Design of Dual-Mode SIW Cavity Filters

Chia-Cheng Chuang, Hung-Hsuan Lin, and Chin-Li Wang Information and Communication Research Laboratories

Industrial Technology and Research Institute 195 Sec. 4, Chung Hsing Rd., Chutung, Hsinchu 310, Taiwan

Abstract- For designing the dual-mode substrate-integrated waveguide (SIW) cavity filters, new methods of generating intra-cavity couplings and transmission zeros are presented. Two adjustment techniques for refining the frequency responses are also presented. Compared with a conventional four-pole quadruplet filter, a proposed four-pole dual-mode coupled-cavity filter can provide two additional transmission zeros, which enhance the side-band rejection. A 24-GHz SIW cavity filter with a symmetrical frequency response was designed using the new methods and fabricated using LTCC multilayer process.

I. INTRODUCTION

In the design of microwave and millimeter-wave filters, a modern type of guiding-wave structure named the substrate integrated waveguide (SIW) has attracted much interest because it is low-profile and easily produced with planar circuit processes, such as PCB and LTCC. The SIW is constructed in a dielectric substrate with a pair of metal plates and two parallel arrays of metallic vias that function as the two sidewalls. Similar to a rectangular waveguide, the SIW supports the TE mode propagation with low loss and high-power handling capability. As shown in Figure 1(a), enclosing the SIW with two additional arrays of vias yields a SIW cavity, which can be a resonator in the design of band-pass filters. Recently, several works about the design of SIW cavity filters [1]-[3] have been proposed.

A four-pole band-pass filter in the configuration of a quadruplet is generally regarded as a canonical topology that can implement an elliptic or pseudo-elliptic filtering function. The conventional realization of a quadruplet utilizes four cavities arranged in a main coupling path. The coupling between the cavities connected to the input and output forms the cross-coupling path that can bring the transmission zeros (TZs) aside the passband. These cavities are usually taken to operate in their fundamental resonance modes, for example, the TE101 mode of rectangular cavities. An alternative topology called the dual-mode filter [4]-[6] utilizes the dual-mode operation, which is the coupling of degenerate modes within a single cavity. This kind of filter is developed for the considerations of lower loss and smaller form factors since fewer cavities are required for realizing the same filtering function as that of a quadruplet. The concept of the dual-mode operation has been applied to the design of SIW cavity filters. Ref. [7] proposed a two-pole filter designed with a dual-mode SIW cavity and suggested a particular feeding structure that ensures two TZs. Based on the two-pole topology of [7], we propose several new methods of generating intra-cavity couplings and controlling the frequencies of the TZs. Two adjustment techniques are then used to refine the frequency response. In the following

(a)

(b)

Figure 1 (a) Structure of a SIW cavity. (b) Electric field distributions of

degenerate modes sections, these methods and adjustment techniques are first demonstrated to modify the two-pole topology and then used to design a 24-GHz four-pole dual-mode SIW cavity filter.

II. CIRCUIT ANALYSIS AND DESIGN

Consider a SIW cavity constructed as Fig. 1(a). The resonant frequency with respect to the TEmnl mode is determined by the equation,

222

2

+

+

=

cl

bn

amV

frr

Cmnl

πππεµπ

, (1)

where m, n, and l are the indices of the mode, a, b, and c are the physical dimensions of the cavity along corresponding axes, µr and εr are the permeability and permittivity of the filled material inside the cavity, and VC is the velocity of light in free-space. Considering the dimension of thickness is much smaller than the others, the index n is assigned to be zero in this work. It follows that the field intensity varies two-dimensionally, which does simplify the analysis of mode operations and coupling mechanism.

