диафрагмированные волноводные фильтры / fc8eb434-629f-4087-b3d9-6202ad54c719
.pdfNovel Ka-band Substrate Integrated Folded Waveguide (SIFW) Quasielliptic filters in LTCC
PinJie Qiu1, ZhiGang Wang2, Bo Yan
1Department of Electrical Engineering, University of Electronic Science and Technology of
China, China
2Department of Electrical Engineering, University of Electronic Science and Technology of
China, China 1maxqiu716@yahoo.com.cn
Introduction
The rapid development of microwave and millimeter wave communication system greatly stimulates the demand on high performance microwave filters for low insertion loss, compact size, high Q-factor, and low cost. Although waveguide filters have excellent insertion loss, they are voluminous and incompatible with other planar circuits. Recently substrate integrated waveguide (SIW), which can be synthesized in a planar substrate with arrays of metallic via by the easy and low-cost standard PCB or LTCC fabrication process, has provided a useful technology to design low cost waveguide filters [1]-[2]. Unfortunately, even with this size reduction of a factor 1/2 they can still be large when compared to their microstrip counterparts.
A lot of efforts have been paid to reduce the size of SIW devices. In [3], the author proposed a compact folded waveguide based on the T-septum waveguide, these types of guides reduce the waveguide width by half. However, they can be difficult to manufacture using laminates and LTCC since they require an internal via to form the central septum. In [4], a guided wave structure of half mode substrate integrated waveguide (HMSIW) for microwave application is proposed. Although its size is nearly half of a SIW, HMSIW merely reduces the cavity width rather than the length which really matters. And what worse is that enough margins have to be added by the open side which makes tiny size reduction actually. Basing on the concept of folded waveguide [5], the authors proposed an improved substrate integrated folded waveguide (SIFW) for the first time. The modification model makes SIFW greatly appropriate for filter design. It could introduce cross coupling between nonadjacent resonator cavities to generate transmission zeros (TZs) to get better selectivity of the filter. Then, two novel millimeter-wave SIFW BPFs on LTCC substrates are proposed, besides better performance the filters are significantly smaller than their unfolded counterparts.
SIFW and Filter Analysis
The fundamental mode of the SIW is TE10 whose field remains constant in the Z direction, thus the height of the SIW is not sensitive to the electromagnetic characteristic. That means height reduction of SIW is possible, and there is a lot of space in vertical direction that we can utilize. On the other hand, although some novel structures which reduced the width of SIW are proposed, no good measures to reduce the length which is most effective in size reduction have been developed. Recently in [5], SIFW, a wide band propagation structure, was proposed, in which the mode resembles TE10 mode of conventional waveguide but folded round under itself. In this paper, the authors proposed an improved SIFW which adds a resonator cavity at the folded area. Figure 1 shows the structure of the improved SIFW and its fundamental mode. Besides greatly reducing the length of SIW devices, this modification makes it more appropriate for filter design at the cost of the propagation bandwidth shrinkage. Moreover, this structure could introduce cross coupling between nonadjacent
978-1-4244-2642-3/08/$25.00 ©2008 IEEE
resonator cavities to achieve quasi-elliptic filters with TZs, and our analyses are confirmed by the simulated result.
E-plane iris filter is first proposed by Konishi in 1974 [6]. Due to its simple structure and good performance, it has been applied widely in millimeter wave band. By means of substituting the metallic via arrays in SIW for the conventional iris inserted in the middle of a RW parallel to the E plane, we have proposed a novel millimeter-wave SIW BPF [7], the structure is shown in Figure 5. Since the equivalent circuit of the iris can be expressed as the inductive reactance T network and is given in Figure 2, the approximate dimensions of the filter can be calculated by the following relations:
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The novel SIFW filter we designed belongs to quasi-elliptic filters by introducing crosscoupling. As is shown in Figure 3, the improved SIFW is divided into two layers by the middle metal conductor, thus five oriented resonators cavities are implemented, in which resonator 1 and 2 locate at top layer, resonator 4 and 5 locate at bottom layer, resonator 3 is playing a common cavity on the corner of the structure. These five oriented SIW resonator cavities are comparted by three E-plane irises which are equivalent by via arrays. The microwave signal in SIFW have two transmission paths: resonator l 2 3 4 5 and the cross-coupled path resonator l 2 4 5. The 2nd and 4th resonators are bent to achieve cross-coupling near the edge of the middle conductor. In Figure 4, it can be seen, when f<f0, the phase of resonators is 90°, the phase difference between the main-coupled and crosscoupled path is 180°, thus a lower stopband TZ could be generated. In addition, the filter has none higher stopband TZ because the phase difference between the main-coupled and the cross-coupled paths is 0° when f>f0.
