диафрагмированные волноводные фильтры / de065904-66ed-498a-a0ed-e9591e7a8b7c

Abstract- In this paper, a novel four-section waveguide E-plane bandpass filter with two pieces of metal cut at both sides is presented. In this filter, in order to improve the performance of the band rejection, loaded cavity is designed instead of air cavity. The insertion loss is less than 1.5 dB in the passband from 91.4 to 94.9 GHz. The lower rejection is larger than 40 dB from 75 to 87.6 GHz, while the upper rejection is larger than 40 dB in the range 103–110 GHz.

I.INTRODUCTION

With a rapid development of space exploration as well as environmental monitoring, imaging and communication, millimeter wave system has made great progress. Bandpass Filter (BPF) plays an important role in communication systems, so it becomes increasingly important. To meet the requirement of modern communication systems, the better performance of millimeter wave BPF is demanded, such as low cost, low insertion loss, high rejection etc. Millimeter wave waveguide BPF is the most popular and widely used due to its low attenuation, high Q factor and closed environment. So, waveguide filter is extensively used in millimeter wave engineering application and system.

Waveguide filter often uses conventional inductive elements such as rods, transverse strip and transverse diaphragms. However, this method is difficult to manufacture and assemble because of its complicate structure, which is not suitable for mass production. So, E-plane metal septum waveguide BPF replaces the inductive elements rapidly has been widely applied when it is appeared due to its advantages of simple structure, high Q factor, convenient design, easy mass production, low cost, etc. Thereafter, in addition to all metal insert, there are varieties of E plane structure waveguide filter such as unilateral fin line based on a dielectric substrate, bilateral fin line based on a dielectric substrate, bimetallic strip insert, a bias of diaphragm, the deformation waveguide, high temperature superconductor.

Although superconducting film dielectric insert has a high Q value, it is difficult to achieve the structure. Furthermore, compared waveguide filter with the fin line, all metal insert waveguide filters have low insertion loss and higher Q factor because there is no influence introduced by medium. The processing of sheet metal can achieve high accuracy, so the metal pieces of E plane filters are widely used. However, when we need a wide bandwidth, the length of first level metal diaphragm

,(((

for single diaphragm E plane filter should be designed with a short length in order to obtain the strong coupling. When the bandwidth increases to a certain value, the processing precision and mechanical strength can’t meet the requirements of design and application, then we will face the problem of difficult processing. Moreover, stopband characteristic of ordinary single diaphragm structure is not satisfactory, especially the upper rejection is not ideal. So in many situations it requires high characteristics of resistance, such as the filter in a duplexer, it is difficult to satisfy the corresponding index. At high frequencies, for example W band and F band, the length of first level metal diaphragm for single diaphragm E plane filter will become very short. Small length makes the processing become difficile and a great difference between measure and simulation results. Especially, the length of first level metal diaphragm has been reduced to the minimum machining accuracy or less when the frequency is up to D band. In order to solve the above problem, many design methods are proposed. Filter with double membrane method is more effective than the one of design methods, but processing and assembling of the filter become difficult.

In this paper, the metal diaphragm was designed as a whole so that each metal diaphragm connected, and two pieces of metal cut were designed at both ends of metal diaphragm. Metal cut could improve the coupling so as to increase the length of first grade metal diaphragm, and the loaded cavity with sheet metal in the central of waveguide could improve the upper stopband rejection, either.

II. EQUIVALENT CIRCUITS

E-plane filters are usually constructed by two divided waveguide components and a metal plate. The metal plate is located in the center of waveguide, parallel to both the traveling direction and maximum electric field of rectangular TE10 mode. Fig. 1 shows the structure of the metal plates for E-plane filters. A traditional type shown in fig. 1(a), the metal plate consists of a pair of support beams and inductive elements. On the other besides, a novel type with metal cut metal plate is shown in fig. 1(b) which is also designed as a whole.

