диафрагмированные волноводные фильтры / 7e456276-7d6e-4d18-ac73-6e05003697bc

Abstract—A exact design method of Ka band H-plane inductance diaphragm waveguide band-pass filter is discussed. It starts from the lumped low-pass prototype, through frequency conversion, summarizing the design formula of the lumped parameter coupled resonator band-pass filter, and then using the microwave structure to realize the coupling structure and the resonator, thus we can get the microwave band-pass filter. And in the design we consider the machining chamfer which leads to frequency offset, sum up the exact design method of diaphragm filter with chamfer which can realize the low loss and band without offset. It overcomes the error and loss which is relatively high compared with traditional method.

Keywords—chamfer; millimeter wave; H-plane inductance diaphragm; low loss;

I.INTRODUCTION

The human security inspection has become the common focus of many governments. Its security, humanization and efficiency are raised with high requirement[1][2]. The conventional X ray human security inspection system may have some harm. As millimeter wave (MMW) electronic technologies have matured, the passive MMW imaging used for human bodies security inspection is emerging as an effective approach to imaging through obscuring materials, such as clothing for concealed weapons detection or plastic mines with no harm for human body. Various passive MMW imaging systems have been reported[3].

The passive MMW imaging technology has high-level compact integration and resolution. The front-ends of their MMW receiver are realized using MMICs while the passive components, such as filters, are often realized off-chip[4]. The band-pass filters are used in the radio frequency receivers for suppression of the image frequency which results in improved noise figure. According to the circuit structure, millimeter filter could be divided into microstrip filter and waveguide filter. Millimeter wave microstrip filter has smaller structure and wider operation band than waveguide filter, but larger insertion loss and poor stopband suppression limit its application especially in higher millimeter wave frequency[5][6]. direct coupled waveguide filter has many advantages, such as high Q value, low insertion loss, good temperature stability, especially suitable for narrow-band filtering applications,and its structure is very strong, easy processing and installation, is a kind of widely used filter in microwave band[7]. In the design of direct coupling waveguide band-pass filter, commonly used inductive irises, pins, small or E insert as coupling network between levels of waveguide resonant cavity. In this work we use H- plane inductance irises structure to design rectangular waveguide band-pass filter[8].

II.ANALYSIS OF CIRCUIT STRUCTURE

According to the theory of microwave filter network, all types of filters, such as the maximum flat, Chebyshev, elliptic function filters, can be mapped into a prototype lowpass filter. Compared with maximum flat filter, Chebyshev filter has advantages such as equal ripple and little error. Although the elliptic function filter has a better performance, but it is generally more difficult to achieve, so this work discusses the Chebyshev band-pass filter[9].

The H-plane waveguide filter as shown in Fig.1 uses the waveguide section of half wavelength as the series resonators, uses the parallel inductance formed by inductance diaphragm as a coupling structure between the resonators. From the Fig. 1 we can see the chamfer that will be produced in the actual processing. The effect of chamfer is discussed in section 3.

Fig. 1. Circuit structure of H-plane waveguide filter

The performance of h-plane waveguide filter is decided by the synthetic resonant effects of these series resonant cavities by tuning the dimension of inserted inductance diaphragm. In order to analyze the waveguide filter character, the metal diaphragm inserted in the waveguide could be equivalent to a T type reactance network as shown in Fig. 2, which is helpful to analyze the coupling effect of adjacent resonate cavities dimension[10].

Fig. 2. T type equivalent circuit of inserted inductance diaphragm

The corresponding equivalent circuit of waveguide filter is shown in Fig. 3.

Fig. 3. Waveguide filter equivalent circuit network

In this kind of waveguide filter, there is only TE10 single mode transmission, we can use approximation transformation of low-pass to the band-pass according to (1) to (3).

λg 0 ,λg1 , λg 2 , λg is Waveguide wavelength when the

By the above formula (11) to (13), the relationship between reactance and K diaphragm is shown as formula (14).

We can get the impedance transformation formula of waveguide filter shown as (8) to (10).

With these impedance transformation, then we can design the inductor diaphragm size and resonator length. In the case of thin diaphragm, the equivalent circuit of diaphragm is shown as Fig. 4[11].

