Analysis , Design , and Actual Fabrication of a Hybrid Microstrip-SIW Bandpass Filter Based on Cascaded Hardware Integration at X-Band

In this paper, the Microstrip-Substrate Integrated Waveguide (M-SIW) bandpass filter is designed, simulated, and fabricated based on the theoretical analysis. The Substrate Integrated Waveguide (SIW) highpass filter and the microstrip lowpass filter are combined in a hybrid design to achieve the M-SIW bandpass filter in the X-band. This design is more comprehensible and easier to achieve a bandpass filter at a desired frequency. The SIW highpass filter and the microstrip lowpass filter are connected in series to achieve the bandpass filter. To the measured results of the fabricated M-SIW bandpass filter, the center frequency is 10.20 GHz and the bandwidth is 2.40 GHz. When the analytical and measurement results are compared, the frequency change in the cut-off frequency is 6.02 % and the frequency change in the bandwidth is 8.74 %. It is generally seen that analytical, simulation, and measurement results are compatible with each other. The M-SIW bandpass filter can be broadly used in radar, Worldwide Interoperability for Microwave Access (WiMAX), and satellite technologies. The filters are simulated in Computer Simulation Technology (CST) Studio Suite.

The main purpose of this study is to design a modular expandable bandpass filter that can be a reference for future studies. The hybrid bandpass filter is designed in three stages. In the first step, a highpass filter is designed. In the second step, a lowpass filter is designed. In the third step, a substrate integrated waveguide highpass filter and a microstrip lowpass filter are combined in a hybrid filter design. The M-SIW bandpass filter with the desired bandwidth and cut-off frequency is obtained by cascaded hardware integration of these two filter designs. This hybrid design approach helps to produce the bandpass filter transparently and quickly. Simple and clear structures are preferred instead of complex structures. Thus, the bandpass filter with desired cut-off frequency and bandwidth can be designed with fewer variables. This method increases the flexibility of the bandpass filter design.
The lengths of the microstrip lowpass filter parameters are determined based on 0.125λ lpf (λ lpf is the wavelength of the microstrip lowpass filter). The transition from microstrip to SIW is a tapered section [18]. Frequency response of fullmode SIW is better than the transmission efficiency of halfmode SIW. Therefore, full-mode SIW is preferred in this bandpass filter design [19].

II. ANALYSIS AND DESIGN OF THE PROPOSED M-SIW BANDPASS FILTER
In this study, the frequency range of the proposed M-SIW bandpass filter is considered 8.39 GHz-11.02 GHz. The Analysis, Design, and Actual Fabrication of a Hybrid Microstrip-SIW Bandpass Filter Based on Cascaded Hardware Integration at X-Band center frequency f C is 9.62 GHz, the -3 dB lower cut-off frequency f L = 8.58 GHz, the high cut-off frequency f H = 11.2 GHz, and the bandwidth BW = 2.63 GHz. The M-SIW bandpass filter is achieved by using a lowpass filter and a highpass filter structures [20], [21]. Firstly, the cut-off frequencies of the lowpass and highpass filters should be determined. In this way, it is possible to get the appropriate bandwidth for a desired cut-off frequency of bandpass filter [20]- [22].

A. First
Step: The SIW Highpass Filter Design SIW mainly performs as a planar rectangular waveguide. A planar rectangular waveguide is a highpass filter [21], [22].
The geometry and parameters of the SIW highpass filter are shown in Fig. 1. The Rogers 4003C substrate material is used in filter design (h = 1.52 mm, ϵ r = 3.38, and tanδ = 0.0027) [20], [23]. The lengths of parameters of the SIW highpass filter are shown in Table I. The dominant TE mode of a planar rectangular waveguide is TE 10 (m = 1, n = 0) mode. The wavelength of the SIW highpass filter (λ hpf ) is calculated as 18.983 mm according to (1), (2). Besides, the CST Studio Suite simulation results of the SIW highpass filter from 6 GHz to 16 GHz are shown in Fig. 2.
As shown in Fig. 2, out-of-band frequency suppression is observed to be good in the simulation solution. It is seen that there is a sharp transition-band in the simulation results. The SIW highpass filter has a good "insertion loss" (IL) performance. However, within the frequency band (passband) from 13.5 GHz to 14.5 GHz, the "return loss" (RL) performance of the SIW highpass filter is a little poor. The cut-off frequency f C is 8.06 GHz in the simulation results. Based on the simulation results, it is seen that the high pass filter design is good.