(a)

(b)

Fig. 1 the structure of the metal plates for (a): traditional type and (b): the novel type

In Fig. 1(b), it is obvious that there are some apertures designed in the metal plate. When the metal plate is inserted into waveguide from the longitudinal direction, the rectangular waveguide develop into two small rectangular waveguides with a new section size (a/2*b) because the thickness of metal plate is very thin compared with long edge width of waveguide. As we know, the condition for rectangular waveguide transmission TE10 mode is a≤λ≤2*a. So two small rectangular waveguides are cutoff waveguide and they are connected in parallel through the metal plate. If the diaphragm longitudinal is w, we can get the transfer parameter matrix of each cutoff waveguide equivalent network.

Where r' is transmission constant of cutoff waveguide, and Z0' is characteristic impedance of cutoff waveguide:

There will be mutations in the section dimension of the waveguide when the longitudinal metal is put into rectangular waveguide. But the cutoff waveguide is unchanged in the longitudinal direction, so loaded waveguide only produces TEm0 higher-order mode when m is an odd number. We do a simple analysis, considering that only TE10 higher harmonic mode exists in the waveguide and then we can get:

As a consequence, the transfer parameter matrix of each cutoff the waveguide equivalent network can be written as

equivalent impedance:

120bΣ

Ze a1 Ο2a 2 (6)

We can get the scattering matrix parameters by using matrix transformation as

The filter synthesis is based on Chebyshev low pass filter prototype which can export the design formula of bandpass filter with lumped parameter coupled resonator by frequency converter, then microwave structure is used to achieve the filter.

III. SIMULATIONS AND MEASUREMENTS

The four stages BPF which has Chebyshev characteristic is used for design specification. The center frequency was 94.35GHz and bandwidth was 3.5GHz. The width of inserted mental strip was 0.1mm, and the waveguide was standard WR10(a=2.54mm, b=1.27mm). According to tchebyscheff’s synthesis and direct coupling, the dimension structure diagram of the metal plate can be obtained and shown in Fig. 2. Then the model of filter was built and simulated in the HFSS. By optimizing the design of simulation software, the optimized design parameters were w1 = w5 =1.22 mm, l1 =l4 =1.49 mm, w2 =w4 =0.76 mm, l2 =l3 =1.51 mm, w3= 1.01 mm, h1 =0.2 mm. The length of the first metal diaphragm was increased by 4 to 5 times compared to the ordinary diaphragm, which made processing easier and less machining errors.

Fig. 2 . The dimension structure diagram of the metal plate

Fig. 3 .Photograph of the filter with two metal cut

The filter was already processed and a photograph of the prototypes is shown in Fig. 3. The filter characteristics were measured by using a vector network analyzer N5244A and its frequency expansion component. The transmission and reflection characteristics are shown in Fig. 4. We could see from the simulation, 0.5dB passband of the filter was 3.5GHz (92.6- 96.1GHz). The measured passband was 3.5 GHz (91.4-94.9 GHz). The largest insertion loss in the passband was about 1.47 dB at 91.7 GHz, and the lowest insertion loss in the passband was about 0.23dB at 93.9 GHz. The lower rejection was larger than 40 dB if the frequency is lower than 87.6 GHz, and the upper rejection was larger than 40 dB in the range 103–110 GHz.

Fig. 4 .Simulated and measured transmission characteristics

The return loss of measured characteristics became worse, and the lowest return loss in the passband was about 11.39 dB at around 91.5 GHz. There appeared a 1.2GHz frequency shift between simulation and calculation in Fig. 4. After analysis, the deterioration of performance was caused by off-center position of the diaphragm. Then the model was simulated in HFSS when the off-center position of the diaphragm was 0.05 mm or 0.1mm. Simulation results are presented in Fig. 5.

Fig. 5 .Simulation results of the off-center position

IV. CONCLUSION

A novel waveguide E-plane filter is presented in this paper. In this filter, the first metal of the diaphragm and the air cavity are replaced respectively by metal cut and loaded cavity. The metal cut can increase the length of the metal diaphragm and make the diaphragm easy to process. And the air cavity can improve the performance of filter. After simulation and processing, we got a filter which 1.5dB passband is 3.5GHz (91.4-94.9 GHz1). The largest insertion loss in the passband is about 1.47 dB at 91.7 GHz, and the lowest insertion loss in the passband is about 0.23dB at 93.9 GHz.

ACKNOWLEDGMENT

This work is supported by the National Nature Science Foundation of China under Grant No. 61301051

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