The design, characterization and comparison of two widely used approaches in realizing slow-wave effect on microstrip transmission lines, that is stub loaded and defected ground structure loaded microstrip lines are presented in a wide bandwidth (10–67 GHz) for the first time. Transparent substrate and dielectric material are chosen to ease the alignment of electrode and ground plane. Thin dielectric layer are applied to make the comparison prominent. The results indicate that defected ground structure loaded microstrip line has better RF performance in terms of compactness and insertion loss than stub loaded method within the whole band especially in thin film applications

Jakoby, Rolf

Polat, Ersin

Tesmer, Henning

Wang, Dongwei

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Wideband evaluation of two types of slow-wavemicrostrip linesD. Wang,✉ E. Polat, H. Tesmer,and R. JakobyInstitute for Microwave Engineering and Photonics, Technishe Univer-sität Darmstadt, Darmstadt, 64283, Germany (E-mail: ersin.polat@tu-darmstadt.de, henning.tesmer@tu-darmstadt.de, rolf.jakoby@tu-darmstadt.de)✉Email: wang@imp.tu-darmstadt.deThe design, characterization and comparison of two widely used ap-proaches in realizing slow-wave effect on microstrip transmission lines,that is stub loaded and defected ground structure loaded microstrip linesare presented in a wide bandwidth (10–67 GHz) for the first time. Trans-parent substrate and dielectric material are chosen to ease the alignmentof electrode and ground plane. Thin dielectric layer are applied to makethe comparison prominent. The results indicate that defected groundstructure loaded microstrip line has better RF performance in termsof compactness and insertion loss than stub loaded method within thewhole band especially in thin film applications.Introduction: A transmission line (TL) is the fundamental and key com-ponent for RF systems. It is not only used in connecting RF components,but also in wavelength λ based millimetre wave (mmW) devices, suchas branchline coupler [1, 2], interference based crossover [3], impedancetransformer, virtual coupling [4], and delay line phase shifters and soforth. Therefore, TLs with high performance are crucial for advanced RFsystems. Microstrip line (MSL) is being widely used in mmW systems,because of its planarity and cost effective fabrication, which makes MSLsuitable for monolithic microwave integrated circuits (MMIC) as well asfor high-speed digital PCB. Compared with waveguides, the drawbacksof MSLs are lower power handling capability and higher loss. Hence,MSL with high compactness, low loss and high power handling capacitywould be of interesting.Stub loaded high impedance microstrip line: Compared to a conven-tional homogeneous 50  MSL, high impedance line loaded with openended stubs (L-MSL), as shown in Figure 1a, performs longer electricallength within the same physical length, which is referred to as slow-wave(SW) effect. This approach has been widely applied in miniaturizing mi-crostrip devices [1, 2]. Its equivalent circuit is given in Figure 2a. ZC andθC represent the characteristic line impedance and electrical length ofFig. 1 3D view of (a) L-MSL and (b) DGS-MSL. Dimensions are given as:W1 = 10 µm, W2 = 40 µm, L = 150 µm, S = 70 µm; W3 = 100 µm, a = b =120 µm, c = a/2 = 60 µm, and g = 20 µmFig. 2 Equivalent circuit of: (a) L-MSL and (b) DGS-MSLMSL, where ZC normally equals to 50 . Similarly, ZH and θH repre-sent a high impedance line. C is the capacitance of the open ended stub.The equivalent characteristic impedance ZE and electrical length θE ofL-MSL loaded with two stubs can be derived from ABCD-matrix:AC =⎡⎢⎣cosθC ZCsinθC1ZCsinθC cosθC⎤⎥⎦, Astub =[1 0ωC 1], (1)AC = AE = Astub · AH · Astub, (2)This gives:ZCsinθC = ZH sinθH (3)cosθC = cosθH − ωCZH sinθH , (4)The SW factor K is defined as:K = θCθH, (5)According to Equation (3), with ZC = 50 , the achievable SW factordepends on the highest possible ZH that can be realized by microstrip TL.Nevertheless, the highest achievable ZH by narrowing the transmissionline width WTL is limited to be around 100 , which limits the SW effect.Moreover, such narrow line causes high metallic loss and low powerhandling capability which largely limits the applicability of the L-MSL.Defected ground structure loaded microstrip line: Instead of L-MSL,defected ground structure (DGS) loaded microstrip line (DGS-MSL) isalso found to be able to provide SW effect, while keeping a low levelloss. The 3-dimensional view of a DGS-MSL with rectangular dumbbellshape etched lattice on the ground plane is shown in Figure 1b, and itsequivalent circuit is shown in Figure 2b. DGS introduces serial induc-tance to the TL by disturbing the return