586 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 8, AUGUST 2007 Quasi-Elliptic Microstrip Low-Pass Filters Using an Interdigital DGS Slot Atallah Balalem, Ali R. Ali, Jan Machac, Senior Member, IEEE, and Abbas Omar, Fellow, IEEE Abstract—This letter introduces a DGS slot with an interdigital shape. The resonant frequency of the slot can be easily controlled by changing the length of the metal fingers, without changing the area taken by the structure. Using this slot, two quasi-elliptic low- pass filters were designed, fabricated and tested. The filters have a cut-off frequency of about 3 GHz. Index Terms—Defected ground structure (DGS), filter, low-pass filters, microstrip filter, transmission zero. I. INTRODUCTION D EFECTED ground structures (DGS) for microstrip lines have been attracting researchers in recent years. They have been presented in a number of different shapes for filter applica- tions [1], [2]. This technique is suitable for periodic structures, and for both low-pass and band-pass filters, e.g., [3]–[8]. The DGS applied to a microstrip line causes a resonant character of the structure transmission with a resonant frequency control- lable by changing the shape and size of the slot. This letter introduces a DGS in the form of an interdigital slot. The resonant frequency of the structure with this slot can be controlled by adjusting the distance between the metal fingers without changing the area occupied by the slot or the aperture. Two quasi-elliptic low-pass filters based on this slot were de- signed and fabricated on an RO4003c substrate 0.831 mm in thickness and with a relative dielectric constant of 3.38. The resonant behavior of the interdigital DGS used here introduces transmission zeroes to the filter response and consequently im- proves its stop-band performance. II. INTERDIGITAL DGS The proposed interdigital DGS slot is shown in Fig. 1. All dimensions in this letter are in mm. The slot is etched in the ground metallization under the microstrip line. This slot has a major advantage in providing tighter capacitive coupling to the line in comparison to known microstrip DGS structures. More- over, the resonant frequency of the structure can be controlled by changing the distance between the metal fingers. The reso- nant frequency of the slot can also be modified by changing the Manuscript received February 7, 2007; revised March 22, 2007. This work was supported by the Czech Technical University, Prague, Czech Republic and by the Czech Ministry of Education, Youth and Sports under the “Research in the Area of Prospective Information and Navigation Technologies” Project MSM 6840770014. A. Balalem, A. R. Ali, and A. Omar are with the Chair of Microwave and Communication Engineering, University of Magdeburg, Magdeburg 39106, Germany (e-mail: atallah.balalem@et.uni-magdeburg.de). J. Machac is with Faculty of Electrical Engineering, Czech Technical Univer- sity, Prague, Czech Republic (e-mail: machac@fel.cvut.cz). Digital Object Identifier 10.1109/LMWC.2007.901769 Fig. 1. Bottom side layout of the interdigital DGS structure. Fig. 2. Equivalent circuit of the proposed interdigital DGS structure. number of metal fingers, so there is in most cases no need to enlarge the overall slot size. The equivalent circuit of the proposed structure, illustrated in Fig. 2, is a combination of the interdigital capacitance equiv- alent circuit [9] and the equivalent circuit of the DGS [1], [2]. The series circuit represents an equivalent circuit of the in- terdigital capacitance [9]. A parametric study was carried out to show the effect of the finger length on the resonant frequency, particularly on the res- onance of the parallel resonator shown in the equivalent circuit in Fig. 2, which represents a transmission zero. This study was done by placing a slot under a transmission line 0.2 mm wide. The total width of the slot was fixed at 4.9 mm, and the two dif- ferent slot lengths 3.75 and 5.65 mm were used. The number of metal fingers was fixed at 6. The width and spacing of the fingers are equal to 0.3 mm, as shown in Fig. 1. Fig. 3 shows the dependence of the resonant frequency on the length of the metal fingers . By increasing , capacitance is raised, and therefore the resonant frequency of the slot, the transmission zero, is shifted down. Capacitance can be as well changed by changing the spacing between the metal fingers. The narrower this spacing is, the higher is the capacitance, and the resonant frequency goes down. III. LOW-PASS FILTER WITH AN ADDITIONAL TRANSMISSION ZERO In general, the cut-off frequency of the low-pass filter can be adjusted by setting proper values of the lumped elements of the 1531-1309/$25.00 © 2007 IEEE