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Microstrip wideband bandpass filter with six transmission zeros using transversal signal-interaction concepts.

1. INTRODUCTION

Planar microstrip wideband bandpass filters with high selectivity and out-of-band rejection are extremely desirable in wide-band RF/microwave communication systems. In [1-8], multi-mode resonators, complementary split-ring resonator (CSRR), and multilayer aperture-coupled patches were used to design various microstrip wideband bandpass filters. In addition, electromagnetic (EM) loaded bandgap, the cascaded low-pass/high-pass filters, T-shaped resonators, and stepped impedance resonators (SIR) were used to extend the upper stopband bandwidth and improve the selectivity of the passband [9-14].

Recently, microstrip wideband bandpass filters based on transversal signal-interaction concepts have drawn a lot of attention [15-21]. High-selectivity filtering characteristics with only a few resonators were realized in [15,16], however, DC suppression was not solved and the circuit size should be further reduced. A wideband bandpass filter using transversal resonator and asymmetrical interdigtial coupled lines with a good out-of-band response was realized in [17]. Another wideband bandpass filters with an ideal DSPSL (double-sided parallel-strip line) 180 phase inversion was introduced in [18]. The filter has high selectivity and good harmonic suppression; however, two via holes were used to realize the ideal 180 swap, causing fabrication difficulty. Moreover, two wideband bandpass filters based on the ideal 180[degrees] phase inversion of the two shorted coupled lines were proposed in [19, 20]. However, there is only one transmission zero (in the upper stopband) closed to the passband, and the 3-dB bandwidth of the filters is not easy to adjust. In [21], another new wideband bandpass filter using Marchand balun based on transversal signal-interaction concepts was introduced, indicating some advantages of compact circuit size, adjustable bandwidth and high selectivity. Moreover, we also proposed several wideband balanced circuits using transversal signal-interaction concepts in [22,23], wide passband for the differential mode and wideband common mode suppressed can be easily achieved due to signals superposition of different transmission paths.

In this paper, a new microstrip high-selectivity wideband bandpass filter using transversal signal-interaction concepts based on T-shaped structure [13] and open/shorted coupled lines is proposed. Six transmission zeros (0-2[f.sub.0], [f.sub.0] is the centre frequency of the passband) can be realized due to the superposition of signals of the two transmission paths with a T-shaped structure and open/shorted coupled lines. A prototype of wideband bandpass filter operating at 3.0 GHz with fifth-order passband is designed. All the structures are simulated with Ansoft HFSS v.11.0 and constructed on Rogers 5880 with dielectric constant 2.2 and thickness 0.508 mm, and tan[delta] = 0.0009. Detailed theoretical design, simulation and experimental results are demonstrated and discussed.

2. DESIGN OF THE PROPOSED MICROSTRIP BANDPASS FILTER

2.1. Basic Theoretical Analysis

Figures 1(a) and (b) show the top view and the equivalent circuit of the microstrip wideband bandpass filter. Two different transmission paths are introduced to realize the signal transmission from Port 1 to Port 2. For Path 1, a T-shaped structure (characteristic impedance [Z.sub.1], electrical length [theta], [theta] = 90[degrees] at the center frequency [f.sub.0]) is located in the center of the two open coupled lines (even/odd-mode characteristic impedance [Z.sub.oe] and [Z.sub.oo], electrical length [theta]). For Path 2, two shorted coupled lines (even/odd-mode characteristic impedance [Z.sub.oe] and [Z.sub.oo], electrical length [theta]) are connected in the center of the two transmission lines with characteristic impedance [Z.sub.2] and electrical length [theta]. In addition, the characteristic impedances of the two microstrip lines at the input/output ports are all [Z.sub.0] = 50 [ohm].

