Arsanjani, Arash

Bösch, Wolfgang

Rezaee, Behrooz

Sarbandifarahani, Hossein

Teschl, Reinhard

English

TUGraz OPEN Library

A Compact Slow-Wave Filter with Double-sidedSelectivity and Wide Out-of-Band RejectionArash Arsanjani1, Hossein Sarbandi Farahani2, Behrooz Rezaee, Reinhard Teschl, and Wolfgang BöschInstitute of Microwave and Photonic Engineering, Graz University of Technology, Graz, Austria1 arash.arsanjani@tugraz.at, 2 sarbandifarahani@tugraz.atAbstract—In this paper, a miniaturized filter with double-sidedselectivity and wide out-of-band rejection is presented. Theslow-wave and negative cross-coupling structures are effectivelyadopted to realize an ultra-compact K-band filter with twosymmetric transmission zeros. Starting from a bandpass filterconfigured for a center frequency of 18.5 GHz and the main-linecouplings that only exhibits a good out-of-band rejection of 30dB up to 30 GHz, the two proposed negative cross-couplingsenable the steep roll-off out-of-band rejections as well as lowin-band insertion losses of 1.7 dB and 2.4 dB. The designedstructures are utilizing both the additive manufacturingand PCB-based technology which in turn, provide a simple,low-profile, and cost-effective fabrication process for highlyintegrated RF systems, particularly in satellite applications.Index Terms—K-band filter, negative coupling, slow-wave,transmission zero.I. INTRODUCTIONFuture generations of telecommunication systems willexhibit a deeper integration – or even convergence – offixed and mobile services. Scenarios have been identifiedwere satellite systems and in particular LEO satellite mega-constellations, can support 5G architectures to improvethe connectivity of IoT devices in remote areas. Besidesmanaging the intersystem handover and security issues, otherchallenges have been identified. One of which is associatedwith the economic utilization of the limited spectrum and theuse of higher frequencies. For that purpose filters are essentialbuilding blocks which demand low-profile, lower losses, strictpassband performance, and good out-of-band rejection [1].Slow-wave microwave components are a branch of metama-terials that provide good performance in terms of out-of-bandrejection and size reduction, making them a good candidate toaddress the strict requirement of satellites. Many investigationshave been made to exploit the benefits of slow-wave structuresin implementing microwave and mm-wave components [2],[3]. One of the worthy mentions is the implementation of slow-wave substrate integrated waveguide (SW-SIW) [2], where theslow-wave effect inside the SIW was created by an array ofperiodic blind vias inside the propagation medium. The SW-SIW reported in [2] achieved a longitudinal size reductionof better than 40% compared to a conventional SIW. There isalso a recent study of a slow-wave filter implementation in theSIW platform [4], [5], where the implementation of inductivepost and iris coupling has been investigated. Both of the filtersexhibit a significant size reduction. However, it is shown in theresults that the insertion loss is high due to the dielectric losses.In some other literature, a type of slow-wave structure is intro-duced, where it is referred to as meta-substrate structure. Thesemeta-substrate structures also show significant improvementin miniaturization of microwave components such as patchantennas [6] or SIW cavities [7]. All of these components aredesigned for the lower spectrum of microwave frequencies.Despite the compactness of these planar solutions, one sig-nificant setback in the implementation of slow-wave structuresin a planar solution were the increased losses, especially athigher frequencies. One method of reducing losses at higherfrequencies was to remove the dielectric substrate to eliminatethe dielectric losses [8], [9]. Reference [9] proposes a semi-planar structure which has the advantage of compactness andhigh quality factor performance. Additionally, having a PCBlayer as the top layer provides opportunities to create pathsfor waves through the PCB layer to create negative coupling.Moreover, the filter component can be used as surface mount-able component which can be interesting