Comparison of post and iris substrate integrated waveguide band-pass filters for X-Band applications

Abstract

Keywords: Band-pass filter; Centered post filter; Iris filter; SIW

Introduction

Microwave filters have been in the picture for a long time and have been playing an important role in civilian applications and military systems such as wireless computer networks, military and government radar applications, including weather monitoring, air traffic control, maritime vessel traffic control, defense tracking and vehicle speed

detection [1-2]. Since the electromagnetic spectrum is limited, the channel bandwidth became a scarce resource [3]. Therefore, the role of microwave filters became very important in the mentioned applications. In these applications, tighter specifications for microwave filters are required. To that end, the most commonly used filters are band-pass

*Corresponding author. *E-mail address: nturker@yildiz.edu.tr This paper was recommended for publication in revised form by Regional Editor N. Özlem Ünverdi Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

filters (BPFs). Depending on BPF application, parameters like insertion loss, return loss, bandwidth and attenuation require careful optimization steps. As frequency increases, transmission loss increases, and the amount of energy emergence decreases [1]. As a result, hollow-pipe waveguides such as rectangular waveguides (RWs) are preferred. RWs and RW filters are necessary and fundamental components in various practical microwave and millimeter-wave applications, especially for communication systems that require low loss and high power handling capability at higher frequencies like radar communications, satellite communications and mobile radio communications [4-6]. However, RW filters have the disadvantages such as bulky size, difficulty of manufacturing, high cost and difficulty of integration with printed structures due to the need of complex transitions [7-8]. Due to the necessity for miniaturization and integration, planar transmission lines like the microstrip line and stripline are used instead of RWs for many applications. However, these structures have high loss and low power handling capability. As a result, to overcome these difficulties, substrate integrated waveguide (SIW), which combines the advantages of waveguides with the advantages of planar transmission lines, has been proposed [8-9]. SIW has crucial properties such as small size, ease of manufacturing, low cost, high performance, high power handling capacity, low interference and simplicity of integration with other microwave components [10]. SIW belongs to the family of substrate integrated circuits (SICs) [11-12]. Its structure consists of two arrays of metallic via-holes parallel to each other embedded in a dielectric substrate that electrically connect two parallel metal plates (upper and lower) [13-14]. These via-holes are equivalent to the two metal sidewalls of the RW, while the top and bottom layers of SIW are equivalent to the upper and lower RW walls [15]. When compared to microstrip and waveguide structures, a SIW has the characteristic of having no additional components and can be easily realized with a conventional standard printed circuit board (PCB) process, thick film process or low-temperature cofiring ceramic (LTCC) process [10, 13, 16]. This makes the integration of SIW and other planar circuitry on the board simple and cost-effective. Besides, it is claimed to perform better than microstrip structures at high frequencies and has the waveguide transmission performance [13]. Because of all of these properties and advantages, SIWs have been applied to several microwave components, including filters [17–18], directional couplers [19], oscillators [20], antennas [21] and circulators [22]. In the last years, many planar filter structures have been proposed in microstrip line and stripline form [2324]. Such filters have advantages like low cost, small size, easy to integrate, but it has a low-quality factor (Q = 50-100). Although RW filters have many disadvantages, their quality factor is high (Q = 5000-10000). When the

requirements such as integrability, low cost, small size, high power handling capability and low insertion loss are considered, SIW filters become a better alternative. SIW filters bridge the gap between microstrip line filters and RW filters by achieving a relatively high-quality factor (Q = 500-1000) [25]. Due to the properties and advantages of the SIW filters, this paper is contributed to design SIW BPFs that work at the X-band frequency range (8-12 GHz); these filters could be used in many applications including radar and satellite communications. The X-band has been chosen because it is used for a wide range of civil and military applications [2627]. Moreover, it has extremely low rates of atmospheric attenuation in comparison to frequencies above 10 GHz, which makes this band suitable for the harshest weather conditions [28]. Although different types of SIW filters can be found in the literature [29-40], the design procedures are not given properly or are suited only for a specific filter order. This work discusses the design procedures of two different types of SIW BPFs in details; in particular the centered post and the iris types. Third and fifth-orders of both types have been simulated by taking into consideration two types of filter responses (Butterworth, Chebyshev). The design and simulation results of a third-order filter on different bandwidths are demonstrated, as well. All the simulations have been done using CST Microwave Studio. Post and iris third-order filters with Chebyshev response have been fabricated and scattering parameters of the prototypes are measured.

