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A compact S-band band-pass filter with ultra-wide stopband

  • Qian-kun Xu , Zhong-nan Zhang , Xiao-hua Wu , Jue-zhe Wang and Lin Peng ORCID logo EMAIL logo
Published/Copyright: May 31, 2022
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Frequenz
From the journal Frequenz Volume 77 Issue 1-2

Abstract

A novel compact S-band band-pass filter (BPF) with ultra-wide stopband is presented. The proposed filter mainly consists of three parts, a novel crossed resonator, parallel-coupled microstrip lines, and defect ground structures (DGSs). The resonator is constructed by three capacitive open stubs, while a defect is etched in the center stub to eliminate high frequency harmonics. The parallel-coupled microstrip lines are mainly used to improve the passband and the roll-off rate (the ratio of height to width of the filter transition band). And the DGSs are utilized to improve the stopband performances. The proposed BPF has the merits of compactness, good insertion loss and good stopband characteristic. The measured 3 dB operating band is 1.80–4.03 GHz (76.5% at f 0 = 2.92 GHz), The upper out-of-band suppression was better than 20 dB for the frequency up to 20 GHz (6.33f 0), and the upper roll-off rate reaches 75.59 dB/GHz.

Keywords: bandpass filter; high rejection level; S-band; ultra-wide stopband

1 Introduction

The S-band regime has widely applications, such as satellite communication, surveillance radar, and meteorological radar, etc. The developing of these systems requires S-band filters with higher performances, such as the characteristics of wide passband, ultra-wide stopband with high/wide rejection features, and compactness. These characteristics are critical in engineering applications.

A microstrip band-pass filter (BPF) can be designed by three parts, coupling lines, resonators, and a metal ground plane [1], [2], [3], [4]. These constitutions usually form a BPF with the characteristics of compactness, good stop/pass-bands, high reliability and easy implementation [5], [6], [7]. Nevertheless, to further improve the roll-off rate, the width of the stopband and the out-of-band rejection level, the developing of the three parts of a filter is required. Firstly, several novel coupling line structures are used to improve the insertion loss (IL) and out-of-band suppressions [8], [9], [10], [11], [12], [13]. By comparing with these filters, it can be seen that the proposed filter has performance advantages. For example, the filter in [8], improves the insertion loss (IL) by triple coupled lines, where the IL is 1.2 dB, and yet the proposed filter is superior in terms of 3 dB fractional bandwidth (FBW), IL, suppression, roll-off rate and compactness. In [9], by using bent and couple-line structure, the filter has an excellent IL < 0.4 dB, Nevertheless, the 3 dB fractional bandwidth (48%) is relatively small. The coupling line in [11] is a multiconductor transmission-line (MTL) structure with short-circuited lines, then, a wide upper stopband with a reject level greater than 15 dB from 5.2 to 8.5 GHz can be obtained, and the first transmission zero reaches 30 dB at 5.4 GHz. In [12], the filter consists of two partially coupled lines separated by a cross resonator, the out-of-band suppression and passband are improved by the strong coupling effect of the gap between the coupling lines, the structure can achieve a 3 dB fractional bandwidth of about 66% at the center frequency of 3 GHz, and the first transmission zero reaches 50 dB at 5 GHz. Secondly, several structures based on improved microstrip line resonators are proposed in [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. The filter in [15] achieves high selectivity by a stepped impedance open-stub loaded ring resonator, where the upper roll-off rate is about 117.39 dB/GHz, and yet the 3 dB fractional bandwidth (58.3%) and size (0.34 λ g × 0.34 λ g) are inferior to the proposed filter. In [17], the filter improves the roll-off rate by a multiple-mode resonator that consisting of the open and short stubs, where the roll-off rate is 78 dB/GHz. In [19], by incorporating the microstrip line resonators into quarter-mode substrate integrated waveguide (QMSIW) structure, the filter has a high/wide stopband rejection from 12.64 to 20 GHz. The third approach is DGS by etching various patterns in the ground plane. The DGS structure functions as a resonator. It can be seen from [26], [27], [28], [29], [30], the loading of DGSs can improve the out-of-band suppression and roll-off rate. For example, in [30], the filter achieves excellent performances by a strip-loaded slot-line structure, where the upper out-of-band suppression was better than 20 dB to reach 6.5f 0, nevertheless, the proposed filter has superior roll-off rate with similar suppression, which is an important indicator of the filter in engineering applications.

In this letter, a novel S-band BPF filter is proposed to fulfil the requirements of compactness, wide passband, ultra-wide stopband with high/wide rejection. The measured results agree with the simulated results, which verify the design.