The second filter is designed to improve the performance at higher stopband basing on the first structure. As is shown in Figure 7, a slot on the middle metal conductor is created to make the energy transmission between resonator 1 and 5 to be possible. Thus, as is shown in Figure 6, two new combinations of main and cross coupling paths are generated: resonator l 5 to resonator l 2 3 4 5, resonator l 5 to resonator l 2 4 5. Similarly, besides the lower stopband TZ this filter gets two higher stopband TZs because the phase difference
between the main-coupled and the cross-coupled paths is 180° when f>f0.
Result and Discussion
The center frequency of the two 5 poles quasi-elliptic SIFW BPFs we designed is 35GHz and bandwidth is 1.8GHz. The substrate chose Ferro-A6M, which has a relative permittivity of 5.7 and 8 layers thickness of 0.752mm. In order to obtain a good performance, the position of metallic via should be optimized with a high-precision.
The two filters are designed with the aid of a 3D full-wave field solver, HFSS, and the simulated results are provided in Figure 8 and Figure 9. From the figures, it can be seen that two filter has a bandwidth from 34.1GHz to 35.9GHz. For the first filter, in passband, return loss is less than -21dB and insert loss is about 1.8dB, including the input and output SIW-to- microstrip transitions. For the second filter, the insert loss is 0.3dB bigger than the first one in passband, but three cross-coupled-induced TZs are found at 33.2, 36.5 and 39.4GHz, which greatly enhance the side-band rejection, and confirms the analyses we did just now. The whole SIFW filter including SIW-to-microstrip transition has a size of
11.7mm×3.0mm×0.8mm. Compare with the unfolded counterpart which has a size of 19.7mm×3.2mm×0.4mm [7], the SIFW filter reduce nearly half of the length of the structure, at the cost of tiny height increase in vertical direction which are used to be wasted in common designs of SIW devices.
Conclusion
An improved SIFW structure is proposed for the first time in this paper. Base on the new structure, two novel millimeter-wave SIFW BPFs have been analyzed and designed in this paper. Through the cross-coupling, the filters could provide TZs on lower and higher stopband which greatly enhance the side-band rejection. LTCC technology is also implemented to realize further compact size of this filter. Moreover, the length of the filter is nearly half of the unfolded counterpart, which has important significance for system's miniaturization.
References
[1]Zhangcheng Hao, Wei Hong, “A broadband substrate integrated waveguide (SIW) filter” Antennas and Propagation Society International Symposium, vol. 1B, pp. 598601, 2005.
[2]Dominic Deslandes, Ke Wu, “Millimeter-wave substrate integrated waveguide filters”
Electrical and Computer Engineering, 2003. IEEE CCECE 2003. Canadian Conference, vol. 3, pp. 1917-1920, 2003.
[3]Grigoropoulos N., Young P.R., “Compact folded waveguides” Microwave Conference, 2004. 34th European, vol. 2, pp. 973-976, 2004.
[4]Hong Wei, Liu Bing, “Half Mode Substrate Integrated Waveguide: A New Guided Wave Structure for Microwave and Millimeter Wave Application” Joint 31st International Conference on Infrared Millimeter Waves and 14th International Conference on Teraherz Electronics, pp. 219-219, 2006.
[5]Nikolaos Grigoropoulos, Benito Sanz-Izquierdo, “Substrate integrated folded waveguides (SIFW) and filters” Microwave and Wireless Components Letters, IEEE, vol. 15, Issue 12, pp. 829-831, 2005.
[6]Y.Konishi, K.Venakada, “The design of a bandpass filter with inductive strip-planar circuit mounted in waveguide” Microwave Theory and Techniques, IEEE Transactions, vol. 22, Issue 10, pp. 869-873, 1974.
[7]Pinjie Qiu, Bo Yan, “Design of a Novel Millimeter-Wave Substrate Integrated Waveguide (SIW) Filter in LTCC,” unpublished.
Figures
Figure 1. Improved SIFW Structure |
Figure 2. SIW BPF Equivalent Network |
Figure 3. Resonator Cavities in SIFW |
Figure 4. The 1st BPF’s Coupling Topology |
Figure 5. Structure of the SIW BPF
Figure 6. The 2nd BPF’s Coupling Topology |
Figure 7. The 2nd SIFW BPF |
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Figure 8. The 1st BPF’s Result
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Figure 9. The 2nd BPF’s Result