2017 9th IEEE International Conference on Communication Software and Networks
Novel Filters Based on Hybrid Structure of Coaxial Resonator and Evanescent Mode Waveguide with Loaded Posts
Kun Li
De-Xin Qu, Xing-Jian Zhong
Microwave Communication Department College of Communications Engineering, PLA UST Nanjing, China e-mail:
[email protected] Microwave Communication Department College of Communications Engineering, PLA UST Nanjing, China e-mail:
[email protected],
[email protected] Abstract—A novel hybrid bandpass filter based on coaxial resonator and evanescent mode waveguide(EMW) with loaded posts is presented. The coaxial resonators are directly coupled to the EMW with posts through air windows. The fundamental TEM mode of the coaxial resonator and the evanescent mode of the EMW constructing the passband of the filter, while the staggered spurious modes of the coaxial rsonator and EMW are suppressed each other to improve the stopband. A fourorder bandpass filter is designed fabricated and measured, with good selectivity and wide stopband. Keywords-wide stopband; coaxial resonator; hybrid structure; evanescent mode waveguide (EMW)
INTRODUCTION
Waveguide filter is widely used for application in communication system due to it has merits such as low insertion loss, large power capability, high-Q values and better stability. Conventional rectangular cavity filter has hardly wide stopband because the presence of higher order modes which decided by the intrinsic structure of resonator are often closed to the passband. The work to improve the harmonic suppression of cavity filter is relatively few currently. The method of changing the width or height of each resonator to make their higher order mode staggered by each other while keeping the fundamental frequency same is mentioned in [1-4]. The approach need carefully design physical dimension of each resonator. Whatever approach is used, the stopband performance of the filter can be further improved by choosing the stagger frequencies of undesired harmonic. This paper proposed a novel filter based on hybrid coaxial resonators and evanescent-mode waveguide with loaded capacitive posts. Due to evanescent-mode waveguide has strong attenuation below the cutoff frequency, the filter can achieve better selectivity. Comparing to conventional rectangular cavity filter [5], the designed filter has good performances with wider stopband, better selectivity and smaller size.
w_x l1
h4
w_z
h2
h1
l4 l2
a1
w1
2r1
l1
w2
(b) Front view
w_x
II.
(a)
l3 h3
I.
through air window. The coaxial cavity resonator is consisted of a cylindrical inner conductor and a rectangle cavity outside, and using rectangle cavity instead of cylindrical cavity here is convenient to adjust the coupling between cavities. The two metal posts loaded in the evanescent mode waveguide is totally same, and their center is in a line. The coaxial probe feed of the filter formed by extending and the inner conductor coaxial line is a relatively sample feed model. And it's easier to achieve appropriate Qvalues by adjusting the probe length and feed position. The front view and top view of the filter are schematically illustrated in Fig.1 (b) and (c).
FILTER DESIGN
l5
A. Framework of the Filter Fig. 1 shows basic framework of the designed hybrid bandpass filter. A length of evanescent mode waveguide with loaded posts are coupled to two coaxial resonators
978-1-5090-3822-0/17/$31.00 ©2017 IEEE
(c) Top view Figure 1. The structure of designed filter (a) structure of the filter (b) front view(c) top view
Evanescent mode waveguide works below the cutoff frequency, and its characteristic impedance is inductive for TE10 mode according to the basic the theory of rectangle waveguide. So if loading some proper capacitive structure in the evanescent mode waveguide, it can form a resonant circuit, if loading several capacitive structure, then it can form a bandpass filter. Based on this principle, we load two metal posts in the evanescent mode waveguide as shown in Fig. 3 (a), and its equivalent circuit is given in Fig. 3 (b). The evanescent mode waveguide with two loaded metal posts can be seen as two resonators. The loaded metal posts have the effect of shunt capacitances, which formed resonant circuits with the waveguide which is equal to a inductance in the evanescent-mode. The value of capacitance C is decided by the size of metal posts, which decided the resonance frequency of resonator. And among the dimension parameters of post, the variation of height h_z influence the resonance frequency mostly. Besides, from the equivalent circuit, the coupling between two resonators is inductive coupling, of which strength is decided by the distance between the posts l2, the longer the distance between the posts, the weaker the coupling strength.
B. Coaxial Cavity and Evanescent Mode Waveguide As shown in Fig. 2, the fundamental frequency of coaxial cavity resonator f0 relatively decreases as the height of inner cylindrical conductor rises, while the correlation between the ratio f1/f0 and the height h2 is positive, the larger the height of inner conductor, the further the lowest spurious frequency. Here f1 is the lowest spurious frequency of coaxial cavity resonator. So increasing the metal cylinder height can keep the harmonic frequency of filter away from the fundamental frequency further in the case of the rectangle cavity unchanged. However, considering the problem of power capacity of filter, a trade-off between the distance of spurious frequency and power capacity need to be weighed in the design. 9
3.6
f0-h2
8
3.2
7
2.8
6
f1 / f0
f0(GHz)
f1/f0-h2
C. Coupling between Cavities The coupling between coaxial cavity and evanescent resonator is realized through bottom air window, whose height and width also influence the coupling. We analysis the relationship between the coupling of coaxial cavity and evanescent mode waveguide resonator by HFSS electromagnetic simulation software. The results is shown in Fig.4 and it can be seen that the coupling coefficient k12 rises at first and decreases latter with the increasing of the window height h3, and gets the maximum near h3 = 8 mm. This is why the energy of electric field is mainly in the top of coaxial cavity while the energy of magnetic field mainly in the bottom. When the height increased to a certain value from the bottom of coaxial cavity, the coupling between coaxial cavity and evanescent mode waveguide resonator changes from magnetic coupling into electric coupling.
