An innovative bandwidth tunable optical filter with controllable bandwidth, center frequency, and transmission zero is proposed in this paper. The proposed filter utilizes a quad-mode resonator to achieve a wideband filter centered at 2.42 GHz. By incorporating varactor diodes into the open branches of the resonator, the proposed filter's center frequency and bandwidth can be dynamically adjusted via the voltage applied to the diodes. This tunable filter exhibits low insertion loss of less than 2 dB, return loss exceeding 10 dB, and a relative bandwidth of up to 40%

Du, Jikun

Li, Jingwen

Zhang, Zihan

English

Electronics Science Technology and Application (E-Journal)

-184-Electronics Science Technology and ApplicationBandwidth Tunable Optical Filter Based on the Quad-ModeResonatorZihan Zhang, Jikun Du, Jingwen LiDalian Jiaotong University, Dalian 116028, China.Abstract: An innovative bandwidth tunable optical filter with controllable bandwidth, center frequency, and transmissionzero is proposed in this paper. The proposed filter utilizes a quad-mode resonator to achieve a wideband filter centered at2.42 GHz. By incorporating varactor diodes into the open branches of the resonator, the proposed filter's center frequencyand bandwidth can be dynamically adjusted via the voltage applied to the diodes. This tunable filter exhibits low insertionloss of less than 2 dB, return loss exceeding 10 dB, and a relative bandwidth of up to 40%.Keywords: Resonator Filters; Microstrip Filters; Varactor Diodes1. IntroductionAs wireless communication technology continues to rapidly develop and the number of network participants andservices increases, the need for efficient utilization of spectrum resources has become increasingly crucial. This has resultedin a growing interest in the research of highly selective filters that possess continuously tunable center frequencies andbandwidths[1][2]. Combine topologies are a commonly used design approach for creating compact filters with wide stopbands.They are characterized by a simple structure consisting of multiple resonators that are arranged in parallel andelectromagnetically coupled to each other[3]. There are some design methods for creating bandpass filters. The first methodproposes a continuously tunable bandpass filter over a wide range of frequencies achieved by manipulating the electric andmagnetic coupling between resonators[4]. The second method suggests several adaptations to the conventional combinedfilter, employing stepped impedance resonators to create filters with constant absolute bandwidth[5]. The final methodproposes that tunability in these filters can be achieved by utilizing variable capacitors based on varactor diodes[6].In this paper, based on a quad-mode resonator that is loaded with open-ended branches, the filter bandwidth is expandedby adjusting the length of the branch lines corresponding to the upper and lower cutoff frequencies. Furthermore, theimplementation of varactor diodes on the three open branch lines of the filter yields a wideband filter with controllablebandwidth, center frequency, and transmission zero. The filter's unique structure contributes to its superior performance andpotential for various applications.2. Design of the Bandwidth Tunable Optical Filter2.1 The Wideband Filter ConfigurationThe quad-mode resonator, depicted in Fig.1, consists of a T-shaped resonator loaded with open branches at the centerand symmetrically loaded with open branch lines on the left and right sides, where the branch lines θ2 and θ4 are not equal.Volume 10 Issue 5-185-Fig.1 Quad-mode resonator structureFig.2 presents the overall design of a wideband filter that is based on the above quad-mode resonator. The filter isimplemented on a Rogers RO4003 substrate with a dielectric constant of εr = 3.55 and has a plate thickness of 0.8 mm, lossangle of 0.0027, and a copper foil thickness of 0.035 mm. The simulation results, displayed in Fig.3, demonstrate a relativebandwidth of 40%, a center frequency of 2.42 GHz, and a return loss of better than 10 dB. Additionally, the filter itselfgenerates two transmission zeros located at 1.82 GHz and 3.10 GHz.Fig.2 Geometry of the Wideband filter. ( L1=15.61 mm, L2=13.8 mm, L3=1.3 mm, L4=27.5 mm，L5=15 mm, W1=0.5 mm,W2=0.3 mm, W3=W4=W5=0.5 mm, g=0.15 mm)Fig.3 Simulation result of wideband filter2.2 The Proposed Filter ConfigurationA terminal grounding capacitor can be employed to substitute a section of the terminal open branch line, which hasimpedance characteristics of capacitance[7]. Fig.4(a) displays a schematic diagram of a section of the terminal open line,characterized by conductance Y and electrical length θ. Fig.4(b) illustrates a terminal grounding capacitor with capacitancevalue C.