IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 23, NO. 6, JUNE 2013 303

Design of Sharp Roll-Off LowpassFilter With Ultra Wide Stopband

Gholamreza Karimi, Ali Lalbakhsh, Member, IEEE, and Hesam Siahkamari

Abstract—A novel microstrip lowpass filter is proposed toachieve an ultra wide stopband with 12th harmonic suppressionand extremely sharp skirt characteristics. The transition bandis from 1.26 to 1.37 GHz with and , respectively.The operating mechanism of the filter is investigated based onproposed equivalent-circuit model, and the role of each sectionin creating null points is theoretically discussed. An overall goodagreement between measured and simulated results is observed.

Index Terms—Micriostrip lowpass filter (LPF), sharp roll-off,ultra wide stopband.

I. INTRODUCTION

M ICROWAVE lowpass filters (LPFs) with small circuitsize, low insertion loss, sharp roll-off, and wide stop-

band are in great demand for wireless communication systemsto suppress harmonic and spurious signals [1]. In order to designa compact LPF with high performance, planar resonators, suchas stepped impedance resonators (SIRs) and their transformedstructures [2]–[6], as well as other newmethods such as defectedground structure (DGS) and electromagnetic band-gap (EBG)have been proposed. However, the former will cause radiationand the latter will increase the size of the filter. Another newmethod to design a microstrip filter is using modified steppedimpedance hairpin resonators [2]–[7]. In order to achieve a widestopband, radial stubs and shunt open-stubs at the feed points ofa center fed coupled-line hairpin resonator in [2] and [3] wereemployed; however, these filters failed to provide a sharp tran-sition. In [6] a single T-shaped resonator in combination with asuppressing cell was utilized to obtain a LPF with around 5thharmonic suppression. In [8] a new transformed radial stubs(TRSs) with an extended stopband was introduced. To achievecompact design, both symmetrically loaded radial shape patchesand meandered main transmission line were adopted in [9], andthen the stopband was expanded to 16th harmonic suppressionin [10] and [11]. In spite of this modification, the filter still suf-fered from inadequate attenuation in the stobpand and gradualroll-off.In this letter, a novel lowpass filter with very sharp transition

band and its equivalent circuit are presented. In order to realize

Manuscript received December 08, 2012; revised March 14, 2013; acceptedApril 08, 2013. Date of publication May 16, 2013; date of current version June03, 2013.G. Karimi and H. Siahkamari are with the Department of Electrical Engi-

neering, Faculty of Engineering, Razi University, Kermanshah 67149, Iran.A. Lalbakhsh is with the Department of Engineering, Islamic Azad Univer-

sity, Kermanshah Branch, Kermanshah, Iran.Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LMWC.2013.2261057

Fig. 1. (a): Layout of the proposed resonator, , ,, , , , , , ,

, , , , ,, , , , , (all in

millimeters). (b): L-C equivalent circuit.

a wider stopband, circular-shaped patches and open stubs areadopted in this design, introducing serial resonance in the cir-cuit. Thus, an ultra wide stopband from 1.37 GHz up to 16 GHzwith large attenuation of more than 20 dB is achieved.

II. MAIN RESONATOR DESIGN

A. Equivalent Circuit and LC Parameters Extraction

The geometric structure of the proposed resonator and itsequivalent LC circuit are shown in Fig. 1. The values of thoseparameters can be extracted by the use of methods discussed in[1]. As it is shown in Fig. 1(b), is the series coupling ca-pacitance while and are shunt capacitances associatedwith the gap. is the sum of the capacitances of the junctiondiscontinuities between the main feed line and three inductancelines. and represent the inductances of the transmissionlines while , , , and , , are inductancesand capacitances of the open stubs, respectively. The calculatedand optimized values for L-C equivalent circuit are summarizedin Table I. To better understand the characteristics of the pro-posed resonator, it is divided into compound resonator depictedin Fig. 2(a) and the shunt-connected series L-C resonator de-picted in Fig. 2(b) that determines the cutoff frequency. The

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304 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 23, NO. 6, JUNE 2013

Fig. 2. Two main parts of the circuit. (a): Compound resonator. (b): Shunt-connected series L-C resonator. (c): Layout of the resonators.

Fig. 3. (a),(b),(c): Three sections of compound resonator. (d): Layout of theresonators.

TABLE ICALCULATED AND OPTIMIZED VALUES FOR L-CEQUIVALENT CIRCUIT (UNITS: C, pF; L, nH)

compound resonator can be divided into three sections corre-sponding to three resonators as shown in Fig. 3.

