диафрагмированные волноводные фильтры / 0576e59a-6b23-464b-9fc2-caf13acf8e72

I. Introduction

Traditional metallic rectangular waveguide bandpass filters such as E-plane filters [1], [2], H-plane filters [3], and fin-line filters [4] have been widely used in microwave and millimeter-wave systems because of their merits of low insertion loss and high power handling capacity. However, classical rectangular waveguide filters are bulky, heavy, high cost, and are difficult to integrate with planar circuit technology, being 3-D structures. Kim and Lee [5] proposed the partial H-plane waveguide, with a cross section that is one-quarter that of conventional waveguides. It has been utilized to design compact waveguide filters called partial H-plane filters [6], [7]. Compared with conventional rectangular waveguide filters, these filters have reduced volume, but they are still 3-D structures, and difficult to integrate with planar circuits. Recently, a new planar waveguide structure, the substrate integrated waveguide (SIW), was proposed [8], [9] in which an equivalent rectangular metallic waveguide is fabricated using the standard printed circuit board process. Such a class of waveguides has the advantages of high Q-factor, convenient integration, mass-producibility, low cost, etc. Diversified filters based on SIW have been realized [10]–[14]. The configuration can be easily integrated into microwave and millimeter-wave integrated circuits. However, compared with microstrip or stripline components, the width of SIW may be too large for some circuits. To reduce the width, the concept and geometry of substrate integrated folded waveguides (SIFW) are presented [15]–[18]. Several

Manuscript received August 14, 2011; accepted December 15, 2011. The authors are with the School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu, P. R.

China (e-mail: wzw1023@126.com).

DOI: http://dx.doi.org/10.1109/TUFFC.2012.2229

II. Filter Design

The structure of an SIW is shown in Fig. 1(a) and that of an SIFW in Fig. 1(b), where a2 is the width of the SIFW and d is the spacing between the H-plane metal vane and the two side walls of metallic via-holes. Fig. 1(c) is the side view of the SIFW. It can be seen that the SIFW is a SIW whose sides have been folded underneath its central part. Metallic via-holes are used to synthesize the conducting side walls. The diameter and spacing of the holes is much shorter than the operating wavelength and therefore is equivalent to a conducting wall [9]. The top, bottom, and central metal layers are formed by patterned metallization on the surfaces of substrates. The thickness 2h of the SIFW of Fig. 1(b) is twice that of the SIW in Fig. 1(a), whereas its width a2 is nearly half of a. The dominant mode of a SIW (TE10) varies sinusoidally in the transverse direction and has a maximum the center and null at the two side walls. The same mode in the SIFW maintains the maximum in the middle of the upper layer, but allows the electric field to shift 180° at both sides before it finally goes to zero on the lower central part of the SIFW [17].

The proposed partial H-plane band-pass filter made using the SIFW is illustrated in Fig. 2. This configuration provides a more compact band-pass filter than a metallic waveguide. The proposed filter consists of resonators alternating with evanescent waveguide sections. Evanescent waveguide sections are implemented by inserting double H-plane septa between the H-plane metal vane and sidewalls of metallic via-holes, as shown in Fig. 2. A tapered transition from SIFW to microstrip is employed for convenience of test and integration. The partial H-plane band-

wang et al.: SIFW H-plane band-pass filter

Fig. 1. Configuration of substrate integrated waveguide (SIW) and substrate integrated folded waveguide (SIFW): (a) SIW, (b) SIFW, and (c) the side view of the SIFW.

Fig. 2. The proposed partial H-plane filter with double H-plane septa: (a) 3-D structure and (b) layout of the central metal layer.

pass filter based on SIFW is a direct-coupled resonator filter like an E-plane double-iris waveguide filter. It is composed of half-wavelength resonators which are terminated with shorted ends. Therefore, the evanescent waveguide can be represented by an impedance inverter (K-inverter) circuit, as shown in Fig. 3. Normalized inverter values K and negative electrical length ϕ are given by [19]

561

Fig. 3. Impedance inverter (K-inverter) circuit for evanescent waveguide.

where Zg is a wave impedance of a partial H-plane SIFW. The design method of the proposed partial H-plane band-pass filter based on SIFW can be summarized in

three steps.

First, according to design requirement, the normalized inverter values for an equal-ripple band-pass filter are confirmed by [20]

where g0, g1, …, gn+1 are element values for an equalripple low-pass prototype and ωn is a normalized cutoff frequency; λg0, λg1, and λg2 are waveguide wavelengths at center frequency and at the lower and upper passband edge frequency, respectively; ωλ is a relative bandwidth; and n is the order of the filter.

