диафрагмированные волноводные фильтры / b0e1a3a5-bd3c-4ad5-9ed1-28399e9dd10c

Abstract—A novel miniaturized and light weight W-band bandpass filter (BPF) is proposed based on the gap waveguide (GWG) concept using the Micro-Electromechanical System (MEMS) technique. The BPF is achieved by cascading four rectangular GWG resonant cavities with designed rectangular and cross-shaped coupling structure. Two standard WR-10 waveguide coupling windows are designed as the input/output couplings, which contributes to the flexible application in various packaging configurations. By utilizing the classical coupling matrix theory, the proposed MEMS GWG BPF is designed to work from 89.8 to 97.4 GHz, which is fabricated by the deep reactive ion etching process using the silicon wafers, and the fabricated silicon wafers are also electroplated and bonded together as a whole structure. The measured insertion loss is within 1.6 dB, and the return losses are larger than 15 dB.

Index Terms—Bandpass filter (BPF), Gap waveguide (GWG), Micro-Electromechanical System (MEMS), Micro-system packaging and integration, W-band.

I. INTRODUCTION

RECENTLY W-band has attracted much attention in various fields, such as Automotive radar, 5-Generation (5G) backhaul, synthetic aperture radar [1]-[3]. In the W-band system, the bandpass filter (BPF) located in the transmitter plays an important part in suppressing the spurious emission. It can also be placed before the low noise amplifier to act as a pre-selection filter, when its insertion loss is low without

deteriorating the noise figure of the receiver seriously. Traditionally, BPFs are generally designed in hollow

waveguide structure, due to low insertion loss and high power handling capability [4]. However, the high precision of the

Manuscript received Jul. 3, 2018, revised Aug. 15, 2018, revised Sep. 15, 2018, accepted Sep. 23, 2018. This work is supported by the National Natural Science Foundation of China under Grant 61601421, 61822110, and Guangdong Innovative and Entrepreneurial Research Team Program (No. 2017ZT07X032). (Corresponding authors: Yongrong Shi; Wenjie Feng)

Y. Shi, J. Zhang, and M. Zhou, are with the Nanjing Electronic Devices Institute, 210016 Nanjing, China. (E-mail:yongrongshi@hotmail.com).

W. Feng, W. Che are with the Department of Communication Engineering, Nanjing University of Science and Technology, 210094 Nanjing, China, and also with School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510006, China. (E-mail: fengwenjie1985@163.com)

B. Cao is with the Nanjing Corad Electronic Equipment Corporation, 211106 Nanjing, China.

metal machining at W-band is very expensive, and it is also too

bulk for integration in a compact space. As an alternative solution, the W-band filter is also commonly made by inserting the E-plane circuit substrate in the waveguide package which is also called waveguide E-plane filter [5]. However, the assembling of the substrate (for example, Rogers5880 substrate with the thickness of 0.127 mm) is challenging. Recently, Gap waveguide (GWG) technique emerges in many millimeter-wave applications including antenna array, passive component [6], noise suppression [7], and interconnection with planar transmission line for MMIC integration [8]. Compared with the above traditional waveguide BPF, GWG BPF provides another attractive solution for the millimeter-wave BPF design [9]-[12]. For example, eight GWG resonant cavities are cascaded in the horizontal direction for Ka-band radio link diplexer application [9]. In [10], three GWG resonant cavities are cascaded in the vertical direction instead of the horizontal direction for Ka-band RF front end application. Another Iris GWG BPF is also proposed for the Ka-band operating frequency [11]. For these GWG BPFs [9]-[11], the coupling WR-28 rectangular waveguide flanges are all designed in the electromagnetic wave propagation direction. Besides these Ka-band GWG BPFs, a V-band end-coupled BPF [12] based on inverted microstrip GWG is also proposed, and the additional two V-band transitions between inverted microstrip GWG and rectangular waveguide are needed. However, there is a lack of W-band BPF based on GWG for W-band applications. To improve the integration level of the W-band system, the input/output coupling waveguide windows need to be designed perpendicular to the electromagnetic wave propagation direction for the surface mounted packaging application.

