IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 12, NO. 11, NOVEMBER 2002 435 New Periodic-Loaded Electromagnetic Bandgap Coplanar Waveguide With Complete Spurious Passband Suppression Ferran Martín, Francisco Falcone, Jordi Bonache, Txema Lopetegi, Student Member, IEEE, Miguel A. G. Laso, Student Member, IEEE, and Mario Sorolla, Senior Member, IEEE Abstract—In this letter, coplanar waveguides (CPW) periodi- cally loaded with shunt capacitances and periodically perturbed by varying the distance between the central strip (of constant width) and ground planes are studied. It is demonstrated that the mul- tiple spurious passbands above the Bragg frequency, inherent to the presence of the reactive elements, can be completely and effi- ciently rejected by means of very simple geometry perturbation. This result is in contrast to previous works, where the rejection of multiple frequencies requires complex layout patterns and reveals that the elimination of spurious frequency bands in periodic loaded CPWs can not be merely estimated from the Fourier transform of the perturbation geometry. Index Terms—Coplanar waveguide (CPW) technology, electro- magnetic band gap (EBG), microwave filters. I. INTRODUCTION C OPLANAR waveguide (CPW) technology has recently at- tracted much attention for the fabrication of transmission lines periodically loaded with reactive elements, the main ad- vantage being the single metallization level required for signal and ground, which eases device grounding and limits line to line coupling. Microwave and millimeter wave low-pass filters, phase shifters [1], and frequency multipliers [2] are some of the applications of this technology that can be achieved by loading CPWs with shunt capacitances. Key to these applications is the presence of the reactive elements, which introduce periodicity and dispersion. The consequence is the presence of passband- stopbands in the frequency response and thus the filtering prop- erties of the structure. If the shunt capacitances are replaced by varactor diodes, propagation velocity can be tailored by means of an external bias and operation as a phase shifter results. Fi- nally, under large signal conditions, nonlinearity and disper- sion combined give rise to soliton-like propagation [3], and har- monics of the fundamental frequency are produced. The relevant parameter for the design of these devices is the Bragg frequency (which delimits the first frequency band). It gives the cutoff of the structure operating as a low-pass filter and determines the Manuscript received April 15, 2002; revised June 10, 2002. This work was supported by DGI and CICYT by Project Contracts BFM2001-2001 and TIC1999-0292. The review of this letter was arranged by Associate Editor Dr. Ruediger Vahldieck. F. Martín and J. Bonache are with the Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, Barcelona, Spain. F. Falcone, T. Lopetegi, M. A. G. Laso, and M. Sorolla are with the Electrical Engineering Department, Public University of Navarre, Campus Arrosadía, Pamplona, Spain. Digital Object Identifier 10.1109/LMWC.2002.805536 operating bandwidth in phase shifters and frequency multipliers. However, due to periodicity, the presence of undesired spurious passbands above the Bragg frequency is unavoidable. In this work, we propose a method to reject these spurious passbands which is based on the electromagnetic band gap (EBG) concept [4] and consists of periodically perturbing the wave impedance of the line. It has been previously demonstrated that a periodic pertur- bation of the wave impedance of a transmission line produces Bragg reflection in some frequency bands. According to cou- pled mode theory [5], [6], these rejected bands are roughly given by the spectrum of the coupling coefficient, which is closely related to the spatial variation of wave impedance and, hence, line geometry. This method has been successfully applied to the elimination of spurious bands in distributed passband fil- ters by periodically modulating device footprint [7]. Also, EBG structures have been used to enhance efficiency in broad-band power amplifiers by harmonic tuning [8], to cite some exam- ples of EBG applications. In this work, we explore the applica- tion of EBG structures to CPWs periodically loaded with shunt capacitances with an eye toward the rejection of the spurious passbands above the Bragg frequency. To this end, the distance between the central strip (of constant width) and ground planes will be periodically varied [9] (different types of geometry mod- ulation in CPWs have been recently proposed [10], [11]). As will be shown, the combination of shunt capacitors and geom- etry modulation enhances the reflection properties of the struc- ture being possible the rejection of all spurious bands with very simple geometry perturbation. II. DESIGN OF PERIODIC LOADED EBG-CPW STRUCTURES For comparison purposes, we have fabricated two structures: one of them with geometry perturbation (from now on referred to as PL-EBG-CPW) and the other with uniform lateral dimen- sions (PL-CPW). Both structures have been designed to be fab- ricated on a Rogers RO3010 substrate ( , thickness mm) and to exhibit a Bragg frequency of GHz. This frequency has been chosen relatively small to be clearly separated from the self-resonant frequency of loading devices (2.2-pF phicom capacitances disposed in parallel pairs), which is in the vicinity of 3 GHz. For the design of the PL-CPW, it is also necessary to set the lower frequency of the first spurious band. A value of 1.2 GHz has been selected in order to ensure that the resonant frequency of capacitors lies above the second 1531-1309/02$17.00 © 2002 IEEE