current on ground plane. ZL andθL represent a low impedance TL, and L is the inductance of the singleDGS lattice. ConsideringADGS =[1 ωL0 1], (6)the SW effect equationZLsinθC = ZCsinθL (7)could be derived similarly to Equation (3). It is realized by using lowimpedance TL with ZL < ZC such that W3 is wider and leads to lessloss than L-MSL. This is a big advantage especially in high frequencyapplications as well as in applications where MSLs are used with thindielectrics of less than 20 µm.Fabrication and measurement: By periodically cascading the elementsshown in Figure 1, L-MSL and DGS-MSL realized in suspended mi-crostrip line topology are designed and optimized in CST MicrowaveStudio (CST), both having ZE = 50 . The cross section and dimen-sions of the suspended microstrip structure are given in Figure 3. Toease the alignment between DGS and electrode, transparent AF32 glassfrom Schott GmbH with εr = 5.1 and tanδ = 0.01 is chosen as the topand bottom substrate, on which electrode and ground are plated by usingstandard photo lithography process described in [5]. Two glass substratesare precisely aligned with a distance of h2 = 20 µm held by spacers.Then a fluid dielectric with εr = 2.4 and tanδ = 0.002 is filled in. Be-sides, a conventional MSL without any loading method and thru-reflect-line (TRL) kits are fabricated together with the proposed demonstratorsas reference TL and post de-embedding kits, respectively. A photographof all the aforementioned devices is shown in Figure 4. Three DGS-MSLs with different lattice dimensions, but same total length, are fab-ricated. Detailed dimensions are given in Figure 1. In order to performon-wafer measurements on suspend microstrip TL with fluid dielectric,156 ELECTRONICS LETTERS February 2022 Vol. 58 No. 4 wileyonlinelibrary.com/iet-elFig. 3 Cross section of the suspended microstrip structure. Dimensions aregiven as: h1 = h3 = 700 µm, h2 = 20 µm. GND and electrode are galvanicgold with 2 µm thicknessFig. 4 Photograph of MSL, L-MSL, DGS-MSL1 (a = b = 120 µm), DGS-MSL2 (a = b = 160 µm), DGS-MSL3 (a = b = 200 µm) and TRL de-embedding kits. The dimensions are given as: lDGS = 16 mm and lMSL = 10mmcoplanar waveguide (CPW) contact pad with via-less CPW to MSL tran-sition is designed [4]. First, all devices are measured from 100 MHz to67 GHz, with two ports line-reflect-reflect-match (LRRM) probe tip cal-ibration. Then, the influence of fixtures is removed by moving the refer-ence plane directly to the device under test (DUT) by performing TRLde-embedding in Matlab. The reference planes after de-embedding areshown in Figure 4 as dashed lines.The de-embedded measurement results of reference MSL, L-MSL,DGS-MSL1, DGS-MSL2 and DGS-MSL3, are shown in Figure 5 interms of the absolute value of transmission coefficient |S21| and reflec-tion coefficient |S11|. All five demonstrators show broadband match-ing where |S11| < −15dB. The raw measurement results before de-embedding also show good matching, indicating well-fabricated 50 devices, which is not plotted here for simplicity. Based on the de-embedded results, unit length loss α, propagation constant β, whichrepresents compactness, and quality factor Q of the demonstrators arecalculated and given in Figure 6. As an overall evaluation of SW effectand loss, Q is calculated as:Q = β2α, (8)which represents how lossy the TL is. The higher the Q, the less lossgenerated by the TL. High Q transmission line is required in wavelengthλ based passive mmW components, such as coupler, impedance trans-former, delay line phase shifters and so forth, to minimize the dimension,loss and heating issues. Within their pass-bands, DGS-MSLs perform al-most the same α as MSL, while L-MSL exhibits around twice the loss.Due to expected SW effect, DGS-MSLs and L-MSL all show higherβ than MSL, among which DGS-MSL3 with largest etched area indi-cates the largest SW factor K of approximately 1.9 times that of MSLat 40 GHz. However, DGS-MSL3 performs low-pass feature and has acut-off frequency at around 40 GHz due to the high lumped serial induc-tance compared to DGS-MSL1, which limits its pass-band according toFigure 5. In contrast, DGS-MSL2 with intermediate DGS size corre-spondingly indicates intermediate performance in terms of α, β as wellFig. 5 De-embedded measurement results on amplitude of transmission co-efficient |S21| and reflection coefficient |S11| of MSL, L-MSL, DGS-MSL1,DGS-MSL2 and DGS-MSL3Fig. 6 Extracted parameters from de-embedded data: (a) unit length loss α,(b) propagation constant β and (c) quality factor Q of MSL, L-MSL, DGS-MSL1, DGS-MSL2 and DGS-MSL3as bandwidth. DGS-MSL1 and L-MSL exhibit similar K. DGS-MSLsshow an outstanding Q in their pass-bands. L-MSL has the lowest Qbecause it provides limited SW effect at the expense of drastically in-creased metallic loss. A comparison of L-MSL and DGS-MSL is sum-marized in Table. 