Int. J. Electron. Commun. (AEÜ) 65 (2011) 864–872 Contents lists available at ScienceDirect International Journal of Electronics and Communications (AEÜ) j our na l ho mepage: www.elsevier.de/a eue Design of low-pass ﬁlters using some defected ground structures A.K. Verma, Ashwani Kumar ∗ Microwave Research Laboratory, Department of Electronic Science, University of Delhi South Campus, New Delhi 110021, India a r t i c l e i n f o Article history: Received 14 July 2010 Accepted 11 February 2011 Keywords: Defected ground structure (DGS) Microstrip Low-pass ﬁlter Planar ﬁlter a b s t r a c t We present characteristics of seven types of simple and complex defected ground structures (DGS) slot to realize the quasi-lumped inductance. A simple DGS follows the Butterworth type response; whereas a complex DGS has quasi-elliptic response. The response of DGS has strong inﬂuence on the performance of a 5-pole Chebyshev type low-pass ﬁlter (LPF). We present a systematic design process to realize the LPF using these DGS slots. The 5-pole open square (OS) DGS based LPF of cut-off frequency 2.5 GHz provides best measured performances-transition region bandwidth 0.53 GHz, 20 dB rejection band-width 14.7 GHz, insertion loss 1.29 dB. Theses performances are better than the reported results of the 5-pole DGS based LPF in literature. © 2011 Elsevier GmbH. All rights reserved. 1. Introduction Defected ground structures (DGS), in place of narrow width microstrip line sections, have been widely employed for the design of compact and high performance microstrip low-pass ﬁlters (LPF). These ﬁlters can handle larger power as compared to the stepped impedance type LPF. The stop-band characteristic of these ﬁlters is also superior as they can effectively suppress the higher order har- monics. Photonic band-gap (PBG) structures are also proposed for such applications [1]. Several kinds of simple and complex shaped DGS slots are proposed in literature [2–9]. A DGS structure offers series connected quasi-lumped inductive element to a microstrip line. Its use reduces length of a LPF. The performances of low pass ﬁlters, i.e. sharpness of the transition region, suppression of har- monics in the stop-band and low insertion loss in the pass-band; are signiﬁcantly inﬂuenced by the type of the DGS slots. Rahman et al. [3] have compared the performances of a few simple DGS slots. However, performances of the complex slots are not com- pared for their judicious application to the low-pass ﬁlters. We have presented such comparison recently [8]. Some efforts have been made in literature to design the DGS based LPF using the prototype design of Chebyshev LPF [9,10]. However, the suggested design processes are not adequate. The ﬁrst method [9] assumes size of the DGS and the design does not start with the speciﬁed cut-off frequency of a LPF. The sec- ond method [10] is valid only for one substrate. Thus the existing design of the DGS based LPF is based on the trial and error process on EM-simulator. The dimension and performance of a LPF are not ∗ Corresponding author. E-mail addresses: anandvrm48@gmail.com (A.K. Verma), ashwanikumar7@yahoo.com (A. Kumar). optimized. It offers scope for further improvement in performance of a LPF. We reported earlier a synthesis process to design Cheby- shev type DGS based LPF that is valid for most of the substrates used in practice. The design process starts with the speciﬁed cut-off fre- quency and number of poles. We have presented the design and results for both 5-pole [11,12] and 7-pole [13] LPF. In these design simple triangular DGS are used. In this paper we present a summary of comparative perfor- mances of several simple and complex DGS slots; so that these can be used as the quasi-lumped inductors. Following our earlier design methodology [11], we present the design of Chebyshev type 5-pole low-pass ﬁlters using the open square (OS) [5] and interdig- ital (ID) [7] type DGS. Using Ansoft HFSS EM-simulator, we have compared performances of these ﬁlters against the LPF based on square headed DGS. In order to compare performances of the DGS slots and also performances of the LPF based on them, we have maintained identical cut-off frequency for all the ﬁlters at 2.5 GHz. We have further examined performances of both the T-and -type LPF topologies for the OS type DGS. Finally the 5-pole ﬁlters are fab- ricated and tested in order to validate the present design process. The results of the LPF, based on our design process, are further com- pared against performances of available design in literature [5,7]. Our LPF show signiﬁcant improvement in performance over the reported design of the DGS based LPF. In brief the present work helps a ﬁlter designer to select an appropriate complex DGS structure in order to synthesize a higher order DGS based LPF at given frequency. In our knowledge such synthesis process is not reported in open literature. 2. Quasi-lumped DGS inductor and comparison of DGS slots We can group all DGS slots in two categories – (i) simple DGS slots and (ii) complex DGS slots [8]. The slot-heads of simple DGS 1434-8411/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2011.02.007