A miniaturized UWB bandpass filter with tunable cut-off frequencies employing rectangular open-loop defected ground structure

Abstract

This article presents a compact rectangular impedance resonator (RIR)–based ultra-wideband bandpass filter with an improved reflection coefficient within the passband and controllable cut-off frequencies. The proposed filter consists of anti-parallel coupled RIR coupled with rectangular open-loop defected ground structure (ROL-DGS) etched on the ground plane. The ROL-DGS ensures better in-band and out-off-band characteristics with reduced size due to its high Q-factor. The short- and open-circuit stubs control the lower (f_CL) and upper (f_CU) 3-dB cut-off frequencies, respectively, by originating transmission zeros independently with the help of variations in design parameters. As a result, the filter having a compact size of 0.216λ_CL × 0.176λ_CL is achieved, where λ_CL is the corresponding guided wavelength at the lower cut-off frequency f_CL of 2.95 GHz. Further, an equivalent circuit is also modeled to explain the working of the proposed filter, and equivalent lumped parameters were extracted. Simulated results reveal that the reported filter passes the frequencies effectively within 2.95–11.44 GHz with fractional bandwidth of 117.9%. The simulated insertion loss values within the passband are less than 1.17 dB, and the reflection coefficient is better than −19.6 dB. Further, the proposed filter is fabricated, and it was found that the measured and simulation results are in well agreement.

Introduction

In the world of digitization, there is a need to procure secure wireless services at high data rates and with robust performance against interference for short-range wireless communication. Ultra-wideband (UWB) technology proffers a suitable environment for such wireless transmission and reception. In February 2002, the US Federal Communications Commission (FCC) authorized the usage of UWB (3.1–10.6 GHz) for commercial purposes. This offers a driving force in developing short-range and long-range UWB technology [1]. A bandpass filter (BPF) is an integral part of the front end to regulate FCC mask emission. In literature, three different design methodologies were reported, including the cascade approach, multi-mode resonator (MMR) approach, and MMR with defected ground structure (DGS)–based UWB BPF to retain passband characteristics [2–17, 20–23]. Conventional cascading of lowpass and highpass filters was adopted to the construct UWB BPF, which yielded good selectivity, but it exhibited a larger electrical size [2–4]. After this, MMR with modified designs, such as rectangular open stub [5], U-shaped stub [6], circular stub [7], funnel-shaped stub [8], split ring resonator [10], fractal tree stub [11], tapered stub [12], were proposed in the literature to enhance passband characteristics by generating and tuning multiple modes. In order to improve passband selectivity and stopband characteristics, modified MMR with DGS-based lowpass filter is discussed in [13]. Furthermore, wide stopband suppression up to 100 GHz is achieved in [14] and [15] by using five-stage stepped impedance resonator and half-mode substrate integrated waveguide, respectively. Moreover, modified stubs were utilized to govern resonant modes within the passband and to improve the stopband suppression level. After reviewing related literature, it has been conceived that more attention is required on the structures that control the resonant modes within passband. There is also a need to control the 3-dB lower (f _CL) and upper (f _CU) cut-off frequencies independently while maintaining the compactness.

In this paper, a simple compact UWB BPF was proposed with the help of rectangular impedance resonator (RIR) and rectangular open-loop DGS (ROL-DGS). Further open- and short-circuit stub is integrated with the feed line in order to regulate the transmission zeros and 3-dB cut-off frequencies. DGSs are employed at the ground plane to sustain wideband characteristics by perturbate surface current distribution, and it offers a configurable resonant mode within passband. The frequency response of the proposed UWB BPF is analyzed through LC equivalent circuit model and verified using parameter extraction. The simulated result offers a return loss greater than 19.26 dB within passband with compact size of 0.216λ_CL × 0.176λ_CL at lower cut-off frequency (f _CL).

The proposed work is organized as follows: “Design evolution theory and geometry” section briefs over the detailed design evolution and its geometry. “Simulation results and analysis” section presents the modeling of electrical equivalent circuit. Parametric study and visualization of surface current distribution is also carried out in this section. “Results and discussion” section presents the experimental results followed by its discussion, and the “Conclusion” section concludes this work.

Design evolution theory and geometry

Design evolution

This section elaborates the structural design evolution of the proposed UWB BPF. Four self-originated configurations with their corresponding scattering parameters are shown in Fig. 1. Initially, an anti-parallel coupled RIR is fed by 50 Ω microstrip line designated as Case 1, shown in Fig. 1a. It offers a wider bandwidth and deeper null at lower frequency, but it exhibits poor in-band (bandwidth and insertion loss within passband) and out-off band characteristics (stop band suppression).