in some applications.The implementation of a semi-planar air-filled SW filter haschallenges of its own and requires further investigation. Thispaper investigates two different methods for implementinga negative coupling in the proposed SW filter configuration.All three proposed structures show low losses due tothe elimination of dielectric loss from the structure. Thetwo proposed negative couplings created symmetricaltransmission zeroes around the passband, thus improvingthe filter’s frequency selectivity. Furthermore, the slow-wavefilters show improved out-of-band rejection because of theSW’s frequency selective characteristic, which is a desirablefactor in practical filter design. In the end, a comparisonbetween the three filter configuration is made and a conclusionbased on the achieved results is drawn.II. SLOW-WAVE FILTER DESIGNA. Slow-wave Unit-cell and resonatorThe working principle of slow-wave structures is basedon reducing the group velocity of the transmission lineby creating an array of periodic structures in the wavepropagation path. The unit cell design is decided based onthe upper out-of-band rejection specification and the requiredminiaturization. The stop-band of the unit cell can be utilizedto achieve an even better out-of-band rejection. The dispersiondiagram of the proposed unit cell is shown in Fig. 1. Thestopband of the proposed unit cell is in the frequency bandof 62 to 95 GHz, while the designated slow-wave passbandis from 17 to 20 GHz. Therefore, an excellent out-of-bandrejection can be achieved with a careful design of unit-cellresonators and input and output coupling.This manuscript is a reproduction of the accepted version of the paper with the full citation:A. Arsanjani, H. S. Farahani, B. Rezaee, R. Teschl and W. Bösch, "A Compact Slow-Wave Filter with Double-sided Selectivity and Wide Out-of-Band Rejection,"2021 IEEE Asia-Pacific Microwave Conference (APMC), 2021, pp. 256-258, doi: 10.1109/APMC52720.2021.9661651.https://ieeexplore.ieee.org/document/9661651, Copyright © 2021, IEEEFig. 1: Dispersion diagram of the slow-wave structure over theirreducible Brillouin zone (Γ−X−M−Γ).Additionally, to demonstrate the slow-wave effect of theproposed structure, the phase velocity was plotted versus thefrequency for different values of pin’s height. It is shownin Fig. 2 that the bigger the height of the pins are withrespect to the cavity, the slower is the phase velocity of theproposed structure. The resonator created with the proposedunit-cell shows a size reduction of equal to 41% compared toa conventional resonator of the same shape without the pins.B. Inline filter and negative coupling implementationA four-pole inline filter was designed using the proposedslow-wave resonators, with an insertion loss of 1.7 dB atthe center frequency of 18.5 GHz and a bandwidth of 400MHz. The proposed filter has a lower roll-off factor of6.1 dB/100 MHz, an upper roll-off factor of 7.9 dB/100 MHzand an out-of-band rejection of better than 30 dB up to30 GHz. The substrate used for the proposed structure isfrom the Rogers company (RO4350B), with a thickness of0.508 mm, dielectric constant of εr = 3.66 and dissipationfactor of tanδ=0.0037. The frequency response of the filter,as shown in Fig. 3c, has an acceptable performance in termsof loss and out-of-band rejection.As a complementary, the filter selectivity was improved byimplementing transmission zeros around the passband. Twopossible solutions for achieving a negative coupling weresimulated: either through the substrate integrated waveguideon top or from a GCPW coupling through the top conductorof the resonator. The non-zero elements of the couplingmatrix of the proposed filter have the following values:MS1 = M4L = 0.91, M12 = M34 = 0.79, M23 = 0.676,M14 = −0.071. The aforementioned couplings and externalquality factor and corresponding physical dimensions of thefilter can then be calculated through the following expressions:K=f2High−f2lowf2High+f2low, Qext =2f0BW−3 dB, (1)The two possible solutions along with the frequencyresponses are shown in Fig. 4a and Fig. 5a, respectively. Thesefilters were simulated through the frequency domain engine10 20 30 40 50 60 70Frequency (GHz)0.511.522.5/c0h2/h3=0h2/h3=0.4h2/h3=0.6h2/h3=0.8Fig. 2: Normalized phase velocity (νφ/c0) versus frequencyfor different relative heights of pins (h2) to the height of thecavity (h3).