The ABCD matrices of the open stub, transmission lines and open/shorted coupled lines are [24]:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

For Path 1, the ABCD matrix is [M.sub.o] x [M.sub.stub] x [M.sub.o] ([M.sub.o]--open coupled line, [M.sub.stub]--open stub [Z.sub.1]); for Path 2, the ABCD matrix is [M.sub.line] x [M.sub.s] x [M.sub.s] x [M.sub.line] ([M.sub.line]--transmission line [Z.sub.2], [M.sub.s]--shorted coupled line). After the ABCD- and Y-parameter conversions, the S-parameters of the wideband bandpass filter can be illustrated as [24]:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

And Y = 1/Z ([Z.sub.0] = 50 [ohm]), when [S.sub.21] = 0 ([Y.sub.21] = 0), the transmission zeros of the circuit of Figure 1(b) can be obtained:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

The solutions [[theta].sub.tz] = 0, n correspond to 0 and 2[f.sub.0], respectively, which are the inherent transmission zeros of the open/shorted coupled lines [24]. In addition, the transmission poles in the passband can be calculated when [S.sub.11] = 0, saying, a fifth-order equation for [theta] versus [Z.sub.1], [Z.sub.2], [Z.sub.oe] and [Z.sub.oo] can be obtained for this case. When Z is fixed, five roots for [S.sub.11] = 0 can be acquired by proper choosing the relationships of [Z.sub.1], [Z.sub.2], [Z.sub.oe] and [Z.sub.oo], and then five transmission poles in the passband can be achieved.

Figures 2(a)-(b) show the simulated results of the circuit in Figure 1(b). Due to the superposition of signals for Paths 1 and 2, four transmission zeros ([f.sub.tz2], [f.sub.tz3], [f.sub.tz4], [f.sub.tz5]) can be easily achieved, leading to a quasi-elliptic function that improves the passband and out-of-band performances for the wideband bandpass filter. The five in-band transmission poles reflect the fact that [S.sub.11] = 0 has five real solutions when [Z.sub.1], [Z.sub.2], [Z.sub.oe] and [Z.sub.oo] are properly selected.

In addition, when [Z.sub.0] and [theta] are fixed, three variables [Z.sub.1], [Z.sub.2], and k (k = ([Z.sub.oe] - [Z.sub.ee])/ ([Z.sub.oe] + [Z.sub.ee])) need to be determined by taking into account not only the characteristics of the filter but also the tunability of the passband. Figures 3(a), (b), (c), (d), (e), and (f) show the transmission zeros ([f.sub.tz2], [f.sub.tz3], [f.sub.tz4], [f.sub.tz5]), 3-dB bandwidth and [absolute value of [S.sub.11]] versus different characteristic impedance [Z.sub.1], [Z.sub.2] and k (k = ([Z.sub.oe]-[Z.sub.oo])/ ([Z.sub.oe] + [Z.sub.oo])). We may see that, the transmission zeros [f.sub.tz2] and [f.sub.tz5] move away from [f.sub.0] as [Z.sub.1], [Z.sub.2] and k increase; the transmission zeros [f.sub.tz3] and [f.sub.tz4] move away from [f.sub.0] as k increases, and the 3-dB bandwidth of the passband decreases as [Z.sub.1], increases as [Z.sub.2] and k increase, in addition, the in-band balance can be also adjusted by [Z.sub.1], [Z.sub.2] and k. In this way, the performances of the wideband bandpass filter can be controlled conveniently by changing the characteristic impedances [Z.sub.1], [Z.sub.2], [Z.sub.oe] and [Z.sub.oo] when [Z.sub.0] and [theta] are fixed. Actually, the 3-dB bandwidth of the passband can be illustrated as [f.sub.3dB] = f ([Z.sub.oe], [Z.sub.oo], [Z.sub.1], [Z.sub.2]). Once Z0 and 0 are determined, we can adjust [Z.sub.oe], [Z.sub.oo], [Z.sub.1] and [Z.sub.2] to satisfy the demand of 3-dB bandwidth [f.sub.3dB], and the transmission characteristic for the passband can be simultaneously obtained and further optimized based on the above discussion.

