диафрагмированные волноводные фильтры / e84ac343-d1f7-41d2-a904-a161e3c20f5d

Abstract—In this paper, a compact diplexer using substrate integrated waveguide (SIW) resonators is introduced. The diplexer consists of two filters and a circulator. The diplexer channels are 4% and 5% relative bandwidth at 32GHz and 33GHz, respectively. The filters are based on SIW resonators, which have higher Q values. This paper also analyzed the factors which affect the Q value of resonators, and describes the designing procedure of Generalized Chebyshev filters, which used in this paper. The circulator also is on account of SIW. It uses inductive windows to increase bandwidth. At last, the paper gives the diplexer’s simulation and measure results. And two results are similar with each other.

Keywords—Diplexer, High selective bandpass filters, circulators, substrate integrated waveguide

In this paper, a compact diplexer using substrate integrated waveguide technology is introduced as an alternative for the miniaturization of the filter and diplexer structure. The diplexer consists of two parts, which are two bandpass filters and a circulator, is shown in Figure 1.

circulator

filter

I.INTRODUCTION

Diplexers or multiplexers are indispensable components in RF/microwave wireless communication systems. Normally, these components are designed using two or multiple bandpass filters with different bandpass frequencies, whether they are contiguous or not. The fast growth of wireless communications technology has brought communication systems operation in millimeter-wave range. Usually, the millimeter wave diplexers are made of metal waveguide, because of its low insertion loss and high isolations. However, the designs suffer disadvantages such as being bulky, costly, and difficult to fabricate, etc [1]- [3]. In particular, the passive components are required to be easily integrated or assembled with active circuits in a highly integrated system. It is difficult to integrate waveguide elements into a planar circuit, and the transition from a waveguide to planar integrated circuits would substantially degrade the performance. Therefore, how to design a bandpass filter and diplexer with low cost and with high performance is currently of great interest [4]-[9]. Microstrip bandpass filters can be easily mounted on a dielectric substrate and can provide a more flexible design of the circuit layout. But The Q value of microstrip resonator is a little low which leads to high cost of band pass filter. So the novel bandpass filters with high Q resonators need to be researched. Substrate Integrated Waveguide (SIW) based on substrate integrated circuits scheme (Dealandes and Wu 2003, Wu et al. 2003 [13]), is proposed. The research of SIW filters is developed greatly [11]-[13].Compared to the microstrip resonators, the SIW resonators have higher Q value, which leads to the low cost of SIW filters.

filter

Figure 1. The diplexer with filters and circulator

All the bandpass filters and circulator are implemented of SIW structure, which is to degrade the cost of diplexer. The Generalized Chebyshev configurations [14] are used to achieve the SIW filters, which can reduce the cost and improve the selective of filters by implementation of transmission zeros using the second channel.

II.BANDPASS FILTERS DESIGN

In order to obtain low cost filters, the Q value of SIW resonator is analyzed. As shown in the Figure 2, the length, width and thickness of cavity are d, a and b which are 4mm and 6mm, 0.254mm alternatively. And the substrate is Rogers 5880.

d

a

b

Figure 2. The dimension of SIW cavity

a b d

We = 4 E Edv

0 0 0

Where E is the electric field, ε is a constant.

Similarly, the loss of conductor wall is shown in equation (2).

pc = Rs Ht ds 2

the

Figure 3. Coupling scheme of the channel filters structure

The coupling matrix is shown in equation (6) and (7).

(2)Where the equation (6) is the coupling matrix of upper channel filter, and equation (7) is the lower one.

and Rs = wu0 / 2 . is the conductivity, and for the power consumption dielectric, it has a complex value.

So the Q value of power consumption conductivity and ideal dielectric can be obtained as followed.

Where l the mode number of resonance frequency and k is 2 / .

As the same, the Q value of power consumption dielectric and ideal conductivity is shown in the equation (4)

Therefore, the actual cavity’Q value of actual can be got.

The prototype of the filter has the structure shown in Figure

(5)4. Its main channel has five SIW cavities, and cross channel has a cavity to realize the transmission zeros.

TABLE I.

T HE Q VALUE OF CAVITY WITH DIFFERENT TAN AND THICKNESS B

After selecting the high Q value resonators, coupling matrix needs to be obtained to according to the filter properties. In this paper, the structure of filters is given from Figure 3. The cross resonator is able to provide a transmission zeros which can reduce the order of filters, and improve the selective of filters.

All the filters simulation results are shown in the Figure 5. The maximum insertion loss is 2.5dB. Because of the narrower bandwidth of lower filters, it has higher insertion loss.

