124 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 21, NO. 3, MARCH 2011
Novel Microstrip Periodic Structure and Its
Application to Microwave Filter Design
Wei-Shin Chang and Chi-Yang Chang, Member, IEEE
Abstract—This letter proposes a new type of microstrip slow-wave structure. The proposed structure has the signal strips and the inserted ground strips periodically loaded in the internal part of the conventional microstrip line. The proposed microstrip line has not only the same outline contour but also a lower phase ve-locity compared to the conventional one, especially for the thick substrate. In addition, the slow-wave factor and the characteristic impedance are easily varied by adjusting the structural parame-ters. A third-order Chebyshev filter with a passband ripple of 0.05 dB and a bandwidth of 10% was designed and fabricated using the proposed microstrip line. The simulated and measured results agree well with each other.
Index Terms—Bandpass filters (BPFs), microstrip line, periodic structures, slow wave structures.
I. INTRODUCTION
S
LOW-WAVE guiding structures have been extensively studied to reduce the circuit size [1]–[8]. Among these slow-wave structures, this letter focuses on the microstrip slow-wave structures where a back side conductor is required. Table I compares several microstrip slow-wave structures [3]–[8]. The ladder microstrip line shown in Fig. 1(a) has more compactness and compatibility than the other microstrip slow-wave structures. It only needs a single layer substrate and has the same outline contour as the conventional microstrip line. Moreover, there is no periodic structure patterned on the ground plane so that the substrate is not required to be suspended. In this study, on the basis of the ladder microstrip line, we propose a novel slow-wave microstrip line with an inserted ground strip between the loaded signal strips. The pro-posed structure has a lower phase velocity, a lower loss, and a lower characteristic impedance than the ladder microstrip line. Finally, we design a bandpass filter (BPF) using the proposed structure to demonstrate its feasibility.II. PROPOSEDPERIODICMICROSTRIPLINE
Fig. 1(b) and (c) depict the top and cross-sectional views of the proposed microstrip line with strips periodically loaded in-side the conventional microstrip line. The proposed structure,
Manuscript received October 09, 2010; revised December 17, 2010; accepted December 27, 2010. Date of publication February 10, 2011; date of current ver-sion March 11, 2011. This work was supported in part by the National Science Council of Taiwan, under Grant NSC 98-2221-E-009-034-MY3 and Grant NSC 99-2221-E-009-050-MY3.
The authors are with the Graduate Institute of Communication Engi-neering, National Chiao Tung University, Hsinchu 300, Taiwan (e-mail: aa494412338@hotmail.com; mhchang@cc.nctu.edu.tw).
Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LMWC.2011.2105250
TABLE I
COMPARISON OFMICROSTRIPSLOW-WAVESTRUCTURES
: wavenumber of the conventional microstrip line with the same width. 1.92: for the substrate with = 3:58, h = 1:524 mm, and W = 6:25 mm.
low, medium, and high mean approximately equal to, from 1.5 to 3 times, and more than 3 times the loss of the conventional microstrip line, respectively.
Fig. 1. (a) Ladder microstrip line. (b) Proposed microstrip line. (c) Cross-sec-tional view of the proposed periodic structure. (d) Analysis of the proposed mi-crostrip line in terms of capacitances.
where there is an inserted ground strip between the loaded signal strips, is basically an extension of the ladder microstrip line shown in Fig. 1(a). Note that one via-hole is added on the middle of each inserted ground strip. It is primarily used for maintaining equal potentials on the top and bottom ground metals. In other words, the via-hole is not used to connect the signal strip to the ground plane so that its inductive effect is not severe and can almost be neglected. Here, the width and length of the loaded signal strip are and , respectively. The width of the out-most signal strip is . For the inserted ground strip, the width is and the length is . The spacing between the signal strip and the inserted ground strip is . Accordingly, the length of the unit cell (i.e., pitch) is , and the total
transverse width is .
It is more convenient to assume that . According to the analytical process in [3] and the equivalent capacitance
CHANG AND CHANG: NOVEL MICROSTRIP PERIODIC STRUCTURE 125
Fig. 2. Characteristic impedanceZ versus slow-wave factor and total trans-verse widthW for the conventional (4), ladder (+), and proposed ( ) mi-crostrip lines withW = 0:5 mm. Solid red line: = 3:58, h = 0:508 mm. Dashed blue line: = 3:58, h = 1:524 mm. Dashed-dotted green line: = 10:2, h = 0:635 mm. Dotted black line: = 10:2, h = 1:27 mm.
representation of the proposed structure in Fig. 1(d), the loaded capacitance per unit length at interval is given by
(1) where denotes the parallel plate capacitance located verti-cally between the signal and ground planes. accounts for the modification of the fringe capacitance from the edge of a single line due to the presence of another line. and rep-resent the fringe capacitances across the loaded signal strip and the inserted ground strip in the air and dielectric regions, respec-tively. For the ladder microstrip line in Fig. 1(a) with the same number of strips per (i.e., two strips), the loaded capacitance per unit length at interval is given by
(2) Comparing (1) and (2), the proposed structure has capacitances and . Since the inductance per unit length is almost the same in the ladder and proposed microstrip lines, the proposed structure has a lower phase velocity and a lower characteristic
impedance as long as . This is
par-ticularly easily achieved in the thick substrate.
