диафрагмированные волноводные фильтры / 2ef40d5d-0630-491d-b840-d2afcb69c243

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J. of Electromagn. Waves and Appl., Vol. 23, 1181–1190, 2009

IMPROVED STEPPED-IMPEDANCE RESONATOR (SIR) BANDPASS FILTER IN KA-BAND

Z. R. He, X. Q. Lin, and Y. Fan

School of Electronic Engineering

University of Electronic Science and Technology of China

Chengdu 610054, China

Abstract—This paper presents an improved bandpass filter based on Stepped-Impedance Resonator (SIR) working in millimeter-wave band. Compared to the conventional E-plane waveguide bandpass filter, it can be integrated in the planar circuits easily with more compact size. A new coupling structure is proposed which has increased the coupling coe cient of the 1st and 5th stages at the limitation of fabricating craft in millimeter-wave band. The distorted structure of the microstrip reduces the insertion loss of the filter. The relative analysis is given, and the simulation results are compared with those of traditional structure. Computer-aided design of such a Ka-band filter has been presented. Good agreement has been observed between theory and measurement.

1. INTRODUCTION

In wireless communication and radar systems, filters are playing important roles. Planar filters are particularly popular because they can be fabricated using printed circuit board technology and are suitable for commercial application due to their low-cost [1]. The filter based on the SIR structure has more steepness response on the stop band and smaller size than conventional filter. But planar filters are usually used in the frequency band below the Ka-band [2–11].

In Ka-band, there are many bandpass filters used in system. Most of them are based on the E-plane waveguide structure which results in big size and complex connection to the microstrip circuits. In order to avoid those shortcomings, some integrated bandpass filters have been

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Figure 1. λ/2 and λ/4 step impedance resonator.

proposed. However, the stopband rejection and insertion loss are not perfect.

In this paper, an improved Ka-band filter is proposed using SIR structure which is appeared in 1970’s [12] and has been broadly used in microwave band, such as satellite communication and radar system etc. Pool coupling from the feed port to SIR structure is solved by using improved connecter in Ka-band. Fig. 1 shows a geometry of SIR.

2. CHARACTERISTIC OF SIRS

Figure 1 shows the typical structures of a SIR with K < 1, where K is an important ratio defined as [12]:

As the same way, there are other structures of SIR with K > 1. The resonance can be described by

Equations (2) and (3) correspond to the evenand odd-mode resonance, respectively. Defining α as the length ratio

where θt = θ1 + θ2, and then substituting (4) into Eqs. (2) and (3) we obtain

Choosing a suitable combination of the impedance and length ratio of the SIR, the fundamental and the higher resonance mode frequencies can be determined.

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Figure 2. Electronic length θt versus α and K.

Various high order resonator frequencies have been calculated with di erent combinations of K and α as shown in Fig. 2. It can be found that θt = nπ in the case of K = 1 (where α = 0 or 2) is a uniformed impedance case. When the total length equals n times the half wave length, the nth resonate mode occurrs.

At the beginning of design, the dimension of each resonator must be obtained. Di erent combinations of K and α should be chosen to get various high order resonator frequency from Fig. 2. In actual design, the capacitance in the step and the dependence of the propagation constants on the width of the transmission lines should be considered. The two parameters should be slightly tuned [13].

3. KA-BAND SIR FILTER DESIGN

To avoid the complex design of via holes connected to the ground in Ka-band, we used the λg /2 resonator to design filter. The Chebyshev prototype is selected for the design of the parallel couple bandpass filter.

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3.1. The Target of the Filter

The specifications of the filter are listed as follows:

3.2. The Parameter to Be Designed

As shown in Fig. 1, the admittance can be obtained as [14]

Yi = jY2[2(K tan θ1 + tan θ2)(K − tan θ1 tan θ2)]

/[K(1 − tan2 θ1)(1 − tan2 θ2) − 2(1 − K2) tan θ1 tan θ2]. (7)

The resonance condition is Yi = 0, in which

If θ1 = θ2 = θ, Eq. (7) is formed as

Yi = jY2[2(1+K)(K −tan2 θ)tan θ]/[K −2(1+K +K2)tan θ +Ktan θ].

(9)

Now the resonate condition is

To get smaller size, we often choose K < 1 and α = 0.5. Here, we choose K = 0.5 and α = 0.5. In the parallel coupled line, we choose

Z2 = 50 Ω. So Z1 = Z2/0.5 = 100 Ω. Substituting in Eq. (10), we get

θ= θ0 = arctan(sqrt(K))

=0.616 rad

=35.3◦.

