Harmonic suppression in parallel coupled microstrip bandpass filter with embedded C‐shaped EBG cells

ABSTRACT: In this article, harmonic suppression of a parallel cou-pled microstrip bandpass filter (BPF) with C-shaped electromagneticbandgap (EBG) cells embedded on the middle layer is investigated. Theembedded C-shaped EBG cells patterned on the middle layer add agood stopband-rejection mode to the second harmonics of the parallelcoupled microstrip BPF with suppression of over 40 dB, without affectingthe center frequency and insertion loss of the original designed BPF. Theexperimented results matched the simulated results well. © 2006 WileyPeriodicals, Inc. Microwave Opt Technol Lett 48: 2244–2246, 2006;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.21918 Key words: electromagnetic bandgap (EBG); bandpass filter (BPF);harmonic; suppression 1. INTRODUCTION Because of their lightweight, low-cost, and easily integration,conventional parallel coupled microstrip bandpass filters (BPFs)are more preferred for use in modern microwave and wireless localarea network systems [1]. However, the existence of harmonicresponses at multiples of the filter center frequency is a well-known problem associated with the use of these filters, whichresults in poor harmonic signal suppression when the parallelcoupled BPFs are used as output components of oscillators andamplifiers. Suppression of harmonic responses is normally essen-tial for conventional microstrip filters utilizing half-wavelength orquarter-wavelength coupled lines. Recently, the electromagneticbandgap (EBG) structures and defected ground structures (DGSs)with periodic arrays of etched patterns on the ground plane havebeen investigated for microstrip circuits to provide the rejection insome frequency band [2–6]. However, most of mentioned EBG orDGS structures have holes in the substrate or etched patterns onthe ground plane, and therefore the disadvantages of the structuresare a package problem and realization of monolithic microwaveintegrated circuits (MMICs) [2–4].In this investigation, simple C-shaped ring EBG cells are pro-posed and applied to the harmonic suppression of the parallelcoupled BPF. The C-shaped EBG cells are embedded on themiddle layer to avoid the package problem and to achieve theout-of-band suppressions. The attenuation pole frequency of theproposed C-shaped cell can be easily determined by the ringlength, corresponding with the half guide-wave length. The exper-imental results of the fabricated filter measured by network ana-lyzer have good agreement with the simulation results.