2 Analysis and design

The configuration of the proposed BPF is shown in Figure 1. The filter consists of three parts, a novel crossed resonator that comprised of three capacitive open stubs in the middle, parallel-coupled microstrip lines (via to the ground), and DGSs. It is fabricated on Rogers RT/duroid 6010 substrate with a thickness of 1.27 mm and a relative dielectric constant of 10.6. The proposed filter occupies a small area of 21.5 × 6.8 mm, which is approximately corresponded to 0.215 λ 0 × 0.068 λ 0, where λ 0 is the free space wavelength of the center frequency.

Figure 1: Configuration of the proposed BPF.
Figure 1:

Configuration of the proposed BPF.

2.1 The evolution of the proposed filter

To reveal the mechanisms of the out-of-band suppression techniques, the four-step structural evolution from Filter 1 to Filter 4 is shown in Figure 2(a), which also properly display the design concept of the proposed filter. The simulated frequency responses of the structures are given in Figure 2(b) and (c). As shown in Figure 2(a), the Filter 1 is the original structure, it is mainly composed of three parts, the feedlines, the parallel coupling lines, and a capacitive open stub resonator. Two stubs of the coupling lines are shorted to the ground to introduce transmission zeros at each edge of the passband [12]. It must be pointed out, the length of the stubs should be l < λ/4. From Figure 2(b) and (c) the Filter 1 has a good relative pass bandwidth of 66%, however, the stopband has obvious harmonics, and the suppression level cannot fulfill many practical applications. Therefore, four DGSs are etched in the metal ground to form the Filter 2. The DGSs can be equivalent to shunt resonant circuits for the purpose of out-of-band suppression improvement and structure compactness. As shown in Figure 2(b) and (c), the overall out-of-band harmonics, the inhibition depth and the rejection level of the BPF are improved greatly by the DGS structures. In the Filter 3, two uniform-impedance narrow stubs with length l < λ/4 are inserted into the open stub resonator. The narrow stubs improve the harmonic suppression level at 9.3 GHz from 25.94 to 41.45 dB. To further suppress the harmonics of the stopband, a rectangular slot is etched in the middle open stub, which forms the proposed filter, the Filter 4. As shown in the figure, the suppression level at 9.3 GHz is increased from 41.45 to 60.22 dB compared with filter 3. Meanwhile, the upper out-of-band suppression from 5.33 to 6.75 GHz and 7.27–10 GHz is better than 57.23 dB for the range up to 1.42f 0 (the ratio of bandwidth to center frequency f 0 for the part greater than 57.23 dB), where f 0 is 2.92 GHz. The geometrical dimensions of proposed BPF are shown in Table 1.

Figure 2: Four steps of the final filter design from Filter 1 to Filter 4. (a) Structural change process. (b) Simulated S21 curves. (c) Simulated S11 curves.
Figure 2:

Four steps of the final filter design from Filter 1 to Filter 4.

(a) Structural change process. (b) Simulated S21 curves. (c) Simulated S11 curves.

Table 1:

Physical dimensions of the BPF (unit: mm).

L 1 L 2 L 3 L 4 L 5 L 6 L 7 W 1 W 2
2.8 3.35 3.2 10 0.9 2.1 8.9 1.3 0.2
W 3 W 4 W 5 W 6 g 1 g 2 g 3 g 4 D
0.1 0.45 1.3 0.8 0.2 0.7 0.2 1 0.8

To investigate the effect on the center frequency and bandwidth if the substrate thickness is changed, the simulated frequency responses of the structures are given in the following Figure 3(a) and (b) by varying the substrate thickness h. To facilitate observation of the variation in center frequency and bandwidth, the performances comparison resulting from different thicknesses of h is shown in the following Table 2. As shown in Figure 3 and Table 2, the center frequency and bandwidth change as the thickness of the dielectric substrate is changed. The bandwidth increased significantly with the increasing of h.

Figure 3: S-parameters for varying substrate thickness h. (a) Simulated S21 curves. (b) Simulated S11 curves.
Figure 3:

S-parameters for varying substrate thickness h.

(a) Simulated S21 curves. (b) Simulated S11 curves.