(a) (b)𝑺MS1R1R2 R3R4 𝑳M23M12 M34M4L(c)Fig. 3: (a) 4-pole filter inline filter, (b) top view of filter, and(c) out-of-band rejection performance of filter.of CST 2020, and the results were confirmed using the HFSSsoftware. The Fig. 4a solution provides the filter responsewith a roll-off factor of 13.8 dB/100 MHz and upper roll-offfactor of 18.9 dB/100 MHz, which provides better selectivitycompared to the inline configuration. The second solution alsoprovides a good roll-off factor of 13.9 dB/100 MHz and upperroll-off factor of 15.7 dB/100 MHz. The negative coupling(a) (b)M14𝑺MS1R1R2 R3R4 𝑳M23M12 M34M4L(c)Fig. 4: First proposed filter (a) negative coupling structure-1,(b) top view, (c) filter response inset with topology.provided in both solutions are realized by transferring thepeak of electrical fields of the first resonators to that of thefourth resonator. The first solution shows a filter response withlower insertion loss than the second solution because the slotson top of the resonator can cause an increased leakage loss.III. CONCLUSIONIn this communication, we described the implementationof two negative coupling solutions which can effectivelyimprove the frequency selectivity, in-band insertion loss,and overall size of an inline slow-wave filter with thesame filtering specifications but more number of resonators.The two proposed filters feature a lower roll-off factor of13.8 dB/100 MHz and 13.9 dB/100 MHz and an upper roll-offfactor of 18.9 dB/100 MHz and 15.7 dB/100 MHz, respec-tively. The proposed filter has the advantages of low-loss,high selectivity, good out-of-band rejection, and low-profile.ACKNOWLEDGEMENT“This project has received funding from the EuropeanUnion’s Horizon 2020 research and innovation pro-gramme under the Marie Skłodowska-Curie grant agreementNo 811232”.REFERENCES[1] H.S. Farahani and R. A. Sadeghzadeh ”Novel compactmillimeter-wave gap waveguide filter with double-sided selec-tivity” Micro. and Opti. Tech. Lett. vol. 60 December 2017.(a) (b)M14𝑺MS1R1R2 R3R4 𝑳M23M12 M34M4L(c)Fig. 5: Second proposed filter (a) negative coupling structure-2, (b) top view, (c) filter response inset with topology.[2] A. Niembro-Martı́n et al., ”Slow-wave substrate integratedwaveguide,” IEEE Trans. Microw. Theory Techn., vol. 62, no.8, pp. 1625–1633, Aug. 2014.[3] Y. Zhang et al., ”Slow Wave Substrate-Integrated WaveguideWith Miniaturized Dimensions and Broadened Bandwidth,”in IEEE Transactions on Microwave Theory and Techniques,2021.[4] A. Parameswaran, P. Athira, and S. Raghavan, ”MiniaturizingSIW filters with slow wave technique,” AEU-Int. J. Electron.Commun., vol. 84, pp. 360–365, Feb. 2018.[5] M. Bertrand, Z. Liu, E. Pistono, D. Kaddour, and P. Ferrari,”A compact slow-wave substrate integrated waveguide cavityfilter,” in IEEE MTT-S Int. Microw. Symp. Dig., May 2015, pp.1–3.[6] X. Xu and J. Wei, ”Miniaturisation design of patch antennausing a lowprofile mushroom type meta-substrate tailored withhigh permittivity,” in IET Microw., Antennas Propag., vol. 12,no. 7, pp. 1216–1221, Jun. 2018.[7] B. Zhang, X. Xu, Y. Wang and J. Wei, ”Miniaturized Sub-strate Integrated Waveguide Resonator Loaded with MushroomMetamaterials,” 2020 Cross Strait Radio Science & WirelessTechnology Conference (CSRSWTC), pp. 1-3, 2020.[8] J. Qiang, F. Xu, L. Yang, and J. Zhan, “Compact emptysubstrate integrated waveguide with high performance and itsapplication in microwave,” IET Microwaves, Antennas Propag.,no. October 2020, p. mia2.12127, Apr. 2021[9] A. Arsanjani, L. Robins, R. Teschl and W. Bösch, ”Implemen-tation of K-band Mushroom meta-material filter for satellite ap-plications,” 2020 50th European Microwave Conference (EuMC2020), pp. 495-498, 2021.

A Compact Slow-Wave Filter with Double-sided Selectivity and Wide Out-of-Band Rejection

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A Compact Slow-Wave Filter with Double-sided Selectivity and Wide Out-of-Band Rejection

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