III.CIRCULATORDESIGN

Usually, the diplexer requires a 3-port device to connect the two filters, the 3-port device is typically a T-junction [16] [17]. A T-junction duplexer has a stopband in the middle of the two passbands which can only obtain the function of frequency division multiplexing. But it cannot acquire time division

multiplexing function. The diplexer uses a circulator to replace the T junction, due to the circulator’s ring characteristics. Thus it not only can realize the function of the diplexer, but also can achieve time division multiplexing by adjusting the filter pass - band range.

Figure 5. The simulation results of two filters

Typically circulator with the form of microstrip line and waveguide has been implemented in [18]-[20]. However, microstrip circulator presents a low Q-factor and high radiation losses especially at the millimeter-wave frequency. Waveguide circulator shows a good performance, otherwise it is heavy, hard to be integrated with planar structure, and it is not suitable for low-cost mass production. In this paper, a duplexer connected by SIW circulator is presented. It gets a high Q- factor and is easy to be integrated with planar circuits. Some metal inductive holes circulators have been researched [21] [22]. But metal inductive holes can only obtain good performance in the case of a blind hole. Generally, at the frequency of millimeter wave, the thickness of the substrate is relatively thin .This will greatly increase the difficulty of processing, and the manufacture also be limited by PCB processing conditions.

In this paper, the proposed circulator is designed with metal inductive window, so that all the metal holes are through holes including the hole that places ferrite. Thereby it greatly reduces the PCB processing difficulty, at the same time the circulator gets a good isolation and a smaller loss.

Metallic-via inductive window structure and its equivalent circuit are shown in Figure 6, the metallic via forms an inductive window with a width of W, and the parameters in the equivalent circuit can be determined using the following equations [23]:

Figure 6. Metallic-via inductive window and its equivalent circuit

scattering parameters of the metallic-via inductance window. Equivalent circuit of metal inductive window shows that it is equivalent to an impedance converter; therefore it can increase the bandwidth of the circulator, improve the isolation of the circulator and reduce the return loss, while fabrication becomes easy. Circulator structure is shown in Figure 7. The height of circulator’s substrate is consistent with the filter which can greatly reduce the loss caused by the discontinuity. Here is the designing process of circulator: firstly, the radius of ferrite is calculated according to the operating frequency using the empirical formula (11), and then the radius is optimized by electromagnetic simulation software.

ferrite

SIW

Metallic-via inductive window

Figure 7. SIW circulator

Where 0 and f are respectively the operation frequency

and the relative dielectric constant of the ferrite [15]. The height of ferrite is about equal to the thickness of SIW. Then, the SIW width and the size of the metal inductive window are optimized according to the empirical formula of SIW and metal inductive window, so that the circulator achieves a good matching and obtains a good ring features. Simulation results without microstrip to SIW transition are shown in Figure 8. The results show that the circulator achieves a good ring characteristic with a s maller insertion loss, which can be used as a three-port duplexer connector.

IV. CIRCULATOR DESIGN

Based on two design filters and a circulator, the diplexer is illustrated in Fig.9. The circulator combines the two passband filters together. According to the magnetic biasing direction of ferrite circulator, the diplexer has different.

Figure 8. Frequency response of SIW circulator

Figure 9. SIW diplexer

(a)

(b)

Figure 10. The simulation results of diplexer, (a) the insertion loss simulation results of two channels; (b) the isolation simulation results of two

channels

The key point of such passive device is isolation because RXand TX-band frequencies are in close vicinity. As the two independent filters presented in Section II and III presented a high rejection level circulator, they should provide a satisfactory isolation. Fig. 10 (a) and (b) illustrate the results from 3D EM simulation of this diplexer. Fig.12 (a) and (b) illustrate the results from measured of this diplexer. The diplexer isolation separation reflects the isolation characteristic of the circulator. As bandwidth compression, isolation of the diplexer is improved.

Figure 11. SIW diplexer with a circulator

(a)

(b)

Figure 12. The measured results of diplexer, (a) the insertion loss measured results of two channels; (b) the isolation measured results of two

channels

V.CONCLUSIONS

In this paper, a compact diplexer using substrate integrated waveguide (SIW) resonators is proposed. Two filters and a circulator are contained in the diplexer. Its channels are 4% and

5% relative bandwidth at 32GHz and 33GHz, respectively. The filters are based on SIW resonators, which have higher Q values. The circulator also is on account of SIW. It uses inductive windows to increase bandwidth. After the measurement, the measure results are similar with simulation results. Therefore, it is promising in the use of communication.

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