To observe the property of the proposed structure, we per-form the full-wave EM simulation by using Sonnet software [9]. In the simulation, the substrates with dielectric constants and 10.2 are used. Since the fabrication capability is taken into account, the minimum spacing between adjacent strips is 0.15 mm and the diameter of each via-hole is 0.3 mm.
Here, we fix , , and
. In the following discussion, is chosen as an example. The ladder microstrip line with the same number of strips is included for comparison.
For the thin and thick substrates, Fig. 2 shows the charac-teristic impedance versus the slow-wave factor defined by and the total transverse width for the conventional,
Fig. 3. Design chart for the proposed microstrip line with = 3:58, W = W = 0:3 mm, S = 0:15 mm, and D = 0:9 mm. Solid red line: h = 0:508 mm. Dashed blue line: h = 1:524 mm.
ladder, and proposed microstrip lines. Here, is the free-space wavelength and is the guided wavelength of the microstrip line. For the same , the proposed microstrip line may have a lower or higher characteristic impedance than the conventional microstrip line. On the other hand, the proposed microstrip line has a lower characteristic impedance compared to the ladder microstrip line. As shown in the figure, for the thin substrate, the characteristic impedance of the proposed microstrip line is always lower than that of the ladder microstrip line but larger than that of the conventional microstrip line. Nevertheless, for the thick substrate, the characteristic impedance of the proposed structure is always the smallest. It is clear that the lower the impedance and the wider the line are, the larger the slow-wave factor is. This is because the loaded capacitance is particularly large for wide lines. Moreover, for the same , the proposed structure has the lowest phase velocity among the three mi-crostrip lines, and the slow-wave factor is especially large for the thick substrate.
Take the substrates with and and
1.524 mm in Fig. 2 as an example. Fig. 3 shows the design chart for the proposed microstrip line. It is seen that mainly con-trols the per-unit-length inductance, and primarily affects the per-unit-length capacitance. Accordingly,
in Fig. 2 is just an example, and a smaller value of can be chosen for a larger slow-wave factor.
At 1 GHz, Table II summarizes the losses of the three mi-crostrip lines with and a loss tangent of 0.0021 for two cases: 1) fixed transverse width ; 2) fixed
impedance for and 28.1 for
. The loss of the proposed microstrip line is greater than that of the conventional microstrip line but smaller than that of the ladder microstrip line. One of the expected ap-plications of the proposed structure is the microwave resonator. Table II also compares the relative areas and lengths of the resonators using the conventional, ladder, and proposed mi-crostrip lines. The proposed resonator always has the smallest area, especially for the thick substrate.
III. CHEBYSHEVINTERDIGITALFILTERDESIGN
To demonstrate the proposed structure, a three-pole Cheby-shev interdigital BPF with a passband ripple of 0.05 dB, a center
126 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 21, NO. 3, MARCH 2011
TABLE II
MICROSTRIPLINELOSSES ANDRELATIVEAREAS/LENGTHS OF THE
HALF-WAVELENGTHRESONATORS FOR THECONVENTIONAL, LADDER,ANDPROPOSEDMICROSTRIPLINES AT1 GHz
Fig. 4. Configuration and photograph of the proposed filter. Filter dimensions: W = 6:25, W = 0:5, W = 0:3, W = 0:3, W = 0:8, W = 0:3, L = 5:25, L = 4:95, L = 21:5, L = 21, L = 4:625, S = 0:2, and S = 0:15 mm. Filter A: conventional resonator with the same width W = 6:25, L = 38:125, L = 37:5, L = 12:9, and S = 0:125 mm. Filter B: conventional resonator with the same impedanceZ = 20:7 , W = 3:75, L = 40:65, L = 39:8, L = 12:325, and S = 0:275 mm.
frequency of 1 GHz, and a fractional bandwidth of 10% was realized using the resonator. It was fabricated on the Rogers RO4003 substrate with a dielectric constant of 3.58, a loss tangent of 0.0021, and a thickness of 0.508 mm using the design procedures based on the coupling coefficient and the external quality factor [10].