Figure 1. (a) Evolution steps of the proposed UWB BPF; (b) transmission coefficient and (c) reflection coefficient for all five cases; and (d) S-parameter response for Case 5 (proposed).

In order to improve in-band characteristics, ROL-DGS is etched on the ground plane. Due to higher Q-factor of U-shaped DGS, it presents a strong coupling with RIR and provides better in-band and out-off band characteristics with small circuit size. This structure is called as Case 2. As compared to Case 1, it offers an improved 3-dB bandwidth ranging from 3.81 to 12.18 GHz. But at the same time, it exhibits poor insertion loss within passband, and poor selectivity at upper band-edge frequencies can be observed from Fig. 1b.

Further two short-circuit stubs with ROL-DGS are shunted symmetrically to microstrip feed line with a goal of improving selectivity near lower band-edge frequencies (f _CL) and tuning the position of TZ1 (transmission zeros) without altering the position of TZ2. This is named as Case 3. Figure 1b depicts filter characteristics as it offers better selectivity at lower 3-dB band-edge frequencies and insertion loss within passband. With motive to enhance selectivity near upper 3-dB band-edge frequency (f _CU), two open stubs with ROL-DGS are shunted with microstrip feed line. This filter structure is designated as Case 4. It exhibits an improved selectivity near f _CU and poor insertion loss within passband.

Finally, the desired outcome can be extracted by taking union of both the designs, Case 3 and Case 4. Eventually, the final proposed filter, shown as Case 5, offers a 3-dB passband range from 2.95 to 11.44 GHz with 3-dB fractional bandwidth (FBW) of 117.9% at 7.195 GHz of center frequency.

The obtained average simulated insertion loss is less than 1.17 dB and return loss is greater than 19.27 dB within passband. The attenuation poles are located at f _p1 = 4 GHz, f _p2 = 6.88 GHz, f _p3 = 8.8 GHz, and f _p4 = 10.7 GHz, while the transmission zeros are located at TZ1 = 0.1 GHz, TZ2 = 13.32 GHz, TZ3 = 14.06 GHz, and TZ4 = 17.29 GHz as shown in Fig. 1d. Moreover, the proposed UWB BPF exhibits better wide stop band suppression 20.69 dB up to 2.38f _o. Case 5 propounds better in-band and out-off-band characteristics and gives more degree of freedom to control the 3-dB bandwidth by optimizing the length, width, and position of SC (lower 3-dB cut-off frequency) and OS (upper 3-dB cut-off frequency) stubs as discussed in the “Parametric analysis” section.

Geometry of the proposed filter

The proposed BPF comprises RIR, coupled in anti-parallel fashion and fed by 50 Ω impedance line on the top, while open- and short-circuit stubs are integrated symmetrically with the feed line at both port 1 and port 2 as shown in Fig. 2a. ROL-DGS is etched on the bottom plane as exhibited in Fig. 2b. The proposed filter is realized using Rogers RT/Duriod 5880 substrate with relative dielectric constant ε_r = 2.2, thickness 0.787 mm, and loss tangent tan δ = 0.0009.

Figure 2. Geometrical layout of the proposed UWB BPF: (a) top view and (b) bottom view.

The characterization of the proposed prototype is accomplished using full-wave Electromagnetic (EM) simulator, high-frequency structure simulator ANSYS Electronic Desktop 2019, and the parameters are as follows (all dimensions are in millimeter): l ₁ = 7.3, l ₂ = 5.1, l ₃ = 3.87, l ₄ = 2.8, l ₅ = 1, W ₁ = 2.42, W ₂ = 3.5, W ₃ = 0.2, W ₄ = 1, W ₅ = 1.4, D ₁ = 7.4, D ₂ = 4.8, D ₃ = 2.1, Dw ₁ = 0.2, Dw ₂ = 0.4, Dw ₃ = 0.4, g = 0.2, and d = 0.25.

Simulation results and analysis

Equivalent circuit

The equivalent circuit model of the proposed UWB BPF with its corresponding distributed counterpart is illustrated in Fig. 3. In order to obtain the intuitive equivalent circuit model, an extensive literature survey [16–20] for several microstip-based UWB filters have been carried out. It was conceived that in order to get a UWB response, several closely spaced linked resonant modes should be aroused, and in order to get a compact structure, the least number of resonant modes should be used. We have selected four modes as optimal number for producing the UWB response with a compact structure. From a circuit perspective, multiple resonant circuits are merged together to form a wider bandwidth. In the equivalent circuit, coupled RIR is modeled by the two combination of inductance L ₂ and C ₆, whereas C ₁ represents the capacitive coupling. ROL-DGS in combination with coupled RIR plays a key role in the proposed filter structure, which can be demonstrated by the parallel combination among inductance L ₆ and series connection of inductance L ₇ and capacitance C ₄. Series inductance L ₁ raised due to the current induced in both of the 50 Ω microstrip feedline. Shunt capacitances C ₅ and C ₆ are associated with the corresponding coupling capacitances between top and bottom layer in the specified region where RIR and feed lines are present. Further, open- and short-circuit stubs are integrated to improve the band-edge selectivity. In shunt, the short-circuit stub is represented by parallel tank circuit formed by inductance L ₃ and capacitance C ₂. Open-circuit stub can be attributed as parallel combination of inductance L ₅ and capacitance C ₃ in series with inductance L _4.

