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IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 5, NO. 1, JANUARY 2015
U-Shape Slots Structure on Substrate Integrated Waveguide for 40-GHz Bandpass Filter Using LTCC Technology Sai-Wai Wong, Senior Member, IEEE, Rui Sen Chen, Student Member, IEEE, Kai Wang, Student Member, IEEE, Zhi-Ning Chen, Fellow, IEEE, and Qing-Xin Chu, Senior Member, IEEE
Abstract— Millimeter-wave (mmW) bandpass filter using substrate integrated waveguide (SIW) is proposed in this paper. The propagation constants of three different types of electromagnetic bandgap (EBG) units are discussed and compared with their passbands and stopbands performance. The slotted-SIW unit shows a very good lower stopband and upper stopband performance. The mmW bandpass filter with three cascaded uniform slotted-SIW-based EBG units is constructed and designed at 40 GHz. The extracted coupling coefficient (K) and quality factor (Q) are used to determine the filter circuit dimensions. To prove the validity, the previous proposed structure is fabricated in a single circuit layer using low-temperature co-fired ceramic technology and measured at 40 GHz, respectively. The measured results are in good agreement with simulated results in such frequency and the measured insertion losses at 40 GHz is 1.42 dB, respectively. Index Terms— 40-GHz, bandpass filter, electromagnetic bandgap (EBG), low-temperature co-fired ceramic (LTCC), millimeter-wave (mmW), substrate integrated waveguide (SIW).
I. I NTRODUCTION
T
HE rapid development of wireless communication systems causes the shortage of the frequency spectrum, so it is necessary to find new frequency resource, such as microwave and the millimeter-wave (mmW). The mmW circuits design have attracted much attention due to its attractive properties for high data rate wireless transmission [1], [2]. In [1], the microstrip line is used to design an mmW filter with
Manuscript received November 14, 2013; revised September 15, 2014; accepted October 31, 2014. Date of publication November 23, 2014; date of current version January 7, 2015. This work was supported in part by the National Natural Science Foundation of China under Grant 61101017, in part by the Program for New Century Excellent Talents in University under Grant NCET-13-0214, in part by the State Key Laboratory of Millimeter Waves under Grant K201327, in part by the Southeast University, Nanjing, China, in part by the Fundamental Research Funds for the Central Universities under Grant 2014ZZ0029, and in part by the National Engineering Technology Research Center for Mobile Ultrasonic Detection. Recommended for publication by Associate Editor L.-T. Hwang upon evaluation of reviewers’ comments. S.-W. Wong, R. S. Chen, K. Wang, and Q.-X. Chu are with the School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510640, China (e-mail:
[email protected];
[email protected];
[email protected]). Z.-N. Chen is with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119077, and also with the Institute for Infocomm Research, Agency for Science, Technology and Research, Singapore 138632 (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2014.2367516
insertion loss 0.97 dB at center frequency. An mmW filter fabricated with rectangular waveguide is introduced in [2], its center frequency is ∼140 GHz, and has very good performance. However, the microstrip circuits have serious loss when the frequency increases to the mmW range. Meanwhile, although the rectangular waveguide has high quality factor, it cannot meet the requirements of miniaturization and lightweight of the communication systems today. Recently, substrate integrated waveguide (SIW) has been widely used to design and fabricate the mmW filters because it has high quality factor, low cost, reduction of size and weight, and can be easily integrated with other elements [3]–[5]. Transmission zeros that are introduced by appropriately using the coupling between the SIW cavities is discussed in [3], the out-ofband rejection level is quite good compared with traditional microstrip filters [4]. It is interesting to insert a microstrip line between the SIW cavities, which described in [5], a transmission zero is obtained to improve out-of-band rejection. The emergent of the SIW accelerate the development of the mmW filters, and then cause the urgent needs of mmW manufacturing technology. Nowadays, most of mmW bandpass filters are fabricated