1.ELECTRONICS LETTERS February 2022 Vol. 58 No. 4 wileyonlinelibrary.com/iet-el 157Table 1. Performance comparison of the two technologiesTechnology K Q fcutoff Fabrication effort VersatilityL-MSL√ √DGS-MSL√ √ √√represents better performanceConclusion: This work presents a comprehensive RF comparison oftwo types of slow-wave microstrip lines, namely L-MSL and DGS-MSLfor the first time. A conventional MSL is fabricated as a referencefor comparison. TRL de-embedding accurately extracts the electricalproperties of the proposed demonstrators to validate the comparison.Compared to widely used L-MSL in miniaturizing microstrip TLs,DGS-MSL shows advantages of less loss, wider SW range and higherquality factor. In addition, the electrode width of DGS-MSL is widerthan MSL, which increases its power handling capability and decreasesthe fabrication tolerance especially in thin film and high frequencyapplications. This gives a higher versatility of DGS-MSL. However, theetched dimension of DGS-MSL should be carefully chosen to providesuitable SW factor and passband: the higher the miniaturization, thelower the cut-off frequency of DGS-MSL. In addition, alignment ofDGSs with electrode on thick, untransparent dielectric is challenging,especially when dimensions of TLs and DGS lattices are small whichmay cause degraded matching. While L-MSL with only one patternedmetal layer is easier to be fabricated. L-MSL’s disadvantage of high ILmight be not critical in thick dielectric situations.The conclusions above are instructional in choosing either of themethods to design compact and high performance mmW microstripcircuits.Acknowledgements: This work was funded by the German ResearchFoundation DFG under Project-ID JA921/65-1.Conflict of interest: The authors declare no conflict of interest.Data availability statement: The data that support the findings of thisstudy are available from the corresponding author, Dongwei Wang, uponreasonable request.Credit statement: Dongwei Wang: conceptualization; investigation;methodology; validation; writing – original draft. Ersin Polat: writing– review and editing. Henning Tesmer: writing – review and editing.Rolf Jakoby: funding acquisition; project administration; supervision;writing – review and editing.© 2021 The Authors. Electronics Letters published by John Wiley &Sons Ltd on behalf of The Institution of Engineering and TechnologyThis is an open access article under the terms of the Creative CommonsAttribution License, which permits use, distribution and reproduction inany medium, provided the original work is properly cited.Received: 18 October 2021 Accepted: 17 November 2021doi: 10.1049/ell2.12389References1 Chun, Y.-H., Hong, J.-S.: Compact wide-band branch-line hybrids. IEEETrans. Microw. Theory 54, 704–709 (2006)2 Gai, C., Jiao, Y., Zhao, Y.: Compact dual-band branch-line coupler withdual transmission lines. IEEE Microwave Wireless Compon. Lett. 26(5),325–327 May (2016)3 Fai-Leung, W., Kwok-Keung, M.C.: A novel, planar, and compactcrossover design for dual-band applications. IEEE Trans. Microw. The-ory 59, 568–573 (2011)4 Zhu, L., Melde, K.L.: On-wafer measurement of microstrip-based circuitswith a broadband vialess transition. IEEE Trans. Advan. Packa. 29, 654–659 (2006)5 Karabey, O.H., Gaebler, A., Strunck, S., Jakoby, R.: A 2-D electron-ically steered phased-array antenna with 2 × 2 elements in LC dis-play technology. IEEE Trans. Microw. Theory 60(5), 1297–1306 May(2012)158 ELECTRONICS LETTERS February 2022 Vol. 58 No. 4 wileyonlinelibrary.com/iet-el

Wideband evaluation of two types of slow‐wave microstrip lines

Abstract The design, characterization and comparison of two widely used approaches in realizing slow-wave effect on microstrip transmission lines, that is stub loaded and defected ground structure loaded microstrip lines are presented in a wide bandwidth (10–67 GHz) for the first time. Transparent substrate and dielectric material are chosen to ease the alignment of electrode and ground plane. Thin dielectric layer are applied to make the comparison prominent. The results indicate that defected ground structure loaded microstrip line has better RF performance in terms of compactness and insertion loss than stub loaded method within the whole band especially in thin film applications

Wang, D.

Polat, E.

Tesmer, H.

Jakoby, R.

TUbiblio

Wideband evaluation of two types of slow-wave microstrip lines

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Wideband evaluation of two types of slow‐wave microstrip lines

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