Figure 3. Equivalent LC circuit model of the proposed UWB BPF.

With the help of Advanced Design System software, the LC equivalent circuit model is validated. Lumped component values are extracted and optimized to meet the proposed UWB BPF characteristics, which are as follows: L ₁ = 0.035 nH, L ₂ = 0.527 nH, L ₃ = 19.96 nH, L ₄ = 0.6273 nH, L ₅ = 1.5659 nH, L ₆ = 0.24 nH, L ₇ = 0.0106 nH, C ₁ = 0.6759 pF, C ₂ = 0.157 pF, C ₃ = 0.2959 pF, C ₄ = 5.985 pF, and C ₅ = C ₆ = 0.01 pF. Further, the variation of S-parameter response for circuit and EM simulation is shown in Fig. 4. Although it offers small deviation, it follows the trend. It can be used for better interpretation of transmission and reflection characteristics of the filter.

Figure 4. Variation of S-parameters of the proposed UWB BPF obtained through EM and circuit simulation.

Parametric analysis

This section presents the geometrical parameter variation effect of SC stub length (l ₄), SC stub width (W ₄), RIR length (l ₂), OS length (l ₃), OS width (W ₃), and vertical length of ROL-DGS (D ₂) on filtering characteristics of the proposed UWB BPF namely transmission poles and zeros position, f _CL, f _CU, and FBW as shown in Fig. 5.

Figure 5. Variation of (a) resonating modes with change in short-circuit stub length l ₄, (b) lower 3-dB cut-off frequency (f _CL) with change in short-circuit stub widthW ₄, (c) upper 3-dB cut-off frequency (f _CU) with change in open stub width W _3, (d) upper 3-dB cut-off frequency (f _CU) and FBW with change in open stub length l ₃, (e) frequency and FWB variation with change in stub length l ₂, and (f) resonating modes with change in D ₂.

The variation of inductance L ₃ emerged from SC stub length l ₄, which yields a change in position of poles. As the length increases, its inductance value also increases, and it leads to change in resonating modes offered within passband. Transmission poles f _p1 and f _p2 shift toward lower side, while f _p3 and f _p4 split as the stub length l ₄ varies from 2.2 to 3.4 mm as shown in Fig. 5a.

Furthermore, the capacitance (C ₂) decreases as the SC stub width W ₄ changes from 0.2 to 1.5 mm, which results in the shifting of f _CL toward lower side as displayed in Fig. 5b, while f _UL remains unchanged and the overall 3-dB bandwidth decreases. In regard to SC stub width, it offers an independent control over the variation of 3-dB lower cut-off frequency (f _CL). The capacitance C ₃ decreases as the OS width (W ₃) varies from 0.2 to 0.6 mm. From Fig. 5c, it can be observed that the position of f _CU shifted toward lower side and the overall 3-dB bandwidth decreased as W ₃ increased. The change in length l ₃ (open stub length) is investigated and shown in Fig. 5d, which leads to varied inductance L ₅ in the equivalent circuit. As the length l ₃ varies from 3.57 to 4.27 mm, its associated inductance L ₅ value increases and upper cut-off frequency (f _CU) shifts toward lower side, while the overall FBW decreases. The design parameter l ₂ (RIR length) plays a vital role in designing UWB BPF. It exhibits rise in inductance L ₂ as the length varies from 3.8 to 6.2 mm and current concentration decreases, which leads to shift f _CL and f _UL toward lower side with increase in FBW as can be seen from Fig. 5e. Furthermore, inductance L ₆ offered by vertical ROL-DGS length D ₂ sustain different resonating modes within passband. As length D ₂ varies from 3.8 to 5.8 mm, transmission zeros TZ2 and TZ3 merged into each, while transmission poles f _p1 and f _p2 split to form two different resonating modes as shown in Fig. 5f.

