This paper describes the design of a new configuration of a low pass filter (LPF). This filter structure is based on a ring topology. The proposed LPF is miniature and low cost because it is mounted on FR4 substrate. The validation of this circuit into simulation was based on many optimization methods integrated in ADS "Advanced Design System" and some tuning methods. At the end, the fabrication and the test of the final circuit permits to have a good agreement between simulation and measurement results reaching a low pass behavior. The proposed filter has an area of 25x25 mm2, with a cutoff frequency of 2GHz and good performances in term of insertion losses, which are around -0.127 dB in the whole bandwidth and a high stopband

Benaïssa, Abboud

Bennis, Hamid

Benriad, Farida

Mediavilla Sánchez, Ángel

Zbitou, Jamal

INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY

English

UCrea

INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,A Novel Microstrip Low Pass Filter Based on Ring TopologyFarida Benriad1,*, Jamal Zbitou2, Hamid Bennis3, Abboud Benaïssa1, Angel Mediavilla Sanchez41 LITEN Laboratory, FST of Settat Hassan 1ST University, Morocco2 LMEET Laboratory, FST of Settat Hassan 1ST University, Morocco3 TIM Research Team, EST of Meknes, Moulay Ismail University, Morocco4 DICOM Laboratory, University of Cantabria Santander, SpainE-mail: faridabenriad@gmail.comAbstract- This paper describes the design of a newconfiguration of a low pass filter (LPF). This filterstructure is based on a ring topology. The proposedLPF is miniature and low cost because it ismounted on FR4 substrate. The validation of thiscircuit into simulation was based on manyoptimization methods integrated in ADS"Advanced Design System" and some tuningmethods. At the end, the fabrication and the test ofthe final circuit permits to have a good agreementbetween simulation and measurement resultsreaching a low pass behavior. The proposed filterhas an area of 25x25 mm2, with a cutoff frequencyof 2GHz and good performances in term ofinsertion losses, which are around -0.127 dB in thewhole bandwidth and a high stopband.Index Terms- microstrip, planar filters, low passfilter, DCS, ISM.I. INTRODUCTIONRF filters have been widely studied and employedextensively as key components in moderncommunication systems. Known, as two portdevices this kind of circuit is exploited tosuppress unwanted signals and to separate signalsof different frequencies. Most oftelecommunication systems require numerouskind filters, microwave receivers and transmittersfor example needs filters for a preselectionfunction such as the case of a receiver front end.They are also utilized to suppress noise producedby mixers and parasitic signals generated bypower amplifier in transmitter. In addition, filtersprovide access to the passband and stop bandcharacteristics in diplexer and multiplexers.Theoretically, an ideal filter lets pass all desiredfrequencies with no attenuation and no shift andrejects all the unwanted frequency bands. Thedesign of a microwave filter begins by theapproximation of its ideal response by a finite-order polynomial function. In fact, someapproximations are commonly used formicrowave filters (Butterworth, Chebyshev,...)[1]. After, the initial element synthesis part, thelumped elements are then transformed intodistributed elements (transmission line).Practically, the distributed structures are obtainedfrom LC equivalent prototypes by usingdistributed equivalents of the lumped reactance.There are other filters designed by exploitinglow-pass prototype and classic network theorybased on specific transforms developed fordistributed structures. It should be noticed that,the rejection is an important parameter in thedesign approach [2-12], in fact improving it canbe obtained by increasing the polynomialfunction order.Because of its easy fabrication, low cost andminiature size, planar microwave filtersespecially low pass filters are widely used inmicrowave communication systems. Thetraditional approach to design a microstrip low-pass filter is generally based on shunt stubs orstepped