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2018 Progress In Electromagnetics Research Symposium (PIERS | Toyama), Japan, 1{4 August

G-band Diplexer Based on E-plane Waveguide Structures

Xiong Chen, Jiang Hu, and Xiang Le

School of Electronic Science and Engineering, University of Electronic Science and Technology of China

2006 Xiyuan Ave., Chengdu 611731, China

Abstract| This paper presents a waveguide diplexer operating at G band based on two waveguide ¯lters, each ¯lter is realized in direct couple topology and coupled by inductive iris. E-T junction is adopted to connect the two waveguide ¯lters to the common input port. The diplexer operating at center frequency 190 GHz and 220 GHz with a fractional bandwidth 2.1% and 2.7% is simulated by full wave simulator HFSS and fabricated using high precision metal machining technology. A reasonable agreement between the measured and simulated results has been obtained.

1. INTRODUCTION

Terahertz technology has gained much popularity in recent years due to its potential applications in security imaging, remote sensing and radio astronomy. As an indispensable part in terahertz communication system, diplexers are widely used in receiver and transmitter circuits to separate TX and RX signals depending on frequencies and provide decent isolation between two channels. Many research works about diplexer have been published in various literatures [1{3].

In this study, a diplexer operating at G band is proposed. The design, simulation and fabrication of the diplexer are discussed. The whole diplexer model is built up and simulated in full EM simulator HFSS. CNC (computer numeric control) machining technique with a minimum error better than 5 microns is employed to fabricate the oxygen free copper based diplexer.

2. DESIGN AND SIMULATION

Standard WR-4 rectangular waveguide (1.092 mm £ 0.546 mm) whose dominate mode frequency 170 GHz to 260 GHz is suitable for the application in G band circuits and thus adopted to design the G band diplexer.

The prime requirements of the diplexer presented in this paper are listed in Table 1. To meet the required out of band rejection of each pass band and isolation between the two channels of the diplexer, each ¯lter is designed to be 5 orders. The size of resonator that is used to determine resonate frequency is calculated by using the method given in [4].

Using the method proposed in [5], the coupling coe±cients of intercoupled resonators and external quality factors of the input and output resonators are calculated and then listed in Table 2, where channel 1 denotes 190 GHz passband and channel 2 denotes 220 GHz passband. QSI and QV L are the external quality factor for input/output coupling, respectively. M12 is the coupling coe±cient between the ¯rst and the second resonators, and so on. The topology of the proposed diplexer is showed in Fig. 1. E-T junction is employed to connect the two channel ¯lters to the common input waveguide.

Two adjacent resonators are coupled by inductive iris, the amount of coupling can be adjusted by changing the width and length of the iris to meet the aforementioned coupling coe±cients. Simulation has been done in HFSS eigen mode, coupling coe±cient between resonator1 and resonator

where f1 and f2 are the ¯rst and second resonant frequencies of a pair of coupled resonators.

As M12 = M45, M23 = M34 in both channel one and channel two, a symmetrical structure can be adopted in the design of these two channel ¯lters, The material of the diplexer is set as copper in HFSS to acquire close to real simulation results.

After ¯ne tuning, the S parameter of the lower passband cavity ¯lter can easily achieve the design target. As shown in Fig. 2, the simulated return loss of 220 GHz passband ¯lter is better than ¡15 dB and insertion loss better than 1 dB in passband from 217 GHz to 226 GHz. And the rejection is better than ¡40 dB at frequency higher than 229 GHz and lower than 213 GHz.

The same manner is employed to design the lower passband cavity ¯lter and its S parameter are shown in Fig. 3, the return loss is better than ¡15 dB and insertion loss better than 1 dB in

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2018 Progress In Electromagnetics Research Symposium (PIERS | Toyama), Japan, 1{4 August

passband from 185 GHz to 192 GHz. And the rejection is better than ¡40 dB at frequency higher than 194 GHz and lower than 182 GHz.

E-T junction is elaborately designed for the purpose of connecting the input waveguide and two channel waveguide ¯lter. The overall structure is optimized and analyzed in HFSS, and the simulated S parameter is shown in Fig. 4, return loss in both passband is better than 10 dB and insertion loss better than 1.5 dB which satis¯es the design requirements.

Channel 1

Figure 1. The topology structure of the proposed diplexer.