Study of current distribution

To understand the operating behavior of existing resonant frequency within passband, simulated surface current distribution at 4, 6.8, 8.8, and 10.7 GHz with both top and bottom view of the proposed UWB BPF is shown in Fig. 6(a)–(d), while Fig. 6(e) and (f) depicts the current distribution in stop band region with transmission zeros generating at 13.3 and 14.05 GHz. Most of the current is concentrated near port 1, and negligible current reaches from port 1 to port 2. As can be seen from Fig. 6a, the current is mainly concentrated at vertical outer edges of RIR, SC stub, and upper side of DGS near open loop and resonate at 4 GHz of frequency. It is observed that at 6.8, 8.8, and 10.7 GHz, current is concentrated at the middle edge of RIR, OS, and lower middle of the horizontal section of ROL-DGS, and no current is reaching toward SC stub.

Figure 6. Surface current distribution on top and bottom of the proposed filter at (a) 4 GHz, (b) 6.8 GHz, (c) 8.8 GHz, (d) 10.7 GHz, (e) 13.3 GHz, and (f) 14.03 GHz.

Results and discussion

For experimental validation, the proposed UWB BPF is fabricated and the photograph is displayed in Fig. 7. The overall filter size is only 0.216λ _CL × 0.176λ _CL (14.85 mm × 12.12 mm) at lower 3-dB cut-off frequency (f _CL) of 2.95 GHz, where λ _CL denotes guided wavelength at lower 3-dB cut-off frequency. Scattering characteristics and group delay are measured on Anritsu made vector network analyzer S820E. The experimental results of the proposed filter are compared with EM simulation results and manifested in Fig. 8.

Figure 7. Photograph of the proposed fabricated UWB BPF: (a) top view and (b) bottom view.

Figure 8. Comparison of the simulated and measured (a) S-parameters and (b) group delay of the proposed UWB BPF.

The measured 3-dB bandwidth ranges from 2.78 to 11 GHz. Within passband, insertion loss varies from 0.5 to 2.5 dB while the return loss is better than 14.84 dB. Measured harmonic suppression in upper stopband is better than 21.4 dB upto 2.4f ₀. Meanwhile, the simulated and measured group delay extends from 0.19 to 0.63 ns and 0.29 to 0.67 ns, respectively, which is considered to be flat within passband as illustrated in Fig. 8b. The experimental results are approximately in good agreement with simulated results of the proposed UWB BPF. Due to fabrication tolerance, SMA connector losses, and undesirable soldering losses, some discrepancies exist between simulated and measured results.

The performance of the proposed work is compared with recent relevant works reported in the literature and is summarized in Table 1. It reveals that the proposed filter is more compact as the electrical size is least among all the works. It is important to note that the electrical size is recalculated for each of the work in terms of their guided wavelength corresponding to the lower cut-off frequency for the sake of uniformity. Moreover, the proposed filter exhibits wider bandwidth with respect to the other relevant works. The presented filter also has four transmission zeros, which is greater than or equal to the transmission zeros as reported by other compared works. The roll-off rate or selectivity can be defined as in [21]:

(1)\begin{equation}\varepsilon = {{{\alpha _{{\textrm{max}}}} - {\alpha _{{\textrm{min}}}}} \over {{f_{\textrm{s}}} - {f_{\textrm{c}}}}},\end{equation}

Table 1. Comparison of the proposed UWB BPF performance with other earlier reported similar work

Note: λ _CL = guided wavelength at lower 3-dB cut-off frequency, f ₀ = passband center frequency.

FBW, fractional bandwidth; RL, return loss; IL, insertion loss; USB, upper stopband; and NA, not available.

where α _max and α _min are the 20 and 3 dB attenuation points, respectively, f _s is the 20 dB stop band frequency and f _c is the 3-dB cut-off frequency.

Comparison table suggest that the work proposed in [5, 10, 16] and [27] shows improved selectivity with the proposed work but at the cost of larger electrical size. The proposed filter also features the configurable lower and upper 3-dB cut-off frequencies, which proclaims the novelty of the proposed work. Owing to the above characteristics, the proposed filter may be a good candidate to be employable in wireless devices for UWB communication applications.

Conclusion

A compact UWB BPF is designed and simulated, which exhibits two degrees of freedom for guiding both lower and upper 3-dB cut-off frequencies independently by OS and SC stubs (two transmission zero). ROL-DGS yields to offer different number of modes (transmission poles) within passband by perturbating current distribution (by tuning its dimension) on ground plane. Lumped equivalent associated with distributive microstrip line helps to analyze the filter characteristics. The design evolution extracts the effect of both SC and OS stub with ROL-DGS. The proposed UWB BPF is of wider bandwidth, improved return loss within passband, and compact size makes the filter suitable for UWB wireless technology as per US FCC.