2.2. Proposed Microstrip Wideband Bandpass Filter

To clarify the proposed filter design, the design procedures of wideband bandpass filter are summarized as follows:

(1) Based on the Equations (1)-(4), choose the desired 3-dB bandwidth [f.sub.3dB] of the bandpass filter, determine the values [Z.sub.1], [Z.sub.2], [Z.sub.oe] and [Z.sub.oo] to meet fifth-order passband with six transmission zeros from the response up to 2[f.sub.0];

(2) Adjust the values of [Z.sub.1], [Z.sub.2], [Z.sub.oe] and [Z.sub.oo] to determine the locations of the four transmission zeros ([f.sub.tz2], [f.sub.tz3], [f.sub.tz4], [f.sub.tz5]) from 0 to 2[f.sub.0]; when [Z.sub.0] and [theta] are determined, further optimize the values of [Z.sub.1], [Z.sub.2], [Z.sub.oe] and [Z.sub.oo] to realize better in-band and out-of-band transmission characteristics of the wideband filter.

Based on the above theoretical analysis, the final parameters for the filter circuit of Figure 1(b) are given as follows: [Z.sub.0] = 50 [ohm], [Z.sub.1] = 12 [ohm], [Z.sub.2] = 100 [ohm], [Z.sub.oe] = 185.6 [ohm], [Z.sub.oo] = 84.6 [ohm], [f.sub.0] = 3.0 GHz. The simulated results of the microstrip wideband bandpass filter in Figure 1(a) (0.70[[lambda].sub.g] x 0.40[[lambda].sub.g], [l.sub.1] = 18.7 mm, [l.sub.2] = 18.7 mm, [l.sub.3] = 16.6 mm, [l.sub.4] = 38.4 mm, [w.sub.0] = 1.5 mm, [w.sub.1] = 0.2 mm, [w.sub.2] = 0.47 mm, [w.sub.3] = 9.2 mm, [w.sub.4] = 0.2 mm, [t.sub.1] = 1.0 mm, [t.sub.2] = 4.4 mm, [g.sub.1] = 0.2 mm, [g.sub.2] = 0.15 mm, [g.sub.3] = 0.5 mm, [g.sub.4] = 0.15 mm, d = 0.5 mm) are shown in Figure 4. Obviously, the simulated insertion loss is less than 0.5 dB while the return loss is over 10 dB from 2.35 GHz to 3.63 GHz (3-dB bandwidth is approximately 44.1%, 2.33-3.65 GHz). Six transmission zeros are located at 0, 1.68, 2.3, 3.66, 4.34 and 5.85 GHz with a fifth-order passband are realized to improve the selectivity and harmonic suppression. Furthermore, over 20-dB upper stopband is achieved from 3.95 GHz to 7.95 GHz (2.65[f.sub.0]). The group delay is less than 1.0 ns in the whole passband.

3. MEASURED RESULTS AND DISCUSSIONS

Figure 4(a) illustrates the photograph of the microstrip wideband bandpass filter with six transmission zeros from 0 to 2[f.sub.0], which is designed on Rogers 5880 with dielectric constant 2.2 and thickness 0.508 mm. The efficient electric size of the proposed filter is 0.70[[lambda].sub.g] x 0.40[[lambda].sub.g]. The measured S-parameters and the group delay are illustrated in Figures 4(b). Good agreement can be found between the measured and simulated results. Six transmission zeros are found to locate at 0, 1.66, 2.3, 3.64, 4.22 and 5.96 GHz, respectively; a sixth-order passband with insertion loss less than 1.1 dB while return loss over 11 dB from 2.36 GHz to 3.62 GHz is realized. Furthermore, over 20-dB upper stopband is achieved from 3.92 GHz to 8.02 GHz (2.67/0). The measured group delay is less than 1.0 ns from 2.6 GHz to 3.36 GHz. The slight frequency discrepancies between the measured and simulated results are mainly caused by the limited fabrication precision and measurement errors.