III.SIMULATION AND EXPERIMENT RESEARCH

The required design specification is a five stage bandpass filter which has Chebyshev response. The center frequency of waveguide filter is 34GHz and the bandwidth is about 0.5GHz. The insertion loss of bandpass is less than 0.5dB and the image rejection is more than 60dB when frequency is 30GHz and 38GHz. The thickness of diaphragm is 0.2mm and the diaphragm edge will produce chamfer which radius is 0.5mm because of processing factor. The waveguide flange dimension is the standard WR-28 with a=7.112mm and b=3.556mm. The top view of waveguide filter is shown an Fig. 5. According to above theoretical analysis of waveguide filter equivalent circuit , we can get the length of cavities and the width of diaphragm shown in TABLE I.

After the optimization of electromagnetic simulation software such as Ansoft HFSS, we can get the simulation result. The simulation result is shown in Fig. 6(a).

Fig. 7. The experiment result of Ka band waveguide filter

Fig. 6(a). The simulation result before adjustment

From the simulation results, we can see that the frequency offset to the right of the 250MHz because of the chamfer. So we need to redesign that change the center frequency to 33.75GHz. After adjustment the length of cavities and the width of diaphragm is shown in TABLE II and the simulation result is shown in Fig. 6(b).

TABLE II. THE PARAMETERS OF FILTER AFTER ADJUSTMENT

(a) (b)

Fig. 8 Photograph of the H-plane filter, (a) is internal structure of (b)

IV. CONCLUSION

The design method of H-plane inductance diaphragm waveguide band-pass filter is presented. The chamfer factors leading to frequency offset is considered. The frequency offset can be corrected by adjusting the center frequency to final realize the precise design of the filter. Ensure that the low loss and the center frequency of the filter is not shifted. Excellent experiment result has been obtained, which agree well with the simulation data.

Fig. 6(b). The simulation result after adjustment

Based on the optimized parameters of inserted strip, the waveguide filter is fabricated and assembled, the experiment result is shown in Fig. 7. The experiment result is in good agreement with the simulation result. The actual picture of the H-plane waveguide filter is shown in Fig. 8.

REFERENCES

[1]Sinclair, G. N., R. N. Anderton, and R. Appleby, “Outdoor passive millimeter wave security screening”, Security Technology 2001 IEEE 35th International Carnahan, 172-179, Oct. 2001.

[2]Sun, L.-N. and P.-X. Yuan, “Research of detection technology in X-ray security inspection equipment”, China Measurement Technology, Vol. 32, No. 3, 20-23, 2006.

[3]Liu, S. and H. Jin, “X-ray security inspection technology”, Journal of Security University: Science and Technology, Vol. 58, No. 4, 78-80, 2008.

[4]Minghua Zhao, Yong Fan, Yonghong Zhang, A W-band low loss E- plane type waveguide band-pass filter, IEEE 2007 International Symposium on Microwave, Antenna, Propagation and EMC Technologies For Wireless Communications, 2007:355-357.

[5]J. Uher, J. Bornemann, and F. Arndt, “Magnetically Tunable LowInsertion loss Microwave and mm-Wave Band-pass Filters with High Power Capability”, in Canadian Conference on El. and Com. Eng. Proceedings, Vancouver/Canada, pp. 450-453, Nov. 1988.

[6]R. J. Wenzel, “The Modern Network Theory and Design of Optimum Quarter-Wave TEM Filters”, IEEE trans, Vol. MTT-13, No. 3, pp. 316327, May(1965).

[7]R. M. Kurzrok, “General Three-Resonator Filters in Waveguide”, IEEE Trans, Vol. MTT-14 pp. 46-47, January (1966).

[8]G. L. Mattaei, “Interdigital Band-pass Filters”, IRE Trans, Vol. MTT-10, pp. 479-491 November (1962)

1620

[9]J. Usher, and W. J. R. Hoefer, “Tunable Microwave and Millimeter Wave Band-pass Filter”. IEEE Trans, Microwave Theory and Techniques, Vol.39, Apr 1991, pp. 643-653.

[10]R. Levy, “Theory of direct coupled cavity filters”, IEEE Trans. Microwave Theory Tech. vol. MTT-15, pp.340-348, June (1967).

[11]S. B. Cohn, “Direct-coupled-resonator filters”, Proc. IRE, vol. 45, pp. 187-196, Feb (1957).

1621