Table 1. Summary of the diplexer speci¯cation.

Table 2. Coupling coe±cients and external quality factors of channel 1 (190 GHz) and channel 2 (220 GHz).

3. FABRICATION AND MEASUREMENT

As shown in Fig. 5, standard WR-4 rectangular waveguide is adopted as the input waveguide. Standard UG-387 waveguide °anges were machined in the block for the purpose of connecting measurement system. The input waveguide and output waveguide are lengthened to make room for the input and output °ange, which would increase the metal loss of the diplexer.

For terahertz waveguide fabrication, there are several micromachining techniques, such as CNC (computer numeric control) micromachining technique, SU-8 photoresist technology [6], and deep reactive ion etching (DRIE) silicon micromachining technique [7]. In this work, CNC micromachining technique with a minimum error less than 5 microns is chosen for its cost e®ective and short processing cycle.

Oxygen free copper is used as the block material which was then coated with a thin ¯lm of gold by electroplating. For convenience of CNC metal machining process, as shown in Fig. 5, the diplexer is split by the center of the waveguide E face. In this way, the current °ows on the surface of the waveguide sidewall will not be disrupted so the machining in°uence on the performance of the diplexer would be brought down to minimum. Meanwhile, in order to be compatible with CNC milling process, the corners of the resonators have a radius of 0.1 mm.

The diplexer is measured using vector network analyzer AV3672C provided by China Electronics Technology Group Corporation, since a 2 port network analyzer is used here to measure the 3 port diplexer, each channel of the diplexer is measured while the other channel is connected with a matched load.

The system are calibrated using TRL standard, the measured results compared with simulated results of the diplexer are shown in Fig. 6. S22, S13, S23 are not shown here for better clarity of the results. Measured insertion loss in both two channels are better than 1.5 dB, for the ¯rst passband,

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2018 Progress In Electromagnetics Research Symposium (PIERS | Toyama), Japan, 1{4 August

Frequency(GHz)

Figure 6. Simulated and measured results of the proposed diplexer.

the return loss is better than ¡10 dB from 185 GHz to 190 GHz. For the second passband, the return loss is better than ¡10 dB from 212 GHz to 220 GHz.

The measured passband of both the lower and higher channel are shifted downward 3 GHz

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2018 Progress In Electromagnetics Research Symposium (PIERS | Toyama), Japan, 1{4 August

compared with the simulated results. The reason for this phenomenon may be ascribed to the fabrication error.

4. CONCLUSION

In this paper, the design, simulation and fabrication of a terahertz diplexer has been described. The diplexer consists of two channel waveguide ¯lters and operates at center frequency 190 GHz and 220 GHz with a fractional bandwidth 2.1% and 2.7%, respectively. High precision CNC metal machining technology is adopted to manufacture the diplexer.

A reasonable agreement between the measured and simulated results has been obtained, which veri¯es that the analysis and design are applicable to high performance THz passive component.

ACKNOWLEDGMENT

The authors would like to thank Xiaotong Guan, Tongbing Yang, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, for the measurement and helpful discussion. This work was supported by the National Natural Science Foundation of China (61371054).

REFERENCES

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2.Lu, Y. and X. Jiang, \Design of a Ku-band cavity diplexer with an E-plane T-junction,"

International Symposium on Antennas Propagation and EM Theory, IEEE, 1105{1108, 2010.

3.Lin, J. Y., S. W. Wong, and L. Zhu, \High-isolation diplexer on triple-mode cavity ¯lters,"

IEEE/MTT-S International Microwave Symposium | IMS. IEEE, 1196{1199, 2017.

4.Pozar, D. M., Mirowave Engineering, 3rd Edition, 372{374, Wiley, Hoboken, NJ, 2005.

5.Hong, J. S. and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, 2nd Edition, Wiley, New York, 2011.

6. Yang, H., Y. Dhayalan, X. Shang, et al., \WR-3 waveguide bandpass ¯lters fabricated using high precision CNC machining and SU-8 photoresist technology," IEEE Transactions on Terahertz Science & Technology, Vol. 8, No. 1, 100{107, 2018.

7.Hu, J., S. Liu, Y. Zhang, et al., \Micromachined terahertz waveguide band-pass ¯lters,"

IEEE/MTT-S International Microwave Symposium | IMS. IEEE, 650{653, 2017.

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