A. Key Factors of the Dual-Mode Operation A key factor of the dual-mode operation is to generate

two orthogonal modes that resonate at the same frequency. Such two modes are typically regarded as a pair of degenerate modes. Suppose the index sets of these two degenerate modes are (m, 0, l) and (p, 0, q). The only additional constraint for the dual-mode operation is that m ≠ p and l ≠ q [5]. An obvious example is a square cavity (the case of a = c) since its TE201 and TE102 mode substantially

resonate at the same frequency. Fig. 1(b) demonstrates the electric field distributions of the two modes. The following analysis and design are based on the characteristics of this kind of cavity.

Another key factor is how to design the coupling structure that provides a wanted passband as well as a good side-band rejection. Fig. 2(a) presents the structure of the two-pole dual-mode SIW cavity filter proposed in [7]. Two metallic vias near particular diagonal corners are set to perturb the two degenerate modes. The two degenerate modes thus become non-orthogonal so as to induce a coupling. Such coupling is usually regarded as an intra-cavity coupling, which accounts for the generation of the passband. The farther the via is pushed apart from the corner, the stronger the coupling is induced. The feed lines are allocated at two near sides of the square cavity with coupling apertures opened on the sidewalls, and the feed points are both at the center of contacted sides. This topology provides two TZs, each on each side of the passband. A validating coupling scheme [6] as shown in Fig. 2(b) characterizes this feature. Node 1 and 4 represent the transitions from microstrip (TEM mode) to waveguide (TE mode). Node 2 represents the natural mode excited by port 1 and also the degenerate mode seen by port 2. On the contrary, node 3 represents the natural mode excited by port 2 and also the degenerate mode seen by port 1. A coupling matrix of the low-pass equivalent prototype [8] corresponding to this kind of filter is

−−−

−−−

082.01.005.082.008.01.0

1.08.0082.005.01.082.00

. (2)

The network has to be anti-symmetrical so that the matrix elements must meet the requirements, M12 = M34 and M13 = M24. The appearance of the two TZs results from the cross coupling path formed by M14, which is typically regarded as a source-load coupling.

An example for inspection was simulated and is described as follows. The size of the cavity is 200 x 10 x 200 mil3. The dielectric material of the substrate is LTCC of dielectric constant of 7.8 and loss tangent of 0.005. The material of the conductor is silver. The distance between the perturbation via and the nearby sidewall is 28 mils. The coupling aperture is 70-mil wide. The simulation responses are shown in Fig. 2(c). It is worthy to note that the response is not well symmetrical. The high-side TZ is much more distant from the passband than the low-side one. This phenomenon is attributed to the weak couplings, M13 and M24, while the feed points are close to the null of electric resonance of the degenerate modes. Note that the null is no longer at the center of the cavity side since the degenerate modes have been perturbed.

B. Shift Feeding Structure and Adjustment Techniques It can be proven that the elimination of M13 and M24

yields a symmetrical response [8]. This paper proposes a modification on the feeding structure of Fig. 2(a) to achieve the elimination. It is done by shifting the feed points toward vertex point A as shown in Fig. 3(a). Note that the coupling

(a)

(b)

(c)

Fig. 2 Two-pole dual-mode SIW cavity filter [7]. (a) Structure. (b)

Coupling scheme. (c) Simulation responses. apertures must be shifted along with the feed points. The shift can effectively reduce those weak couplings and move the two TZs to lower frequencies. A critical shift can relocate the feed point at the null of the electric resonance of the degenerate mode so as to completely eliminate these couplings, making the response symmetrical. An over shift yet alters the sign of these couplings and results in an opposite asymmetrical response as well. Fig. 3(b) displays the effect of the shifts on the frequency responses in the cases of 10-mil and 20-mil shift distance. The shift feeding technique is therefore useful in the control of these TZs. Unfortunately, after the shift, both the external loading to the input/output and the intra-cavity coupling are spoiled; so are the responses. Therefore, we propose two adjustment techniques, reducing the perturbations and widening the apertures, to optimize the response. The shift feeding and adjustment techniques were used to refine the previous example for a symmetrical frequency response, as described as follows. The feed points were shifted by 12 mils. The perturbation distance was reduced to 24 mils. The width of the apertures was extended to 75 mils. Fig. 3(c) shows the optimized frequency responses.