SIW Filter Design Methodology

In this part, the design steps of the centered post and iris SIW BPFs are discussed. In general, the design of the SIW itself starts from an equivalent RW. In this work, the equivalent RW is the WR-90 waveguide that works on the desired frequency band (X-band). Fig. 1 shows the structure and dimensions of the SIW. In SIW design, we used copper and RT/Duroid 5870 dielectric material, which has a relative permittivity (εr = 2.33) and a loss tangent (tan δ = 0.0012 at 10 GHz). The procedure discussed in [10] is used to calculate the dimensions

of the corresponding SIW. To minimize the radiation loss of the SIW while designing it, the pitch length (p) and the via-hole diameter (d) should achieve the following conditions d < λg/5 and p ≤ 2d, where λg is the guided wavelength [25, 32]. As a result, SIW dimensions are calculated as follows: L = 59.421 mm, W = 20 mm, as = 15.9028 mm, d = 1.3205 mm, p = 1.9807 mm. Note that the thickness of the substrate (h) is 1.57 mm. Centered post SIW BPF design procedure The design procedure of the centered post SIW filter shown in Fig. 2.a begins from the derivation of the equivalent RW filter. A post is a shunt inductance in the waveguide, which acts both as a cavity wall and as an impedance inverter coupling the adjacent cavities and it can be used to build RW BPFs [41]. Each post inside the RW has an equivalent circuit, as shown in Fig. 2.b [42]. It is known that the BPFs have a center frequency called (f0) and the bandwidth (BW) of the filter is the difference between upper and lower cutoff frequencies (BW = f2−f1). Eq. (1) describes the guide wavelength inside the RW in inches,

where, a is the width of the RW and f is the frequency in GHz. The guide wavelength at the center frequency (λg ) is 0 obtained by taking the arithmetic average of the guide wavelength at the lower frequency (λg ) and upper fre1 quency (λg ). The equations for the frequency fractional 2 bandwidth (w) and the guide wavelength fractional bandwidth (wλ) are given in Eqs. (2) and (3), respectively.

For the nth order filter, we can calculate the element values of the filter (g0, g1,…, gn, gn+1) using the methods described in [42], or we can take it from the ready tables that are made for that. Note that, in our work, we are dealing with two responses of the filters, which are maximally flat (Butterworth) response and equal ripple (Chebyshev) response. After determining the element values of the desired filter, we have to calculate the impedance inverter parameters (Kj,j+1) using Eqs. (4)-(6):

where, Z0 is the guide impedance. The shunt reactance (Xb) can be obtained as follows: X j , j +1 Z0

Figure 2. Centered post SIW filter (a) Its structure; (b) Equivalent circuit of a single post.

Figure 3. Circuit parameters of a centered inductive post in RW [42].

Figure 4. Iris SIW Filter (a) Filter structure with conducting walls; (b) Equivalent circuit of a single iris.

Circuit parameters of a centered inductive post in RW are exhibited in Fig. 3. λ0/a value determines the approximate curve to be used. By using that curve and the (Xbλg )/ 0 (Z0(2a)) ratio, the diameter (dj,j+1 where, j = 0,1,…,n) of each post is calculated. From the value of d/a and the inverse curve of (−Xaλg )/(Z0(2a)), Xa is calculated. 0 To calculate the distances between the centers of the posts (Li) inside the filter, Eqs. (8)-(10) are used: 1 θ j = π + φ j −1, j + φ j , j +1  2

 2X X  X  φ = − tan −1  b + a  − tan −1  a  Z0   Z0  Z0 

Once post diameters and the distances between them are determined, centered post RW BPF can be converted to centered post SIW BPF by dividing all the dimensions by the square root of the relative permittivity of dielectric material used to design the SIW.

Figure 5. Shunt reactance of symmetrical inductive window in RW [42].