To further demonstrate the performances of the filters, the comparisons of measured results for several wideband filter structures [1521] are shown in Table 1. The proposed wideband bandpass filter has more transmission zeros to further improve the passband selectivity and harmonic suppression compared with the other wideband filter structures in [15-21], and the upper stopband of the proposed wideband bandpass filter stretches up to 2.67/ due to the multiple transmission zeros in out-of-band. Moreover, to extend the upper stopband, some lowpass/bandstop networks can be cascaded to further improve the upper stopband performance of the two wideband bandpass filters [9-14], and to achieve tighter coupling and wider passband for the filter, patterned ground-plane technology [21,22] can be used in the proposed wideband bandpass filter.

4. CONCLUSION

A microstrip high-selectivity wideband bandpass filter using transversal signal-interaction concepts is designed and fabricated. Six transmission zeros with a fifth-order passband are achieved to improve the selectivity and harmonic suppression of the filter. Compared with former wideband differential structures [15-21], the proposed wideband filter has higher selectivity and wider-band harmonic suppression. Good agreements between simulated and measured responses of the filter are demonstrated, indicating the validity of the design strategies.

ACKNOWLEDGMENT

This work is supported by the National Natural Science Foundation of China (60971013), 2010 Graduate Innovative Project of Jiangsu Province (CX10B_125Z), China, and in part by the Major State Basic Research Development Program of China (973 Program: 2009CB320201).

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[10.] Lin, W. J., C. S. Chang, J. Y. Li, D. B. Lin, L. S. Chen, and M. P. Houng, "Improved compact broadband bandpass filter using branch stubs co-via structure with wide stopband characteristic," Progress In Electromagnetics Research C, Vol. 5, 45-55, 2008.

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S.J. Xue (1,3), W.J. Feng (2), H.T. Zhu (3), and W.Q. Che (2,3) *

(1) University of Electronic Science and Technology of China, Chengdu 610054, China

(2) Department of Communication Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

(3) State Key Laboratory of Millimeter Waves, City University of Hong Kong, Hong Kong, China

Received 28 September 2012, Accepted 16 October 2012, Scheduled 19 October 2012

* Corresponding author: Wen Quan Che (yeeren_che@163.com).

Table 1. Comparisons of measured results for several wideband
filters.

Filter       Transmission    Transmission     [DELTA]
Structures       zeros          poles       [f.sub.3dB]
              ([absolute      ([absolute        (%)
               value of        value of
             [S.aub.21]],    [s.sub.11]],
             0-2[f.sub.0])     in-band)

Ref. [15]          6              1            10.7%
Ref. [16]          6              3            27.6%
Ref. [17]          4              5             75%
Ref. [18]          4              4            123%
Ref. [19]          3              3           117.6%
Ref. [20]          4              3           124.6%
Ref. [21]          4              3            110%

This work          6              6            43.3%

Filter              Upper            Group
Structures         stopband          delay
                     (dB)            (ns)

Ref. [15]    > 15 (1.4[f.sub.0])      --
Ref. [16]    > 15 (1.5[f.sub.0])    < 0.70
Ref. [17]    > 20 (2.5[f.sub.0])    < 0.70
Ref. [18]    > 20 (2.3[f.sub.0])    < 1.50
Ref. [19]    > 15 (2.4[f.sub.0])    < 0.50
Ref. [20]    > 15 (2.5[f.sub.0])    < 0.30
Ref. [21]    > 13 (2.4[f.sub.0])    < 0.60

This work    > 20 (2.67[f.sub.0])    < 1.0
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Author:Xue, S.J.; Feng, W.J.; Zhu, H.T.; Che, W.Q.
Publication:Progress In Electromagnetics Research C
Article Type:Abstract
Geographic Code:1USA
Date:Jan 1, 2013
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