The physical layout and dimensions of the proposed filter are depicted in Fig. 4, together with the photograph of the proposed and conventional filters with the same specifications. In Fig. 4, the resonators of the conventional filters have the same trans-verse width (i.e., filter A) and the same impedance (i.e., filter B) as those of the proposed filter, respectively. The size of the proposed filter is 19.15 mm 21.5 mm, which is
, where is the guided wavelength of the conven-tional 50- microstrip line on the substrate at the center fre-quency. Compared to filters A and B, the size reductions of the proposed filter are 43.2% and 14.2%, respectively. Fig. 5 shows its simulated and measured responses. For easy compar-ison, the measured responses of the two conventional filters in Fig. 4 are also included. The measured results show that the pro-posed filter has a center frequency of 0.998 GHz. Within the passband, the return loss is better than 17.2 dB, and the min-imum insertion loss is 1.522 dB, which is slightly larger than those of the conventional filters. The measured 3 dB fractional bandwidth is 13.53% from 0.9296 to 1.0646 GHz. The first spu-rious response is at 3.037 GHz (i.e., ), and the rejection level is better than 20 dB from 1.156 to 2.87 GHz. As shown
Fig. 5. Simulated (dashed line) and measured (solid line) results (jS j and jS j) of the proposed filter. The measured responses of filter A (dashed-dotted line) and filter B (dotted line) in Fig. 4 are also plotted for comparison.
in Figs. 4 and 5, the proposed filter has the smallest size and the similar response compared to the two conventional filters.
IV. CONCLUSION
In this letter, a novel microstrip line with periodic structures has been presented. By alternating the loaded signal strips and the inserted ground strips, the proposed microstrip line has a lower phase velocity than the conventional and ladder microstrip lines with the same transverse width or character-istic impedance. Due to the inserted ground strip, the proposed structure can have a lower characteristic impedance than the conventional and ladder microstrip lines. A compact three-pole BPF has been designed and experimentally verified to demon-strate the miniaturization of the proposed structure. Thus, it is very appropriate for monolithic microwave integrated circuits (MMICs) and compact microwave components.
REFERENCES
[1] T. S. D. Cheung and J. R. Long, “Shielded passive devices for silicon-based monolithic microwave and millimeter-wave integrated circuits,”
IEEE J. Solid-State Circuits, vol. 41, no. 5, pp. 1183–1200, May 2006.
[2] D. Kaddour, H. Issa, A. L. Franc, N. Corrao, E. Pistono, F. Podevin, J. M. Fournier, J. M. Duchamp, and P. Ferrari, “High-Q slow-wave coplanar transmission lines on 0.35m CMOS process,” IEEE
Mi-crow. Wireless Compon. Lett., vol. 19, no. 9, pp. 542–544, Sep. 2009.
[3] J. S. Hong and M. J. Lancaster, “A novel microwave periodic struc-ture-the ladder microstrip line,” Microw. Opt. Technol. Lett., vol. 9, no. 4, pp. 207–210, Jul. 1995.
[4] F. R. Yang, K. P. Ma, Y. Qian, and T. Itoh, “A uniplanar compact photonic-bandgap (UC-PBG) structure and its applications for mi-crowave circuits,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 8, pp. 1509–1514, Aug. 1999.
[5] G. A. Lee, H. Y. Lee, and F. De Flaviis, “Perforated microstrip structure for miniaturising microwave devices,” Electron. Lett., vol. 38, no. 15, pp. 800–801, Jul. 2002.
[6] C. K. Wu, H. S. Wu, and C. K. C. Tzuang, “Electric-magnetic-elec-tric slow-wave microstrip line and bandpass filter of compressed size,”
IEEE Trans. Microw. Theory Tech., vol. 50, no. 8, pp. 1996–2004, Aug.
2002.
[7] Y. Yun, “A novel microstrip-line structure employing a periodically perforated ground metal and its application to highly miniaturized and low-impedance passive components fabricated on GaAs MMIC,” IEEE
Trans. Microw. Theory Tech., vol. 53, no. 6, pp. 1951–1959, Jun. 2005.
[8] C. W. Wang, T. G. Ma, and C. F. Yang, “A new planar artificial trans-mission line and its applications to a miniaturized butler matrix,” IEEE
Trans. Microw. Theory Tech., vol. 55, no. 12, pp. 2792–2801, Dec.
2007.
[9] “Em User’s Manual,” Sonnet Software, Liverpool, NY, 2004. [10] J. S. Hong and M. J. Lancaster, Microstrip Filters for RF/Microwave