B. Characteristics of the Main Resonator

The fact that the proposed resonator exhibits the stopband in-cluding four transmission zeros is verified by the theoretical pro-cedures. As shown in Fig. 3, each section works as a resonatorand creates a transmission zero. The resonance frequency for se-ries L-C resonators in Figs. 2(b) and 3(b),(c) are calculated by(1), but for the resonator shown in Fig. 3(a), (2) is extracted asits resonance frequency. Practically, when these resonators areconnected together to form the compound resonator, they are af-fected by each other. For example, the calculated resonance fre-quency for resonators in Fig. 3(b) and (c) change slightly from5.48 and 6.22 GHz to 5.9 and 6.7 GHz, respectively. These areshown in Fig. 4(b).The most significant deviation of the calculated resonance

frequencies is related to the resonator shown in Fig. 3(a).Adding two shunt-connected series L-C resonators causes theresonance frequency to shift from 5.4 GHz to 3.7 GHz. Thisis because at 3.7 GHz two L-C resonators act as a capacitorwith capacitance of , so the (3) for the resonancefrequency is theoretically extracted from the circuit depicted in

Fig. 4. (a): Configuration of Fig. 3(a) at 3.7 GHz, after adding two shunt-con-nected series L-C resonators. (b): Comparison between LC simulation resultaccording to optimized values in Table I and EM simulation result on substratewith relative dielectric constant of 3.38 and thickness of 0.508 mm.

Fig. 5. (a): Cutoff frequency as a function of scaling factor. (b): Frequencyresponse of the main resonator.

Fig. 4(a). The EM and equivalent circuit simulation results areshown in Fig. 4(b)

(1)

(2)

(3)

One of the features of the proposed resonator is that its cutofffrequency can be modified for different applications, as shownin Fig. 5(a), by increasing , and decreasing thenear band transmission zero moves to lower frequencies andthe passband gets narrower. This is because by increasing ,

the capacitance of increases and by decreasingthe inductance of increases. These changes cause smallercutoff frequency. Moreover, resonators shown in Fig. 3(a)–(c)can control transmission zeros at 3.7, 5.9, and 6.7 GHz, respec-tively. So the proposed resonator can introduce four adjustabletransmission zeros in the stopband.

III. LOW PASS FILTER DESIGN

In order to extend the stopband, transmission peaks shouldbe suppressed by adjusting some transmission nulls created bybandstop filters or appropriate suppressing cells. So, three band-stop filters are designed based on a one-pole Butterworth low-pass prototype with . The element values of the

KARIMI et al.: DESIGN OF SHARP ROLL-OFF LOWPASS FILTER WITH ULTRA WIDE STOPBAND 305

Fig. 6. (a): Photograph of fabricated LPF. (b): Simulated and measured -pa-rameters of the proposed LPF.

TABLE IIPERFORMANCE COMPARISONS AMONG PUBLISHED

FILTERS AND PROPOSED ONE

lowpass prototype are , and the relateddesign equations are described in [1]. The bandstop filters aredesigned to have fractional bandwidths , 0.39,0.49 at midband frequencies 8.09, 12.9, 19 GHz, respec-tively. The widths and the lengths of the open stubs designed asbandstop filters are assigned as 0.2, 0.5, 1.3 mm and 5.9, 3.6,2.2 mm, respectively. It can be seen from Fig. 5(b)-dotted line-that despite suppressing spurious band from 12 to 16 GHz andcreating a transmission zero at around 8 GHz by using band-stop filters, there are still some spurious frequencies at 7 and10.5 GHz. Therefore, to eliminate these frequencies and pro-viding a stopband with more attenuation, circular suppressingcells with radiuses of 2.9 and 1.7 mm and width and lengths ofinductance lines of 0.4 mm and 0.5, 0.9 mm are designed to haveattenuation poles around the spurious frequencies. Finally, twoequilateral triangles with side length equal to 2.4 mm are addedto the main resonator to achieve a better performance.

IV. SIMULATION AND EXPERIMENTAL RESULTS

The proposed LPF has been fabricated on a substrate with arelative dielectric constant , thickness ,and loss tangent . Fig. 6(a) shows a photographof the proposed filter. Simulations are done by an EM-simulator(ADS Momentum). The -parameter is measured by an Agilent

network analyzer N5230A. It can be seen from the Fig. 6(b) thatthe filter has 3 dB cutoff frequency equal to 1.26 GHz, inser-tion loss less than 0.8 dB in the passband from dc to 1.1 GHz,return loss better than 9 dB, and suppression level higher than20 dB from 1.37 up to 16 GHz. Thus, the proposed filter hasa property of twelve-harmonic suppression. The relative stop-band bandwidth of the LPF is about 165%. Furthermore, thesize of the proposed filter is only 15 35 which corre-sponds to . The return loss in the stopband re-gion is very small, indicating negligibly small radiation loss.The transition band from 1.26 to 1.37 GHz with andrespectively, is 0.11 GHz, showing that the filter has excellentskirt performance. This is because a deep transmission zero islocated at 1.5 GHz very close to the cutoff frequency. For com-parison, Table II summarizes the performance of some pub-lished lowpass filters based on several specifications explainedin [9]. As shown in Table II, the proposed filter exhibits a highfigure-of-merit (19931) among the quoted filters.

V. CONCLUSION

A novel microstrip lowpass filter with very sharp roll-off andultra wide stopband is presented. A prototype filter with 3 dBcutoff frequency is designed, fabricated, andtested. Results indicate that the proposed filter here achievesextremely high figure-of-merit of 19931. With all these goodfeatures, the proposed filter is applicable to modern communi-cation systems.

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