B. Second Step: The Microstrip Lowpass Filter Design
An easy way to design a microstrip lowpass filter is to use alternative sections that are part of a very high or very low characteristic impedance line. These filters are often described as stepped-impedance because of a good performance of group delay. Therefore, we prefer to use the stepped impedance method to design a microstrip lowpass filter [17], [24]. The Rogers 4003C substrate material is used in filter design.
The geometry and parameters of the microstrip lowpass filter are shown in Fig. 3. The values of the microstrip lowpass filter's parameters are shown in Table II. The microstrip lowpass filter is designed by using the Insertion Loss Method. The element values of the equivalent electrical circuit of microstrip lowpass filter are theoretically calculated (R 0 = 50 Ω, N = 6). The circuit elements of the microstrip lowpass filter are C 1 = 0.1471 pF, L 2 = 1.0053 nH, C 3 = 0.5493 pF, L 4 = 1.3732 nH, C 5 = 0.1177 pF, and L 6 = 0.3679 nH. The equivalent electrical circuit of the microstrip lowpass filter is shown in Fig. 4(a). The wavelength (λ lpf ) of the microstrip lowpass filter is calculated as 11.697 mm. Furthermore, the microstrip lowpass filter response between 7 GHz and 15 GHz in CST Studio Suite simulation and analytical results are shown in Fig. 4(b). As shown in Fig. 4(b), the out-of-band frequency suppression in the simulation is similar to the analytical solution. It is clearly seen that there is a wide transitionband in analytical and simulation solutions. The cut-off frequency, f C , is 12.62 GHz in the simulation and 13.94 GHz in the analytic results. Based on theoretical calculations, the frequency change in f C is 7.17 %. However, within the frequency band (pass-band) from 7 GHz to 8.5 GHz, the "return loss" (RL) performance of the microstrip lowpass filter is a little poor. The microstrip lowpass filter has a good "insertion loss" (IL) performance generally. According to all results, it is seen that the low pass filter design is good.

C. Third Step: The M-SIW Bandpass Filter Design
The geometry and parameters of the M-SIW bandpass filter with two ports are shown in Fig. 5. The lengths of parameters of the M-SIW bandpass filter are shown in Table III. The following equations are used for Bandpass Filter Transformations [17]:  The center frequency, ω 0  The fractional bandwidth of the pass-band Δ = (ω 2 ¯ ω 1 )/ω 0 .
 A series inductor (L k ) is converted to L' k and C' k (Series LC circuit):  A shunt capacitor (C k ) is converted to L' k and C' k (Shunt LC circuit): The Insertion Loss Method is applied in this bandpass filter design. The element values of the equivalent electrical circuit of the bandpass filter are theoretically calculated (N = 6 and R 0 = 50 Ω).
The wavelength (λ bpf ) of the M-SIW bandpass filter is calculated as 11.69 mm by using (1), (2). The pass-band is 2.63 GHz. In addition, the comparison of the analytical and simulation results is given in Fig. 6(b). The value of BW is 2.63 GHz in the analytical solution, and it is 3.30 GHz in the simulation solution. In general, the out-of-band frequency suppression in the simulation is close to the analytical results. The center frequency, f C , is 10.61 GHz in the simulation, and it is 9.62 GHz in the analytical solution.