Second, the simulation model of unit cell can be established using a commercial full-wave 3-D FEM simulator such as HFSS (Ansys Inc., Canonsburg, PA) or CST Studio Suite (Computer Simulation Technology AG, Darmstadt, Germany). The model is composed of an evanescent waveguide in the middle and one section of SIFW on each side of the unit cell. The S-parameters of the unit cell with different widths (Wj) of the H-plane septa can be gained easily by the simulator HFSS. The S-parameters gained are put into the Advanced Design System (ADS; Agilent Technologies Inc., Santa Clara, CA). The equivalent circuit of the evanescent waveguide cell is shown in Fig. 3. By parameter extraction, the impedances Xp and Xs are extracted for different widths (Wj) of the H-plane septa. Then, the relations between the normalized inverter value (K/Zg) and the width, Wj, of the H-plane septa, and between negative electrical length ϕ and Wj, can be confirmed using (1) and (2). The length of the half-wave-

(2)Thus, the initial physical parameters of filter can be determined.

Fig. 4. Photograph of the fabricated three-pole partial H-plane filter (including test cavity).

Finally, HFSS is used to analyze and optimize the filter after the initial design. To validate the proposed partial H-plane filter based on SIFW, a three-pole narrowband band-pass filter is designed and fabricated on a six-layer LTCC substrate, which has a relative dielectric of 5.9; each layer thickness is 0.1 mm. The specifications for the filter are three-pole, 0.01-dB passband ripple, center frequency f0 = 9.5 GHz, and the fractional bandwidth ∆ ≈ 3.2% (300 MHz). From (3)–(6), the normalized inverter

values (K/Zg) are K0,1/Zg = K3,4/Zg = 0.178 and K1,2/Zg = K2,3/Zg = 0.0415; then, the initial geometric dimensions

can be obtained according to (1), (2), and (7). To enable measurement, a tapered transition to microstrip has been developed, as shown in Fig. 2. The geometric dimensions are W0 = 0.45 mm, W1 = 1 mm, W2 = 5.1 mm, W3 = 0.2 mm, W4 = 2 mm, R1 = 5.6 mm, R2 = 5.6 mm, Q = 0.2 mm, P = 0.5 mm, L = 4 mm, d = 0.8 mm, and a = 5.6 mm. A photograph of the fabricated partial H-plane filter is shown in Fig. 4 (including a pair of test cavities with 2.92-mm connectors).

III. Measured Results

The measurements were taken using an Agilent E8363B vector network analyzer (VNA). The simulated and measured responses of the filter are shown in Fig. 5. It can be seen from Fig. 5 that the operational bandwidth of the measurement results is a little smaller than that of simulation results, and the insertion loss and group delay are larger; these differences could be attributed to tolerance of manufacturing and relative permittivity, high-order modes, material, and radiation loss. In LTCC multilayer, all metallic vias and metal surfaces are assumed to be perfectly conducting in our design, which could also result in the discrepancy between the measured and simulated inband insertion losses. In spite of these differences, the fullwave EM simulation (HFSS) and measurement results are still in good agreement. The fabricated filter exhibits 2.5- dB insertion loss at center frequency 9.5 GHz (including the loss of transitions), a 0.3-dB bandwidth of 230 MHz, and return loss of better than 18 dB. The measured out-

Fig. 5. Simulated and measured results of the fabricated three-pole partial H-plane filter: (a) S parameters and (b) group delay.

of-band rejection is better than 54 dB and 21 dB at 1-GHz lower and higher separation from the center frequency of 9.5 GHz, respectively. The measured group delay is better than 8 ns in the passband.

IV. Conclusion

A novel H-plane SIFW band-pass filter using double H-plane septa has been developed and investigated in this paper. The filter is considerably smaller than its metallic waveguide counterpart, and has frequency responses similar to those of traditional E-plane double-iris bandpass filters. Furthermore, the filter performance was determined by the layout in the LTCC multilayer. Transitions to microstrip were demonstrated, allowing SIFW to be easily integrated with other planar circuits. A three-pole filter working in the X-band was designed, fabricated using LTCC technology, and measured. Good agreement between the simulated and measured results was presented.

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wang et al.: SIFW H-plane band-pass filter

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Zhengwei Wang received the M.Sc. degree from the University of Electronic Science and Technology in 2007. Currently, he is studying toward the Ph.D. degree at the same university. His research interests include millimeter-wave TR modules and high-temperature superconductor passive devices.

Shirong Bu received B.Sc., M.Sc., and Ph.D. degrees from the University of Electronic Science and Technology (UESTC) in 1999, 2002, and 2009, respectively. Currently, he is an associate professor at UESTC. His research interests include microwave measurements and microwave passive devices.

Zhengxiang Luo graduated from Sichuan University in 1968 and received the M.Sc. degree from the University of Electronic Science and Technology (UESTC) in 1983. He is a professor at UESTC, and a member of the Chinese International Superconductor Technical Commission. His research interest includes superconductor microwave electronics.