C. Rectangular SIWs for Intra-Cavity Couplings Making two degenerate modes non-orthogonal is a key

factor of generating the intra-cavity coupling. Based on the concept, this paper proposes an alternative design of intra-

(a)

(b)

(c)

Fig. 3 (a) Demonstration of the shift feeding. (b) Frequency responses of

shift distances of 0, 10, and 20 mils. (c) Frequency responses of the optimized two-pole cavity filter.

cavity coupling without the use of perturbation vias. The alternative design adopts rectangular cavities, of which two main dimensions (a and c) are different, instead of square ones. TE201 and TE102 are still used as the degenerate modes, but their resonant frequencies are separated. As a result, the two modes become non-orthogonal at any frequency, and the desired intra-cavity coupling is induced. The more difference in the dimensions results in the farther separation in frequency and also the stronger coupling.

D. Proposed Four-Pole Dual-Mode Filter Cascading two above mentioned rectangular cavities

creates a four-pole filter. A four-pole dual-mode cavity filter was designed in this way and optimized using the shift feeding and adjustment techniques. Fig. 4(a) demonstrates the structure of the four-pole filter. All of the inter-cavity

(a)

(b)

(c)

(d)

Fig. 4 Four-pole SIW cavity filter(a) Structure. (b) Structure of the stacked

cavities. (c) Coupling scheme. (d) Low-pass equivalent responses. couplings are generated through the iris between the two cavities. For taking advantage of multilayer lamination of LTCC substrates, stack up the cavities to save half of the circuit area, as shown in Fig. 4(b). The iris for inter-cavity couplings is then replaced by the aperture slot in the common conductor plate of the cavities. The structure looks more compact, and the feed lines are laid in the same line. Fig. 4(c) demonstrates the coupling scheme of the four-pole filter. The coupling matrix of the corresponding low-pass equivalent prototype is

−

−

098.0014.00003.098.0079.0032.028.00

014.079.0076.0032.000032.076.0079.0014.0028.0032.079.0098.003.000014.098.00

. (3)

Fig. 4(d) shows the equivalent low-pass responses. A wide and deep side-band rejection can be obtained by the four TZs, two of which are on the low side of the passband and the other two on the high side. The generation of these TZs is attributed to two cross coupling paths. The coupling path that generates the inner pair of TZs is formed by M25, and the other that generates the outer pair is formed by M16. M13 and M46 are again weak due to the shift of feed lines. Other weak couplings, M24 and M35, are the couplings between degenerate modes of different cavities. It is evident that these weak couplings tend to spoil the symmetry of the frequency response.

The four-pole SIW cavity filter has a center frequency of 23.4 GHz and a bandwidth of 2 GHz and was fabricated using LTCC process. The dimensions are detailed as follows. The size of each cavity is 212 x 10 x 176 mil3. The diameter of all the vias is 4 mils. The interval between two adjacent vias of the sidewall is 16 mils. The aperture for feed lines is 100-mil wide. The feed lines were shifted from center points of the corresponding sides by 40 mils. The size of the aperture slot is 68 x 10 mil2. The distance between the edge of the aperture slot and the nearby sidewall is 6 mils. Fig. 5 displays the measurement results.

III. CONCLUSIONS

The new methods of designing dual-mode SIW cavity filters, including the use of rectangular cavities for generating the intra-cavity couplings and the shift of feed points for controlling the TZs, have been presented. Two adjustment techniques for optimizing the filter responses also have been presented. Using the method of shift feeding and the adjustment techniques has effectively modified the frequency response of the two-pole filter topology being symmetrical. The new methods and adjustment techniques were then used to design a four-pole SIW cavity filter that

Fig. 5 Measurement results of the four-pole SIW cavity filter.

can provide a symmetrical response and four TZs aside the passband. This filter has a small form factor due to the stacked cavities and was fabricated using LTCC process. The measurement results are good enough to validate the design.

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