Figure 6. Iris SIW BPF structure using small via-holes. Figure 7.a Third order SIW Post filter dimensions. Iris SIW BPF design procedure An iris is a conductive diaphragm placed transverse to a waveguide aperture that causes a discontinuity and generates shunt reactance and it can be used to build RW BPFs [41]. The design procedure of the iris SIW filter shown in Fig. 4.a begins from the derivation of the equivalent RW filter. The following procedure discusses the use of a thin iris, which has purely inductive characteristic, to build a BPF. Each thin iris inside the RW has an equivalent circuit, as shown in Fig. 4.b [42]. For purely lumped inductance iris having shunt reactance (Xb), the shunt reactance can be calculated using Eq. (7). Shunt reactance of symmetrical inductive window in RW are demonstrated in Fig. 5. a/λ0 value determines the approximate curve to be used. By using that curve and the (Xbλg )/(Z0(a)) ratio or the (Xb (a))/(Z0 λg ) ratio, the window 0 0 width (dj,j+1 where, j = 0,1,…,n) of each iris is determined. The distances between the irises (Li) are calculated by using Eqs. (11) and (12):  2 X j −1, j   2 X j , j +1   1 + tan −1  θ j = π −  tan −1    2  Z0   Z 0   Li =

Figure 7.b Third order SIW Iris filter dimensions. When iris window width and distances between them are determined, iris RW BPF can be converted to iris SIW BPF by following the same procedure of post filter. Since the structure of SIW depends on using via-holes, copper walls in the iris SIW filter is rebuilt by the use of a line of small via-holes, as shown in Fig. 6. To keep the pure inductiveness of iris, the via-holes should have a small diameter of 0.2 mm with 0.6 mm separation between them.

SIW Filters Analysis

In this section, the design and simulation results of both centered post and iris SIW BPFs are demonstrated.

Third-order and fifth-order filters with maximally flat and equal ripple responses are considered. Note that the simulated filters have a center frequency of 10 GHz and a bandwidth of 500 MHz. Moreover, the equal ripple response is assumed to be 0.1 dB ripple level. Fig. 7 shows the parameters of post and iris filters. Corresponding values for third and fifth-orders of the post and iris SIW BPFs dimensions are listed in Tables 1-2. The simulation results of third-order filters are exhibited in Fig. 8. The maximally flat third-order centered post SIW filter achieves an insertion loss (IL) of 0.68 dB and a return loss (RL) of 45 dB at the center frequency. The IL and RL are equal to 6.15 dB and 2 dB at the upper frequency, respectively, whereas they are 3.5 dB and 5.16 dB at the lower frequency. For the maximally flat third-order iris SIW filter, it achieves IL and RL of 0.52 dB and 20 dB at the center frequency. The pit of the RL curve, which is at approximately 35 dB level, is shifted to 9.9 GHz for this case. The RL and IL are 5.4 dB and 2.8 dB at the upper frequency, respectively. They are 6.88 dB and 4.5 dB at the

Table 1. Third-order post and iris SIW BPFs dimensions Centered post SIW filter dimensions (mm) d1

lower frequency. When RL and IL curves of post and iris filters are examined, wider bandwidth with lower insertion loss is observed for post filter. The equal ripple third-order centered post SIW filter achieves IL and RL of 0.78 dB and 23 dB at the center frequency. RL of the filter is below 17.5 dB at the upper half-band, whereas it is lower than 21.9 dB at the lower half-band. For the equal ripple third-order iris SIW filter, IL and RL are 0.89 dB and 21.23 dB at the center frequency, respectively. RL of the filter is below 24.5 dB at the upper half-band, whereas it is lower than 14.14 dB at the lower half-band. RL and IL curves of post and iris filters show that wider bandwidth with lower insertion loss is observed for post filter. Besides, lower insertion loss is observed at the lower and upper frequencies of the band with equal ripple response. The transmission characteristics (insertion loss and return loss) of the demonstrated SIW filters are almost identical to the equivalent RW filters’ characteristics as well. The numerical simulation results for third-order SIW filters are given in Table 3 clearly. The equal ripple fifth-order centered post SIW filter achieves IL and RL of 0.2 dB and 32 dB at the center frequency, respectively. It achieves RL better than 17.44 dB Table 2. Equal ripple fifth-order SIW BPFs dimensions

Iris SIW filter dimensions (mm) Maximally flat response Equal ripple response

Figure 8. Scattering parameters of the 3rd order centered post and iris SIW BPFs (a) Maximally flat response; (b) Equal ripple response.