Rogers RT/5880 is chosen as the substrate. The width of line is 750 μm with the impedance of 50 Ω, and the length is 0.5 mm. At the same time, the line width is 200 μm with the impedance of 100 Ω, and its length is 1.5 mm.

The main parameters of Rogers RT/Duroid 5880 laminate are that the thickness is 0.254 mm; dielectric constant is 2.2; the thickness of the copper foil is 0.034 mm and the dissipation factor is 0.001.

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To design a bandpass filter with SIR whose resonators are coupled in parallel, it is necessary to find out the relationship between the even and odd mode impedance of the parallel coupled section and the admittance invert parameters [15].

Using the correlation theory and Chebyshev element, the size of the slot is determined [16]. The slot is 0.05 mm and 0.4 mm. But imitated by the technology of print plant, the slit can only be manufactured to 0.1 mm. If the slit can be manufactured smaller than 0.1 mm using another technology at the first stage, the insertion loss will be improved in traditional structure. In this paper, we propose an improved structure to improve the coe cient of the slit at the first stage.

3.3. Design of the Coupling Structure

In conventional bandpass filter, characteristic impedance of the input port is usually chosen to be 50 Ω usually with side coupling structure at the first and last stages. But limited by the craft of the microstrip circuits, the coe cient is poor, and the insertion loss is deteriorated in Ka-band. So the bandpass filter based on the SIR structure is hardly used in Ka-band.

In our design, wider input microstrip of the filter is presented. Discontinuity is taken, which results in larger return loss. However, design of the detailed sizes of improved coupling structure, strong coupling will be achieved. In other word, the S12 will be improved after increasing the strip width properly in required frequency-band.

It is hard to design detail size of such coupling structure just using presented experimental formula for the larger distribution of equivalent capacity in Ka-band. So we use the HFSS v11 to simulate the di erent sizes of the stage. At last, we choose the best size in the simulation results about the S-parameters and compare with the traditional structure simulation result (as shown in Fig. 3). Based on the simulation result, the size can be confirmed. The length of the distorted microstrip is 1.2 mm, and the width is 1.4 mm.

Based on the analysis and simulation results, the circuits of the filter are manufactured. Traditional structure is used in the 2nd to 4th stages. And in the output and input ports, the distorted structure of microstrip is used to improve the coe cient of the 1st and 5th stages.

3.4. Design of the Improved Filter

After obtaining all sizes of the filter, the whole structure is simulated in Ansoft HFSS 11 and fabricated. The S-parameters of the simulation

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Freq [GHz]

(a) improved coupling structure

XY Plot 1

dB(S(WavePort1,WavePort2))

Freq [GHz]

(b) conventional coupling structure

Figure 3. The simulation results of the di erent structure of the first stage.

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results are showen in Fig. 5. Using the Agilent 8757D network analyer, the filter’s S-parameters are measured as shown in Fig. 6.

Figure 5. Simulation result of the filter.

Figure 6. The measurement result.

The insertion loss of the center frequency is 3.9 dB. The 3 dB bandwide is 1.6 GHz. At the frequency of 32.5 GHz and 35.5 GHz, the stropband rejection is greater than 20 dB. During the filter measurement, two transitions from waveguide to microstrip are used. This transition has about 1 dB insertion loss which is included in the measurement. The length of the microstrip in the output and input

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Figure 7. The filter and transition.

is about 1.5 cm. The insertion loss of the microstrip is about 0.5 dB. Fig. 7 shows the photograph of the filter and the transition.

4. CONCLUSION

This paper has presented a simple and e cient design method for the microstrip integrated filter in Ka-band. An improved structure with larger coupling coe cient of the first and last stages has been proposed to decrease the insertion loss of the filter. With the combination of three uniform SIR structures, the filter has been fabricated which can be easily integrated and used in millimeter wave radar and communication system because of its smaller size. Deducting the insertion loss of the transition and the microstrip, the insertion loss of the filter is 2.4 dB.

We also remark that the larger return loss in working frequencyband can be decreased when detailed sizes of three di erent SIR structures are optimized.

REFERENCES

1.Pozar, D. M., Microwave Engineering, 2nd edition, Ch. 8, Wiley, New York, 1998.

2.Zhang, J., J. Z. Gu, B. Cui, and X. W. Sun, “Compact and harmonic suppression open-loop resonator bandpass filter with tri-section SIR,” Progress In Electromagnetics Research, PIER 69, 93–100, 2007.

3.Wang, R., L. S. Wu, and X. L. Zhou, “Compact folded substrate integrated waveguide cavities and bandpass filter,” Progress In Electromagnetics Research, PIER 84, 135–147, 2008.