impedance lines. However, filters usingthis kind of topologies need a large size to obtaina low cutoff frequency. This can be attributed tothe physical size of one section of the filterstructure, which is comparable to a half-wavelength of the design frequency. Recently,researchers and designers are in the pursuit ofcompact filters with low insertion loss and highrejection. For this reason, several topologies anddesign techniques to develop low-pass filtershave been proposed in literature.17IJMOT-2016-7-1089 © 2017 IAMOTVOL.12, NO.1, JANUARY 2017INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,In this paper, a novel design of low pass filterusing ring resonator associated with open stubs isproposed. This low cost filter is very compact andpresents interesting electrical performances.To validate the proposed circuit, ADS Agilentsolver was used in order to verify the simulationresults and to perform all the parametric studies.The circuit was fabricated and measured, a goodagreement between the simulation results and themeasured ones was obtained, which confirms thedesign and the optimization method used in thiswork.II. DESIGN PROCEDURESTEM structures microstrip lines are ideal forLPFs. To design a microwave LPF, we have tostart our study by using the idealized lumped-element circuit. Fig.1 presents the relationshipbetween a microstrip low pass filter and a lowpass filter prototype [13]. A series inductance isreplaced by a short section of a high-impedancetransmission line. And a shunt capacitor isequivalent to a low-impedance transmission line.(a)(b)Fig.1. Stepped impedance microstrip LPF: layout of 5thorder filter (a); equivalent circuit (b).If the length  of a high-impedance section is lessthan λ/8. Where the electrical lengths of theinductor sections to be calculated as [1]:0    (inductor)hL RZ  (1)and the electrical length of the capacitor sectionsas [1]:0   (capacitor)lC ZR  (2)Where 0R is the filter impedance, L and C are thenormalized element values. In addition, the actualvalues of hZ and lZ are usually set to the highestand lowest characteristic impedance that can bepractically fabricated.But not always simple to achieve a LPF by usingthe equivalent between microstrip lines andlumped elements, therefore, the design of acompact low pass filter is based often onoptimization, tuning and numerical methods.To validate the proposed LPFs, we haveconducted the first study by using Momentumintegrated in ADS, this electromagnetic solver isbased on the method of moments. We havestarted our study by optimizing the ringmicrostrip topology in order to have a low passbehavior taking into account the miniaturizationof the final circuit and to obtain at the end goodperformances in term of insertion loss and theattenuated band. After many series ofoptimization, we have obtained the LPF structuredepicted in Fig.2.Fig.2. The configuration of the proposed low passfilter.As mentioned in Fig.2, the final circuit iscomposed from a microstrip ring associated tofour stubs and two identical microstrip feed lines.The different stubs and the inner and outer radiuspermit to adjust the bandwidth and to enlarge the18IJMOT-2016-7-1089 © 2017 IAMOTVOL.12, NO.1, JANUARY 2017INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,attenuated frequency band. The dimensions of theoptimized LPF are listed in Table 1.Table 1. Dimensions of the filter in mmL W a b c r8.55 1.55 1.82 1.82 1.65 5.7As shown in Fig.3 we have a LPF with a cutofffrequency of 2 GHz and an insertion loss around -0.12 dB and a wide attenuated band. To study theelectric field, Fig.4 presents the Electric Field at1GHz in the bandwidth, which presents that wehave a flow of energy between port 1 and 2. Inaddition, the attenuation of energy for thefrequency at 5 GHz illustrating the attenuatedband.Fig.3. S-parameters versus frequency.