Table 2:

Performances comparison for varying substrate thickness h.

h (mm) 0.9 1.0 1.1 1.2 1.3
f 0 (GHz) 3.14 3.16 3.15 3.13 3.14
Bandwidth (GHz) 2.07 2.17 2.24 2.30 4.35
FBW (%) 66.03 68.78 71.11 73.48 74.96

2.2 Theoretical analysis

To reveal the operating mechanisms of the proposed filter, an equivalent circuit model is derived and shown in Figure 4. A DGS structure can be equivalent to a L 1 C 1 resonator. According to the transmission line theory, the input impedance of the short-ended transmission line with the length l < λ/4 has the property of inductance, while an open-ended transmission line with the length l < λ/4 has the property of capacitance. Therefore, the parallel-coupled microstrip lines are equivalent to shunt L 2 C 2 resonators. The stub inductance and the coupling between the parallel coupling lines and open stub resonator form the series L 3 C 3 resonators. In the open stub resonator, the shunt L 4 C 4 resonators are formed due to the high impedance line and the coupling effect between three capacitive open stubs, Meanwhile, the open stubs are equivalent to shunt capacitors C 5 and C 6 to the ground.

Figure 4: Equivalent circuit of the proposed BPF. The optimized circuit parameters are: L 1 = 6.0094 nH, L 2 = 5.001 nH, L 3 = 0.36766, L 4 = 1.4237 nH, C 1 = 0.41364 pF, C 2 = 0.2505 pF, C 3 = 1.4237 pF, C 4 = 0.42264 pF, C 5 = 0.611 and C 6 = 2.3456 pF.
Figure 4:

Equivalent circuit of the proposed BPF. The optimized circuit parameters are: L 1 = 6.0094 nH, L 2 = 5.001 nH, L 3 = 0.36766, L 4 = 1.4237 nH, C 1 = 0.41364 pF, C 2 = 0.2505 pF, C 3 = 1.4237 pF, C 4 = 0.42264 pF, C 5 = 0.611 and C 6 = 2.3456 pF.

The short-ended transmission line with the length l < λ/4 is equivalent to an inductor, and the inductance is proportional to the characteristic impedance of the transmission line (assuming that the transmission line has no loss) as indicated in the formula:

(1) Z i n ( l ) = j Z c   tan   θ

where, Z c is the characteristic impedance of the transmission line, θ = βl = 2πl/λ is the electric length of the transmission line. The equivalent inductance can be derived as:

(2) L = Z c   tan   θ ω = Z c tan ( 2 π l λ ) ω ,   ( l < λ / 4 )

The terminal open-end transmission line with the length l < λ/4 can be used as a capacitor, i.e.:

(3) Z i n ( l ) = − j Z c   cot   θ

The equivalent electric capacity is:

(4) C = Y c tan θ ω = Y c tan ( 2 π l λ ) ω ,   ( l < λ / 4 )

The inductors and the capacitors of the shorted and open stubs can be evaluated by equations (2) and (4), respectively. The initial parameter value of the other capacitances and inductances are derived following [31, 32]. Then, the equivalent circuit model can be derived.

3 Simulated and measured results

The proposed BPF was fabricated as shown in Figure 5(a). The filter was measured by a vector network analyzer, Keysight N9918A. The simulated and measured results are presented in Figure 5(b) and (c). The measured operating band is 1.80–4.03 GHz with a 3 dB fractional bandwidth (FBW) of 76.5%, which covers the whole S-band (2–4 GHz). The measured typical passband insertion loss (IL) is 1.09 dB, while the passband return loss is higher than 11.24 dB. The first transmission zero is obtained at 5.04 GHz, and it reaches −76.35 dB. The upper out-of-band suppression was better than 20 dB for the range up to 20 GHz (6.33f 0), while the upper roll-off rate reaches 75.59 dB/GHz. Moreover, the measured and simulated group delays are 0.62 and 0.77 ns at the center frequency (f 0 = 2.92 GHz). The measurements are in good agreement with the simulations, and the discrepancies could be due to machining accuracy and SMA coaxial welding.

Figure 5: (a) Photograph of the proposed BPF. (b) Comparison of S parameters between the simulation and measures. (c) Comparison of Group delay between the simulation and measures.
Figure 5:

(a) Photograph of the proposed BPF. (b) Comparison of S parameters between the simulation and measures. (c) Comparison of Group delay between the simulation and measures.