III. EXPERIMENTAL VERIFICATION AND MEASUREMENT RESULTS
A Computer Numerical Control (CNC) milling machine is used for filter production. The M-SIW bandpass filter, which is fabricated by using the Rogers 4003C material, is shown in Fig. 7(a) and Fig. 7(b). The conductive copper wire with a thickness of 1 millimeter is passed through the holes of the SIW filter and it is soldered to the material.
The M-SIW bandpass filter is measured by using the Anritsu handheld vector network analyzer (VNA Master MS2028C) in Antenna and Microwave Lab. of Electrical and Electronic Engineering Dept. at Yasar University. The M-SIW bandpass filter is measured on 4001 points of the frequency range (sampling rate is 10 kHz; the whole frequency band is from 7 GHz to 15 GHz). The M-SIW bandpass filter in the measurement is represented in Fig.  7(c).
The simulated and measured results of the M-SIW bandpass filter are given in Fig. 8. As shown in Fig. 8, the out-of-band frequency suppression in measurement is as good as in simulation. At the same time, the frequency response in the pass-band is close to each other in both simulation and measurement results. The bandwidth in the simulation is 3.30 GHz. The center frequency, f C , is 10.61 GHz in the simulation. According to the results of the measurement shown in Fig. 8, the bandwidth of the M-SIW bandpass filter is 2.40 GHz. The -3dB lower cut-off frequency (f L ) is 9.08 GHz, the -3dB upper cut-off frequency (f H ) is 11.48 GHz, and the center frequency (f C ) is 10.20 GHz. The "return loss" (RL) is commonly greater than 10 dB in the measured pass-band. In addition, the "insertion loss" (IL) is less than 3 dB in the measured pass-band.
Analytical, simulated, and measured values are provided in Table IV. The fabricated bandpass filters with some previous Microstrip-SIW, SIW, Complementary Split Ring Resonators (CSRRs)-SIW, and Spoof Surface Plasmon Polaritons (SSPPs)-SIW are compared with measured performance and type of design in Table V. It is shown that the bandpass filters in [21] and [25] have a very wide passband. The filters in "this work" and [18] have approximately the same "insertion loss" (IL) and "return loss" (RL). The filter in [19] has a very narrow pass-band. However, its "insertion loss" (IL) seems to be the best among them. The filters with different cut-off frequency in "this work" and in [9] have similar BW.

IV. DISCUSSION
According to the hybrid M-SIW bandpass filter results, the center frequency is 9.62 GHz in analysis, 10.61 GHz in simulation, and 10.20 GHz in measurement. The bandwidths of the M-SIW bandpass filter are 2.63 GHz, 3.30 GHz, and 2.40 GHz, respectively.
The result of the S-parameter of the M-SIW filter depends on the diameter of the SIW's embedded metallic post, connector loss, and soldering errors. Therefore, such problems may cause bandwidth deflection according to Table IV. Comparison results between the proposed M-SIW bandpass filter and several manufactured bandpass filters based on SIW are presented in Table V. It is seen that the size of the designed full-mode M-SIW filter (20 mm × 30 mm) is smaller compared to SIW filters for X-band applications.
The hybrid design approach (one inside another) is applied to highpass and lowpass filters' structure based on 0.125λ. This approach is used in the lowpass structure to improve the bandpass filter responses. The measured "return loss" (RL) is generally greater than 10 dB, and the measured "insertion loss" (IL) is less than 3 dB on the pass-band. The performance of the M-SIW bandpass filter is satisfactory as represented in the RL and IL results. Similar to analytical results, the out-of-band frequency suppression is found to be satisfactory in both measurement and simulation. The simulated bandwidth (BW) of the M-SIW bandpass filter is 3.30 GHz and the measured BW is 2.40 GHz. The reason for the low BW in the measurement results compared to the simulation is thought to be due to the manufacture of the M-SIW bandpass filter.

V. CONCLUSIONS
In this study, the theory and hybrid design of the bandpass filter, which has a cascaded structure, are discussed. Unlike the design of the conventional bandpass filter, the M-SIW bandpass filter is clearly presented as a combination of the microstrip lowpass and SIW highpass filters. The period of filter design is shortened and the lowpass/highpass filter effects are clearly seen with this hybrid method. Thus, the bandpass filter can be designed and implemented easily for the desired frequency range. The Rogers 4003C material is seen to be a proper substrate for the application of the M-SIW bandpass filter. With the Rogers 4003C material, successful results can be obtained in the C-band, as well as in the X-band. However, it is predicted that losses will increase in the Ku-band.
It is concluded that analysis, simulation, and measurement values were found to be consistent with each other. By using this cascaded hardware integration, it is also possible to design new bandpass filters having different cutoff frequency. It is evident that the sensitivity of the CST Studio Suite simulation program based on the Finite Integration Technique (FIT) is satisfactory [26], [27].

CONFLICTS OF INTEREST
The authors declare that they have no conflicts of interest.