Table 3. Numerical simulation results for (a) 3rd order post SIW filters; (b) 3rd order iris SIW filters (a) Frequency response

Figure 9. Scattering parameters of the 5th order centered post and iris SIW BPFs with equal ripple response. in the whole band. For the equal ripple fifth-order iris SIW filter, IL and RL are observed as 0.21 dB and 28 dB at the center frequency. It achieves a RL better than 14 dB in the whole band. IL at band extremes is lower for post filter. Although the IL of the 5th order filter is higher at the upper and lower frequencies of the band compared to 3rd order filter, steep-skirt selectivity of the 5th order filter is easily observed. The numerical simulation results for equal ripple fifth-order SIW filters are given in Table 4 clearly. Finally, simulation results of the third-order iris SIW BPFs with 0.1 dB equal ripple response, designed by applying the above mentioned procedure, are shown in Fig. 10 for different bandwidths. The figure clearly shows that the increase in the bandwidth results in higher insertion loss at the stop-bands of the filter. To demonstrate the usefulness of small via-holes instead of copper walls, third-order iris

SIW filter with equal ripple response is designed with both structures and simulation results are exhibited in Fig. 11. Filter with via-holes achieves IL and RL of 0.81 dB and 30 dB at the center frequency, respectively. Although insertion losses of the two filters are very close to each other at the pass-band of the filter, its level is lower at the stop-bands of the filter with via-holes. Thus, using small via-holes instead of copper walls does not affect the general performance and characteristics of the filter considerably. Table 5 includes a comparison of iris and post filters with other filters in the literature. In general, the simulation results of the proposed filters achieve an insertion loss and a return loss better than the filters found in [3132, 43-45]. Furthermore, two types of frequency responses (Butterworth, Chebyshev) are included and we have proposed filters with fifth-order which is not included in the literature.

Experimental Verification

This section shows and discusses the measured results of two fabricated SIW filters. One of them has been fabricated using post type, and the other using the iris type. The fabricated filters are third-order filters with 0.1 dB equal ripple response whose dimensions are given in Table

1. Prototypes are fabricated by chemical etching technique

with via-hole coating. Rogers RT/Duroid 5870 (εr = 2.33) substrate with 1.57 mm dielectric thickness and 35 μm copper thickness is used. Tapered microstrip line is used for the transition between 50 Ω SMA (sub-miniature assembly) connector and SIW filter. The size of the filters is 96.4 mm × 20 mm (lenght × width). Fabricated prototypes are given in Fig. 12. The top and bottom sides of post filter is shown at the upper side, whereas iris filter is given below. Both filters have the advantages of small size, low cost, integrability,

Table 4. Numerical simulation results for 5th order SIW equal ripple filters Filter type

Figure 10. Scattering parameters of the 3 order iris SIW BPF with different bandwidths. rd

Figure 11. Comparison of scattering parameters for the iris SIW BPF with copper walls and conductor via-holes.

Table 5. Comparison of iris and post filters with other filters in literature Reference [31]

Conclusions

In this paper, the design procedure for the widely used post and iris RW filters in SIW form is given. The methodology is applied to post and iris SIW filters with different orders, responses and bandwidths. By following the procedure, very similar reflection and transmission characteristics to that of post and iris RW filters are obtained with SIW filters. The designs are simulated by means of a full-wave simulator. Both types of SIW filters work on 10 GHz center frequency and 500 MHz bandwidth. Moreover, in simulations of third-order Chebyshev filters, the iris SIW filter achieves lower than 0.89 dB insertion loss and better than 21.23 dB return loss in the pass-band. Whereas the post SIW filter achieves lower than 0.78 dB insertion loss and better than 23 dB return loss in the pass-band. The characteristics of the post and iris SIW filters are compared by both simulation and measurement results. The filters exhibit low insertion loss and high reflection loss at the stop-band of the filter. With their low cost, small size, ease of manufacturing, high performance and integrability with other components, both of the demonstrated X-band SIW filters are very versatile and well suited for a variety

of market applications, including radars and satellite communications. Especially the post filters because they have an easy fabrication process comparing to the fabrication of iris filters. This work can be extended by adjusting the size of the posts tunable, which enables the user to change the center frequency and the bandwidth slightly, or the shape of the filter response.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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