Performance comparison between the proposed BPF and the state-of-the-art filters is shown in Table 3, which show that the proposed BPF has the best stopband width and compactness to the references, while the insertion losses are similar. It should be pointed out that since the insertion loss (IL) and suppression are not given in [4] and the upper roll-off rate is not given in [14], Hence, comparisons with the other performances of these two filters. it can be seen that the filter in [4] has an excellent FBW, but the roll-off rate and compactness are inferior to the proposed filter. And the proposed filter has superior FBW, IL, and compactness compared to [14] with similar suppression. Moreover, the proposed design is superiority in terms of FBW and roll-off rate than [12, 19, 25, 26]. Although the structures in [11, 17] have better FBW and Group Delay, the proposed design has a greater advantage in terms of suppression and compactness. The roll-off rate of [27] is relatively higher, Nevertheless, the proposed design has better performances in terms of insertion loss (IL), FBW, suppression and compactness.

Table 3:

Comparison with the state-of-art filters.

Ref. f 0 (GHz) FBW (%) IL (dB) Suppression Upper roll-off rate (dB/GHz) Area (λ 0 2) Group delay variation (ns)
[4] 2.5 200 31.79 0.083
[11] 3.5 84 0.6 15 dB@0.94f 0 52 0.027 0.23–0.70
[12] 3 66 1.0 20 dB@1.5f 0 50 0.017 0.60–1.20
[14] 0.350 13.7 1.6 19.4 dB@6.86f 0 0.0819
[17] 7.1 117 3 20 dB@1.28f 0 78 0.1603 0.25–0.70
[19] 10 22.7 1.2 36 dB@2f 0 42.61 0.3844
[25] 2.45 22.8 1.1 20 dB@2.04f 0 41.94 0.283 4.8–5.70
[26] 3.08 45 0.6 20 dB@5.8f 0 17.35 0.024
[27] 1.5 6 2.4 30 dB@1.55f 0 83.02 0.12
Pro. 2.92 76.5 1.09 20 dB@6.33f 0 75.59 0.0146 0.32–0.74

4 Conclusions

In this letter, a novel compact BPF with high selectivity and stopband rejection level is proposed. Firstly, the structural design principle and theoretical analysis of the proposed filter are discussed. Then, the proposed filter is verified by simulations and measurement. The results show that the proposed filter has excellent performances by comparing with the recently published filters. The proposed filter can cover the entire S-band frequency range, and the excellent performances make it a good candidate for communications and radars.

Corresponding author: Lin Peng, Guangxi Key Laboratory of Wireless Wideband Communication and Signal Processing, Guilin University of Electronic Technology, Guilin, 541004, China, E-mail:

Funding source: Natural Science Foundation of Guangxi Province

Award Identifier / Grant number: 2021JJA170177

Funding source: Innovation Project of GUET Graduate Education

Award Identifier / Grant number: 2022YCXS040

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: This work was supported in part by Guangxi Natural Science Foundation under Grant No. 2021JJA170177, and in part by Innovation Project of GUET Graduate Education under Grant No. 2022YCXS040.

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Received: 2021-11-09
Accepted: 2022-05-11
Published Online: 2022-05-31
Published in Print: 2023-01-27

© 2022 Walter de Gruyter GmbH, Berlin/Boston

Articles in the same Issue

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  2. Research Articles
  3. Compact low-pass filter (LPF) with wide harmonic suppression using interdigital capacitor
  4. Design and analysis of a multiple notched UWB-BPF based on microstrip-to-CPW transition
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https://doi.org/10.1515/freq-2021-0278
Keywords for this article
bandpass filter; high rejection level; S-band; ultra-wide stopband

Articles in the same Issue

  1. Frontmatter
  2. Research Articles
  3. Compact low-pass filter (LPF) with wide harmonic suppression using interdigital capacitor
  4. Design and analysis of a multiple notched UWB-BPF based on microstrip-to-CPW transition
  5. A compact S-band band-pass filter with ultra-wide stopband
  6. Design and fabrication of an ultra compact Gysel power divider with harmonic suppression by using U shaped resonators
  7. A high angle stable and polarization symmetric dual band reconfigurable frequency selective surface
  8. Characterization of dual-band circularly polarized mushroom-shaped monopole antenna with modified ground plane
  9. Novel multilayer antenna array with metamaterial structures for 5G applications
  10. Compact quadband two-port antenna with metamaterial cell-inspired decoupling parasitic element for mobile wireless applications
  11. Gain enhancement of a SIW H-plane horn antenna using of metamaterial array
  12. Optimized SIW antipodal Vivaldi antenna array using Fourier series equations for C-band applications
  13. Design and performance analysis of a compact, wideband dual polarized antenna for WLAN & WiMAX applications
  14. Miniaturised ultra-wideband rectangular shaped slot antenna for ground penetrating radar applications
  15. Performance analysis and rain attenuation modelling of RoFSO link for hilly region of India
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