(a)(b)Fig.4. The E filed of the proposed LPF: (a) at 5 GHzand (b) at 1 GHz.Our design approach is based on optimization andelectromagnetic characterization of the low passfilter. Therefore, it was necessary to achievevarious parametric studies to obtain the desiredfrequency response. For that, the proposed lowpass filter was simulated under different values ofthe length of the open stubs associated to the ringresonator. The obtained simulated results areillustrated in the Fig.5.Fig.5. Frequency response of the proposed low passfilter under different values of the length of open stubsassociated to the ring resonator.We can clearly observe that, this geometricalparameter influences significantly the frequencyresponse of the proposed filter. In fact, increasing19IJMOT-2016-7-1089 © 2017 IAMOTVOL.12, NO.1, JANUARY 2017INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,the value of open stubs length from 1.32 mm to2.32 mm set the cut-off frequency at higherfrequencies and enlarges the passband bandwidth.In other hand, the variation of this parameter caninfluence the location and the attenuation of thetransmission, which will enlarge the attenuationband and suppress the unwanted resonantfrequencies.III. FABRICATED DEVICE ANDMEASUREMENTAfter the validation of the proposed LPFs, wehave passed to fabrication. Fig.6 presents thephotograph of the achieved filter having a volumeof 25x25x1.6 mm3. This circuit was tested byusing a VNA of R&S using 3.5 mm calibrationkit.Fig.6. A photograph of the proposed fabricated LPF.As shows in Fig.7, the tested LPFs present a goodagreement between simulation and measurementresults. The measured insertion loss in thebandwidth is around -0.22 dB with a cutofffrequency of 2.13 GHz and a wide attenuatedfrequency band.Fig.7. Simulation and measurement results comparisonof the S-parameters.After the validation of the proposed structure intosimulation and realization, a comparison betweenthe realized filter and other low pass filtersproposed in literature is introduced.As mentioned in Table 2, we can clearly observethat the proposed filter presents remarkableelectrical performances with low insertion loss,large band stop with good rejection and compactsize with 25x25 mm2.Table 2. Comparison between the achieved filter and other filters proposed in literature.Published literature Circuit size(mm2)Cut off frequency(GHz)|S12|/|S11|(dB)Stop band range Average S21-magnitudeof stop band (dB)[14] 50x17 0.9 0.2/20 Up to 3 GHz 20[8] 20x20 2.33 0.15/15 Up to 7 GHz 10[15] 54x40 1.2 0.1/15 Up to 3 GHz 20[16] 25.6x9.78 3.12 0.3/11.5 Up to 19 GHz 20[17] 91x17 1 0.1/30 Up to 2 GHz 15Proposed filter 25x25 2 0.13/15 Up to 7 GHz 1020IJMOT-2016-7-1089 © 2017 IAMOTVOL.12, NO.1, JANUARY 2017INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,IV. CONCLUSIONThis paper has presented the design of a newsimple compact microstrip LPF based on a ringconfiguration. The final circuit is validated byusing optimization methods and tuningtechniques. This structure is miniature andsuitable for GSM, DCS and UMTS applicationdue to the cutoff frequency, which is around2.13 GHz. The LPF fabricated structure presentsgood performances in term of insertion loss and awide stopband. The different steps followed toachieve such circuit can be used to match thisconfiguration to other applications.ACKNOWLEDGEMENTWe thank Mr. Angel Mediavilla Sanchezprofessor & Director of DICOM Laboratory inCantabria University in Spain, for allowing us touse all the equipment and electromagnetic solversavailable in his laboratory.REFERENCES[1] David M. Pozar, Microwave Engineering, 4thEdition of JohnWiley & Sons, Inc, 2012.[2] J.-S. Hong and M. J. Lancaster, Microstrip Filtersfor RF/Microwave Applications. New York, NY,USA: Wiley, 2001.[3] C. Hunter, L. Billonnet, B. Jarry and P. Guillon,“Microwave filters–Applications and technology”,IEEE Trans. Microw. Theory Techn., Vol. 50, No.3, pp. 794–805, Mar. 2002.[4] S. Y. Huang and Y. H. Lee, “Compact U-shapeddual planar EBG microstrip low-pass filter”, IEEETrans. Microw. Theory Techn., Vol. 53, No. 12,pp. 3799–3805, Dec. 2005.[5] M. K. Mandal and S. Sanyal, “A novel defectedground structure for planar circuits”, IEEEMicrow. 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A novel microstrip low pass filter based on ring topology

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