To promoting the GWG applications in the future miniaturized microsystem above 100 GHz, S. Rahiminejad have done some GWG researches (including 90° bend GWG transmission and antenna working at 100 GHz) based on the Micro-Electromechanical System (MEMS) technique [13]. Previously, the MEMS technique, such as deep reactive ion etching silicon micromachining process, is mainly applied to design and fabricate the passive components in Terahertz [14] due to its high precision. In addition, the MEMS technique has an advantage of light weight design, which makes it possible that the proposed MEMS GWG BPF can be designed as surface

(b)

Fig. 1. Schematic diagram of the proposed GWG BPF for (a) traditional waveguide packaging application with the input/output on the different layers, and (b) surface mounted packaging application with the input/output on the same layer.

mounted device to improve the integration of the W-band system.

Inspired by the MEMS GWG concept, a novel miniaturized and light weight W-band MEMS GWG BPF is proposed with the two directly input/output coupling WR-10 windows perpendicular to the electromagnetic wave propagation direction. Using the classical theory of the BPF based on coupled resonators, the proposed MEMS GWG BPF is designed to work from 89.8 to 97.4 GHz by cascading four MEMS GWG rectangular resonant cavities in the horizontal direction. Two standard WR-10 coupling windows are designed in the vertical direction instead of the additional transition structures in [12], and this special feature contributes the flexible using in various packaging systems. As depicted in Fig. 1(a), it can be assembled in the metal housing for transitional waveguide packaging application. Moreover, as shown in Fig. 1(b), it can also be surface mounted on the other substrates by using coupling transition between WR-10 and planar transmission lines. In this paper, both of the packaging applications are studied. Firstly, the MEMS GWG BPF is designed according to the dispersion diagram and the BPF theory based on the coupling matrix. Then, the W-band MEMS GWG BPF prototype is fabricated and measured for traditional waveguide packaging application. In this scenario, only few metal losses are introduced and the measured results can be referred as the filter performance of the MEMS GWG BPF itself for simplicity.

Good agreement can be achieved between the simulated results and the measured ones. The measured results show that the insertion losses of the proposed MEMS GWG BPF are lower than 1.6 dB, and the return losses are mostly above 15 dB. In addition, the size of the proposed MEMS GWG BPF is 20 mm×10 mm, and its weight is as light as 0.5 g. Finally, the surface mounted packaging application is introduced on the Ferro A6M LTCC substrate with the coupling transition between WR-10 and microstrip line for MMIC integration with the proposed MEMS GWG BPF in the final part of the paper.

(b)

Fig. 2. Proposed W-band MEMS GWG BPF (Fig. 1(a) for traditional waveguide packaging is chosen for demo). (a) 3-D View with the GWG unit cell of the rod inserted. (b) Top view and the geometry parameters.

TABLE I

GEOMETRICAL PARAMETERS OF THE PROPOSED MEMS BPF (UNIT: MM)

II. PROPOSED W-BAND MEMS GWG BPF FOR WAVEGUIDE

PACKAGING APPLICATION AND FILTER PERFORMANCE

MEASUREMENT

A. Proposed W-band MEMS GWG BPF Structure

Figs. 2(a)-(b) show the 3-D view and top view of the proposed W-band MEMS GWG BPF structure, respectively. The GWG is located on the bottom silicon wafer. Four GWG rectangular resonant cavities R1-R4 are cascaded symmetrically, as shown in Fig. 2(a). Two input/output coupling WR-10 windows are designed on the top and bottom silicon wafer for waveguide packaging application, and filter performance measurement. As shown in the Fig. 2(b), the width and the period of the GWG rods are w and l2, respectively. The distance between the terminal rods and the inner wall is l1. The location and the lengths of the rods in the four resonate cavities are designed as l3, l4, l5, l6, to obtain the certain coupling coefficients. The distance between the WR-10 coupling window and the inner wall is l7. All of the geometry parameters are listed in Table I.

