performances. By combining SIRs and scaled lines, the rejection level of the filter is greatly improved and a more compact size is achieved. And the VIP structures help to bring wide passband as well as suppressed harmonics. Then, a 3rd-order filter is designed to verify the validity of the proposed method and topology. The simulated and measured results demonstrate that the proposed design has a wide passband (FBW = 52.9%), low insertion loss (<0.4 dB), and suppressed harmonics with the level of 20 dB up to 7.1f 0 . K E Y W O R D S band-pass filters, filters | INTRODUCTIONBandpass filters (BPFs) are essential components in modern wireless communication systems. Over the past decades, BPFs with various in-band performances, such as wideband [1] and dual/multiple-band [2, 3] characteristics have been proposed and fully analysed. Meanwhile, the out-of-band characteristics of BPFs have equal importance in practical applications. As one of the classical methods, defected ground structure [4, 5] cells can easily achieve a wide stopband, while the signal integrity may be deteriorated for the incomplete ground plane. Then, hairpin structures [6], stepped-impedance resonators (SIRs), and coupled lines [7][8][9] were designed to expand the stopband. However, due to the circuit discontinuity, their design procedures became more complex.Recently, multiple novel techniques have been applied to improve the rejection level of BPFs, such as substrate-integrated defected ground structures [10,11], coaxial resonators [12], multilayer structures with slot lines [13,14], and transversal signal interactions [15]. Of these, multilayer structures require extra fabrication cost, which may be not preferred in industrial applications. Furthermore, the passband bandwidths of the most reported filters with suppressed harmonics are limited, and there are few reported works on wideband BPFs with Harmonic suppression. In ref. [16], a topology combining lumpedThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.