(a)Terminal open stub (b)Terminal ground capacitanceFig.4 Open stub and terminal ground capacitanceBased on this feature, a bandwidth tunable optical filter is designed by incorporating varactor diodes on the open-186-Electronics Science Technology and Applicationbranches L4 and L5, as depicted in Fig.5.Fig.5 Geometry of the bandwidth tunable optical filter ( L1=16.25 mm, L2=12 mm, L3=0.5 mm，L4=4.85 mm, L5=11.25mm, L6=8 mm, W1=0.5 mm, W2=0.3 mm, W3=W4=W5=0.5 mm, W6=0.3 mm, g1=g2=0.15 mm)2.3 Design GuidelinesIn the above theory and analysis process, the design of the proposed filter is mainly divided into the following steps.1） Design a resonator with four resonant modes.2） Add open branches to the resonator to obtain the ideal wideband filter by bending and changing the distancebetween these branches.3） Add varactor diodes and apply the appropriate voltage.4） Optimize the relevant parameters to improve the performance3. Results3.1 Voltage V1 is fixed while voltage V2 regulates the upper sideband and the right transmission zeroThe results of the simulation, depicted in Fig.6, demonstrate the effect of gradually increasing voltage V2 from 6 V to14 V while voltage V1 is held constant at 7 V. As voltage V2 increases, the upper sideband of the passband widens towardsthe right and the passband bandwidth increases from 651 MHz to 1129 MHz. The return loss S11 in the passband remainsbelow 10 dB during the tuning process, and the right side transmission zero point shifts from 3.18 GHz to 3.66 GHz at thehigh-frequency end. It can be inferred that voltage V2 has the ability to control the upper sideband of the passband and thetransmission zeros on the right side, leading to an expansion of the bandwidth in that direction.Fig.6 Bandwidth varies with V23.2 Voltage V2 is fixed while voltage V1 regulates the lower sideband andthe left transmission zero.The decrease in voltage V1 from 15V to 6V, while voltage V2 is fixed at 10V, results in a change in the S21 and S11curves as depicted in Fig.7. As voltage V1 decreases, the lower sideband of the passband expands towards the left, leading toan increase in the passband bandwidth from 764 MHz to 1099MHz. Additionally, the S11 value remains above 10 dB, and thetransmission zero point on the left side shifts from 3.17 GHz to 3.55 GHz towards the right. It can be inferred that voltage V1has the ability to regulate the lower sideband of the control passband and the transmission zeros on the left side, leading to aVolume 10 Issue 5-187-widening of the bandwidth towards the left.Fig.7 Bandwidth varies with V13.3 The simultaneous alteration of voltages V1 and V2 serves to regulate thecenter frequency.Fig.8(a) illustrates the relationship between S21 and S11 and the simultaneous changes in voltages V1 and V2. Thevariation in the filter center frequency and passband bandwidth is observed as the values of V1 and V2 are altered. Forinstance, when V1=15 V and V2=15 V, the center frequency is 2.79 GHz and the bandwidth is 994 MHz, whereas, whenV1=10 V and V2=10 V, the center frequency is 2.71 GHz and the bandwidth is 977 MHz . A further change in the values ofV1 and V2 to V1=7 V and V2=8 V results in a center frequency of 2.62 GHz and a constant bandwidth of 989±10 MHz . Thisindicates that by adjusting the values of voltages V1 and V2, a wideband filter with an adjustable center frequency and aconstant bandwidth can be achieved.Fig.8(b) demonstrates the impact of voltages V1 and V2 on the passband bandwidth. For instance, when V1=15 V andV2=5 V, the minimum bandwidth is 431 MHz with a maximum interpolation loss of 1.53dB and a return loss S11 better than15dB. On the other hand, when V1=6 V and V2=13 V, the widest bandwidth is 1119 MHz with a maximum interpolation lossof 1.16dB and a return loss S11 better than 11dB.(a) Effect of V1 and V2 on the passband bandwidth (b) Effect of V1 and V2 on the center frequencyFig.8 Effect of V1 and V2 on the frequency response4. ConclusionIn this paper, an innovative bandwidth tunable optical filter with controllable bandwidth, center frequency, andtransmission zero is proposed. The bandwidth, center frequency, and transmission zero can be controlled by adjusting thevoltage of the varactor diodes, respectively. According to the simulation results, the measured insertion loss is less than 2 dB,the return loss exceeds 10 dB, and the relative bandwidth reaches 40%.References[1] Helaoui M, Boumaiza S and Ghannouchi FM, "Reconfigurable platform for software/hardware co-design of SDRbase band pre-processing module", International Symposium on Antenna Technology and Applied Electromagnetics andCanadian Radio Sciences Conference, pp. 1-4, Jul. 2006.[2] Chen R. and Hashemi H, "Reconfigurable SDR receiver with enhanced front-end frequency selectivity suitable for-188-Electronics Science Technology and Applicationintra-band and inter-band carrier aggregation", IEEE International Solid-State Circuits Conference (ISSCC), pp. 1-3, Feb.2015.[3] Wünsche R., Collmann R., Krondorf M and Forster J, "Microstrip Combline Bandpass Filter with Tuning RangeEnhancement and Bandwidth Tunability Using Resonator Loaded Series Varactor and SLR," 2022 14th German MicrowaveConference (GeMiC), Ulm, Germany, 2022, pp. 148-151.[4] Lin F and Rais-Zadeh M, "Continuously Tunable 0. 55-1. 9-GHz Bandpass Filter With a Constant Bandwidth UsingSwitchable Varactor-Tuned Resonators[J]", IEEE Transactions on Microwave Theory and Techniques, vol. 65, no. 3, pp.792-803, 2017.[5] Kim BK and Yun SW, "Varactor-tuned combline bandpass filter using step-impedance microstrip lines", IEEETransactions on Microwave Theory and Techniques, vol. 52, no. 4, pp. 1279-1283, 2004.[6] Sanchez-Renedo M and Gomez-Garcia R, "Small-size planar tunable combline filter using decoupling walls",Electronics Letters, vol. 43, pp. 532-534, 02 2007.[7] Zhu D, "Research on miniature multimode filter[D]". Nanjing: Nanjing University of Science and Technology, 2015.

Bandwidth Tunable Optical Filter Based on the Quad-Mode Resonator

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Bandwidth Tunable Optical Filter Based on the Quad-Mode Resonator

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