At the initial design stage, the unit cell of the GWG should be calculated by the dispersion diagram to confirm that the GWG working frequency range can cover the passband of the W-band BPF. Fig. 3 shows this dispersion diagram of the GWG unit cell. The dispersion diagram is simulated by 3-D full-wave

Fig. 3. Dispersion diagram of the periodic rod used in the GWG.

commercial software with the periodic boundary, and the Brillouin zones along − − of the unit cell is also added in Fig. 2 for clarity. The stopband is from 85 to 118 GHz. It means that electromagnetic wave can be forced to resonate in the desired cavities by means of grooves in the GWG. Though these cavities are not closed resonator, they show the similar resonance electromagnetic filed pattern like that in the substrate integrated waveguide (SIW) structure. By the eigen mode simulation in HFSS or CST, the resonant frequencies of cavities R1-R4 can be designed around the center frequency fcenter of the BPF, as shown in Fig. 4(a). For fcenter=93.6 GHz, 0.14 dB-ripple with fractional bandwidth (FBW)=8.5%, and the return loss>15 dB, the designed coupling matrix M and the external quality factors are given as [9]-[10], [16]-[17]

M12=M34=0.0686, M23=0.0550), mij is the so-called normalized coupling coefficient, Qe is the external quality factor (Qe=13.9), and f01/f02 are the higher and lower resonant frequencies of the two coupled resonators, respectively. To determine the geometry parameters, the coupling coefficient (M12, M23) and external quality factor (Qe) design curves are figured as shown in Figs. 4 (b)-(d). After obtaining the initial design geometry parameters, an optimization process is usually required to obtain the satisfactory results, because the couplings as well as resonant frequencies are slightly changed when the four resonators are cascaded together.

B. Prototype fabrication and experimental

The proposed W-band MEMS GWG BPF is fabricated and assembled as following. Firstly, the top and bottom silicon

Fig. 4. The calculated design curves: (a) fcenter, (b) M12, (c) M23, (d) Qe.

Fig. 5. The fabricated silicon wafers. (a) Top silicon wafer, (b) bottom silicon wafer, (c) two silicon wafers are bounded and diced as single MEMS GWG BPF chip component, (d) the details of the proposed W-band MEMS BPF and its measured metal housing.

wafers are fabricated by the deep reactive ion etching process using the high or low resistance silicon wafers, as shown in Figs. 5(a)-(b). Then, the two fabricated silicon wafers are electroplated and bonded together as a whole structure, and the whole structure is diced into MEMS GWG BPF components [17], as shown in Fig. 5(c). Then the single MEMS GWG BPF component is assembled in the metal housing with two WR-10 rectangular waveguide flanges as the device under test (DUT), as shown in Fig. 5 (d). The inner X-ray image is inserted in Fig. 5(d) for clarity. For measuring with the WR-10 waveguide

(a)

(b)

Fig. 6. BPF performance (a) Measured insertion loss and return loss of 10 samples of the proposed W-band MEMS GWG BPF, (b) MEMS process variation analysis.

flanges, the vector network analyzer (VNA) extender (FEV-10-TR, Farran Technology) and the N5245A VNA are used. The measured results of the fabricated 10 samples are shown in Fig. 6(a) compared with the simulated ones. Good agreement can be found between the simulations and the measurements, and the discrepancies between the 10 DUT samples are acceptable. There is almost no frequency shifting between the measured and simulated ones. From the measured results, there is an excellent low loss passband (insertion losses between 0.9 dB and 1.6 dB) from 89.8 to 97.4 GHz, in which frequency band the return losses are almost above 15 dB. The measured insertion losses are 1 dB higher than the simulated ones, due to the metal losses of the BPF itself and the copper mounted housing. Its flat low insertion losses make it suitable for both the transmitter and the receiver. Finally, the influence of the MEMS process

Fig. 7. The coupling transition structure between the WR-10 and the microstrip line. (a) 3-D View in details, (b) its transition performance.

variation on the filter performances are investigated by the HFSS simulations as shown in Figs. 6(b)-(d) (gary lines). A total of about 39 simulations including every dimension of the filter within ±5um in the x-/y- direction, and ±10um in the z-direction. As can be seen, the performances of the filter are not very sensitive to the process variation. The higher cut off frequency fH of the passband becomes higher (about +2.5 GHz), while the lower cut off frequency fL shows stability.