by low-temperature co-fired ceramic (LTCC) [6], [7] and CMOS technology [8]. An mmW SIW filter buried in LTCC has been analyzed and designed in [9]. It uses the metallic via arrays to control the couplings to produce a good inband performance. The via-holes magnetic coupling structures cannot provide strong coupling, which limits the bandwidth of the filters, so a slot etching on the top metal of SIW cavity used to introduce a electric coupling is presented in [10] and [11], a good performance also obtained with low insertion loss and wide passband. To further improve the property of the filters, the magnetic and electric mixed coupling structures are introduced to provide strong coupling intensities, and the coexistence of electric and magnetic coupling can also develop transmission zeros [12]. The U-shaped defected ground structures are used in [13] to generate transmission zeros above the passband. In this paper, U-shape slots etched on single layer SIW cavity are used to design mmW bandpass filter at 40 GHz using LTCC technology. First, three different types of EBG units are studied, and the frequency-dependent propagation constant is extracted, respectively, to show the band response of each EBG unit. Then, the U-shape slot is introduced and studied,
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WONG et al.: U-SHAPE SLOTS STRUCTURE ON SIW FOR 40-GHz BANDPASS FILTER
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Fig. 1. Four different types of EBG unit cells with same L p = 925 μm (L 1 = 1850 μm, L 2 = 1400 μm, L 3 = 700 μm, T1 = 225 μm, and T2 = 250 μm).
some characteristic will be presented. Finally, using the slotted-SIW EBG unit, an mmW bandpass filter is designed at 40 GHz with three cascaded EGB units. The three resonators are formed by U-shaped slots on the top metal plane of the SIW transmission line and the filter is investigated with the theory of coupled resonator circuits. The design curves of coupling coefficient K and quality factor Q are extracted subject to the slot height, the width, and the slots spacing [14]. II. S TUDY ON T HREE D IFFERENT T YPES OF EBG C ELLS AND SIW C AVITY W ITH U-S HAPE S LOT Fig. 1 shows three different types of EBG unit cells, their passband and stopband are studied and analyzed. The EBG-I structure is formed by SIW transmission line. The EBG-II structure is constituted with an SIW transmission line and an via window, the EBG-III structure is formed by etching additional U-shaped slot on the base of EBG-II. These three EBG unit cells have the same cavity size, and their dimensions are marked and shown in Fig. 1. For further studying their transmission behaviors, the guided-wave propagation constants (γ = α + jβ) are presented and numerically extracted from the electromagnetic (EM) simulator (CST) [15]. The propagation constant is generally expressed in terms of two ABCD-matrix elements, A p and D p [16] A P + DP 2 γ = α + jβ
cosh(γ L P ) =
(1) (2)
where L p is the length of each EBG cell and γ = jβ is on behalf of the lossless case, while α represents the attenuation constant, thus the nonzero value of α implies a stopband, while zero value represents a passband. The extracted normalized attenuation constant (α/κ0 ) and phase constant (β/κ0 ) of each EBG cell are shown in Fig. 2 by virtue of the EM simulation [15], where κ0 is the phase constant in the free space. Fig. 2(a) and (b) shows the frequency responses of the normalized attenuation constant α/κ0 and the normalized phase constant β/κ0 of four different types of EBG cells. It can be seen that the α/κ0 of 50- transmission line is close to zero at all frequencies, which implies that 50- transmission line is an all-pass filter. The α/κ0 of EBG-I structure is nonzero at low frequency below f1 , but closed to zero at high frequency, this implies that EBG-I has a highpass characteristic. The cutoff frequency of f 1 is the cutoff
Fig. 2. Extracted frequency-dependent propagation constant of four different types of EBG units. (a) Normalized attenuation constant. (b) Normalized phase constant.
frequency of the fundamental mode (TE01 mode), which is controlled by the L 1 (a half-guided wavelength at f 1 ). The EBG-II structure is similar to the EBG-I, it also has a high-pass characteristic, the difference between them is due to the two extra via-holes that decrease the width of the SIW cavity and thus increase the cutoff frequency to f 2 . For the EBG-III, the α/κ0 is nonzero value
