диафрагмированные волноводные фильтры / ab235b08-3371-4e91-8eef-5ed3a79e6155

Abstract- In this paper, a WR-4.3 band waveguide bandpass filter (BPF) based on a singlet and an extracted pole resonator is presented. The singlet based on TE301 mode and extracted pole resonant cavities is developed to generate a transmission zero (TZ) on the lower side of the passband in order to achieve high selectivity. Furthermore, the TZ position can be controlled independently by changing the dimensions of resonant cavities. The simulation results show that the filter has a passband range of 212-220GHz and the return loss is better than 20dB. The lower stopband range is 200-210GHz and the suppression is more than 30dB. This high-performance filter is capable for improving the single sideband communication receiver.

Keywords—terahertz (THz), Bandpass filter (BPF), waveguide filter, high frequency selectivity

I.INTRODUCTION

In the electromagnetic spectrum, terahertz (THz) waves with frequencies ranging from 0.1 THz to 10 THz are in a special position between the frequency bands of electronics and photonics. During the development of science and technology, the research and utilization of this frequency band developed later than others because of the limitation of scientific research and technical conditions at that time. It was once called a "gap" in electromagnetic spectrum. Because of its special position, terahertz wave can show many unique characteristics which are different from other kinds of electromagnetic radiation[1-2]. These characteristics determine that terahertz wave has broad and good application prospects in many fields. With the development of terahertz technology, it shows great potential in radio astronomy, object imaging, communication, medical diagnosis, environmental monitoring, anti-terrorism security and military radar[3-4].

As an indispensable component of front-ends in THz systems, the filter is very important in the solid-state terahertz research. Filter has a long history of development in the low frequency of electromagnetic spectrum, and has a set of mature and complete design and manufacturing study procedure. For the filter working in terahertz band, due to the special wavelength characteristics of this band, the physical size of the filter becomes the bottleneck of research and processing. Among many forms, waveguide cavity filter has advantages in performance and cost. In addition, the cavity filter has simple structure and good symmetry, which is convenient for optimization and analysis. Due to small dimensions of THz devices, different micromachining

techniques, such as low-temperature cofired ceramic (LTCC) technology [5], Si deep reactive ion etching (DRIE) [6], and SU-8 photoresist technology [7], have been developed and reported for fabrication of THz filters.

At present, super-heterodyne receivers usually exhibit double-sideband (DSB) operation, that is, signals from lower sideband (LSB) and upper sideband (USB) are simultaneously converted to the intermediate frequency. However, in many cases, the transmission performance of DSB operation will be unstable due to incoherence of transmitter and receiver, and rejection of the unwanted sideband is required. Considering the USB and LSB are relatively close, in order to achieve sharp stop-band rejection, the great rectangular factor of the filter is required. The corresponding curve of the Generalized Chebyshev function is similar to the corresponding curve of the elliptic function, which has the characteristics of equal ripple in the passband and finite number of transmission zeros in the stopband, so it is also called the pseudo elliptic function. Compared with the traditional Chebyshev filter, the pseudo elliptic filter has higher rectangular coefficient, better out-of- band rejection and is more flexible.

In this paper, a waveguide filter for the SSB communication system has been presented. In order to realize broadband responses and high selectivity in lower stopband, a fifth-order filter which is symmetry about the geometric center of the singlet is designed. The extracted pole resonator is then loaded at the filter terminal to improve selectivity in the lower stopband. The simulation results show that the filter has a passband range of 212-220GHz and the return loss is better than 20dB. The lower stopband range is 200-210GHz and the suppression is more than 30dB, which can effectively suppress the LSB signals.

II.DESIGN OF THE FILTER

A.Analysis of Transmission Zeros Generation

The purpose of the filter is to suppress the LSB of the double sideband signal generated by the up conversion of the subharmonic mixer and the passband is set at 210~220 GHz. Therefore, the band below 210 GHz needs to be suppressed, the transmission zero needs to be set near 210 GHz.

The center of the operation frequency of the filter is 216GHz. The standard waveguide WR-4.3 is chosen with the size of 1.092mm ×0.546mm, the mode TE102 is selected as the main mode of the resonant filter, and the band-pass

characteristics are realized by using the TE301 high-order mode. The magnetic field distribution diagram and topology structure of the resonant cavity structure are shown in Figure. 1.

characteristic and generate a TZ in the lower stopband. Meanwhile, due to the good symmetry, the spurious passband of BPF is restrained effectively. The whole structure of the proposed BPF is symmetric in relation to the E-plane, which will reduce fabrication complexity and be good for system integration and minimization.

Figure. 1. Magnetic field distribution of the TE301 resonances and the TE102 resonances.

By tuning the dimensions of these two resonant cavities, the location of TZs can be controlled independently. After optimizing the resonator, and the final HFSS model and the simulation results are as shown in Figure. 2. It can be seen that the transmission zero appears at 210.8 GHz, which is near 210GHz at the center of passband and stopband. The two poles of mode TE102 and mode TE301 appear at 212.6 GHz and 218.2 GHz respectively.

Figure. 2. Simulated results of the proposed singlet. The inset shows the geometry of the proposed singlet.

B. 220GHz Waveguide Filter Designing

According to the above analysis, a fifth-order filter, which is composed of an extracted pole resonator and a singlet with four TE301-mode-based resonators, is proposed using the standard techniques in [8]. Figure. 3 shows the whole structure of the simulated model of the 220GHz waveguide BPF.

In this design, the symmetry about the geometric center of the singlet, is designed to implement the passband

Figure. 3. The simulated model of the 220GHz waveguide BPF.

The simulation results are shown in Figure 4. The results show that the filter has a passband range of 212-220GHz and the return loss is better than 20dB. The lower stopband range is 200-210GHz and the suppression is more than 30dB, which can effectively suppress the LSB signals.

Frequency (GHz)

Figure. 4. The simulated results of the 220GHz waveguide BPF.

III. UNITS

Terahertz technology is a hot research field at present. In this paper, a 220GHz BPF with low insertion loss, high selectivity and great response is designed. By utilizing the singlet resonator and extracted pole resonator, the TZ is obtained, and the simulation results show the great performance. This THz bandpass filter has a great 30dB rectangular factor of 0.7. Due to simple and symmetrical structure, the method used in this paper is especially desirable for high-frequency applications. This simple and highperformance BPF would have a great potential in THz applications. This high-performance filter is capable for improving the single sideband communication receiver.

ACKNOWLEDGMENT

The authors wish to thank the teachers and students in School of Electronic Science and Engineering for their sincere support and helpful suggestions. This work is supported by the National Key R&D Program of China under Grant No. 2018YFF0109702 and the National Natural Science Foundation of China under Grant No.61771116 and No.91738102.

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IEEE Access, vol. 7, pp: 54705-54711, 2019.