Compared with the former GWG BPF [9]-[11] in Table II, the chebyshev topology and resonators based on periodic pin GWG are applied in all of them including this work. However, the input/output coupling waveguide windows are designed perpendicular to the electromagnetic wave propagation direction only in this work, which gives more potential in various packaging applications. In addition, the proposed MEMS GWG BPF can work at the highest frequencies (i.e. W-band) with the lightest weight 0.5 g, without deteriorating the filter performances of insertion/return losses. The cost of the MEMS GWG BPF is lower than that fabricated by the machining due to the massive unit cells in the 6/8/12 inch wafer. The device size of the proposed MEMS GWG BPF is smaller than these in [9], [11]-[12], but a little larger than that of the GWG BPF in [10]. However, the proposed MEMS GWG BPF achieves a low-profile design due to its smallest device size in the z- direction.

In addition to the waveguide packaging application, the proposed BPF can also be used as surface-mounted chip component on the surface of other substrates due to its light weight/low profile design and the vertical input/output directly couplings. It should be pointed out that the waveguide flanges can be replaced by the alignment marks in the surface mounted packaging, which contributes to a much more miniaturization design.

(a) (b)

Fig. 8. Surface mounted packaging application. (a) The 3-D View of the proposed MEMS GWG BPF combining with two coupling transitions. (b) filter performance comparison in the two packaging.

III. PROPOSED W-BAND MEMS GWG BPF FOR SURFACE MOUNTED PACKAGING APPLICATION ON LTCC SUBSTRATE

For LTCC surface mounted packaging application, a W-band coupling transition structure between the WR-10 and the microstrip line is firstly needed with integration of MMIC. Fortunately, such a structure was proposed by the author (B. Cao) in [15], as depicted in Figs. 7(a)-(b). It can be seen that the operating frequency range of this coupling transition structure is from 86 to 101 GHz with the insertion losses less than 1 dB and its wide operating bandwidth can cover the passband of the proposed MEMS GWG BPF, as depicted in Fig. 7(b).

Due to the symmetry of the proposed MEMS GWG BPF, the two WR-10 waveguide coupling windows can be designed on the same layer (top layer or bottom layer). To evaluate the filter performance in such a surface mounted packaging application, a full wave co-simulation is investigated by combined two LTCC coupling transitions and the proposed MEMS GWG BPF, as shown in Figs. 8(a)-(b). The results of the co-simulation are given, and the filter performance in the waveguide packaging in Section II is also added for comparison. It can be seen that the passband frequencies are almost not changed. The lower stopband is improved in the surface mounted packaging, while the upper stopband deteriorates little compared with that in the waveguide packaging. In addition, the reflection zeros shift to lower frequencies in the passband. These slight changes of the filter performances are all caused by the introduced coupling transitions. The additional SIW cavity resonators in the LTCC coupling transitions modify the input/output coupling and change the external quality slightly. However, the investigation of the co-simulation proves it that the proposed W-band MEMS GWG BPF can also be used in the surface mounted packaging which can effectively improve the integration of the W-band LFMCW SAR or 5G backhaul systems, and the system weight will be reduced evidently.

IV. CONCLUSION

A novel W-band MEMS GWG BPF is proposed for both traditional waveguide and surface mounted packaging applications in this paper. Four rectangular GWG resonant cavities are cascaded in the horizontal direction and the input/output direct waveguide coupling windows are located in the vertical direction, forming the BPF with the operating frequency range from 89.8 to 97.4 GHz. This novel W-band BPF is fabricated by using the high precision MEMS technique.

The measured results show a good agreement with the simulated ones in the W-band. The proposed W-band MEMS GWG BPF has a potential application in the traditional waveguide transceiver system. Moreover, it can also be used as a surface-mounted device on the other substrates using coupling transition between WR-10 and planar transmission lines for MMIC and antenna integration.

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