2.4 5 2.0 4
6
7
8
9
10
11
h2(mm) Figure 2. f0 and f1/f0 variation as functions of parameter h2
w_x
0.0175
coupling coefficient k12
w_z
l2 (a) Evanescent mode waveguide resonator
Lc
/
&
/
&
0.0170 0.0165 0.0160 0.0155 0.0150 3
(b) Equivalent circuit
4
5
6
7
h3(mm)
8
9
10
11
Figure 4. Coupling coefficient variation as a function of parameter h3
Figure 3. Evanescent mode waveguide resonator and equivalent circuit
TABLE I. THE FILTER DIMENSIONS
0
Parameter
Value(mm)
Parameter
Value(mm)
a1
13
h1
10.8
b1
10.8
h2
9
w1
15
h3
10.8
w2
5.6
h4
3.1
l1
3.2
w_x
7.2
l2
15.8
w_y
8
l3
12.8
w_z
8
l4
3.4
r1
1.3
L5
0.7
S21(dB)
-50
-100
h3=10.8mm
-150
h3= 5.5mm -200
6
8
10
12
14
16
18
20
frequency(GHz)
Figure 5. Measured results of coupling coefficient and S-parameter
To prove the validity of the proposed filter of hybrid structure, a four-order bandpass filter with the centre frequency 6.2 GHz and fractional bandwidth 2% is designed by the method based on evaluations of external quality (Q) factors and coupling coefficients [6], [7]. The coupling coefficients calculated by the software CoupleFil are given as follow: ks1 = k4l = 0.020 , k12 = k34 = 0.017, k23 = 0.014
The fabricated filter was measured by Agilent N5230C, and the narrow band and wide band of S-parameters of the filter are plotted in Fig. 7 (a) and (b), respectively. The measured insertion loss is about 0.9 dB at the centre frequency of 6.2 GHz, and 1.45 dB at the band edge. The measured return loss is below 19 dB and bandwidth is 110 MHz. Compared with the simulated results, the centre frequency of filter have a little shift and it mainly due to the machining tolerance. In addition, the rejection can achieve 60 dB in both and upper stop-band at frequencies 400 MHz away from the pass-band boundary. The first spurious is generally located at 2.8 times of the centre frequency, which is limited by the high modes of the coaxial resonator. Note that the rejection in the spurious band is almost lower than 20 dB from 16.5 GHz to 20 GHz, which is the result of two different staggered harmonic frequencies of the coaxial resonator and evanescent-mode waveguide. Overall, the designed hybrid filter has a wide stop-band and high stopband suppression.
(1)
Note that the amplitude of k12 variation is small with h3 ranging from 5.5 mm to 10.8 mm. In other words, the height of window has little influence on pass-band performance of the filter, but smaller window height can improve the suppression of the first spurious. The simulated results of h3 = 5.5 mm and h3 =10.8 mm are illustrated in Fig. 5, and it could be seen that the stop-band performance of filter when h3 = 5.5 mm is obviously better than h3 = 10.8 mm at the range from 16 GHz to 20 GHz. III.
FABRICATION AND EXPERIMENT
A four-order bandpass filter with hybrid structure of coaxial cavity and evanescent mode waveguide is designed and fabricated. Here the height of coupling window is chosen as 10.8 mm for easy fabrication, which is equal to the height of the coaxial cavity and waveguide. Fig.6 shows the photograph of fabricated filter and the filter's dimensions are given in Table I.
0 -10
S11&S21(dB)
-20 -30 -40
measured S21 measured S11 simulated S11 simulated S21
-50 -60 -70 -80 5.8
5.9
6.0
6.1
6.2
6.3
frequency(GHz)
(a) Narrowband results
Figure 6. Photograph of fabricated filter
6.4
6.5
It's easy to realize mode excitation through directly connecting the coaxial probes of SMA connectors to the vertical feeding posts, and the filter has smaller size in comparison with conventional air cavity filters.
0
S21( dB)
-20 -40
ACKNOWLEDGMENTS
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This work was supported in part by the National Natural Science Foundation of China (NSFC) under Grant 61271105 and in part by the Chinese NSFC under Grant 61271103.
-80
REFERENCES [1]
-100 5
10 15 frequency(GHz)
20
[2]
(b) Wideband results
[3]
Figure 7. Simulated and measured narrowband results and wideband
results
IV.
[4]
CONCLUSION
[5]
A Novel bandpass filter with hybrid structure have been proposed, exploiting the coaxial resonators and evanescent mode waveguide with loaded metal posts in this paper. Staggered harmonics and different working modes of the two kind of resonators improve the selectivity and widen the stop-band. A four-order bandpass filter has been designed and fabricated to validate the traits of the proposed filters. Measured results confirmed the simulation results. The proposed filter has better performances of selectivity, stopband and also size in comparison with conventional cavity bandpass filters.
[6]
[7]
A. E. Atia, A. E. Williams, and R. W. Newcomb, “Narrow bandpass waveguide filters,” IEEE Trans. Microwave Theory Tech., vol. 20, no. 4, pp. 258–264, Apr. 1972. R. V. Snyder, “New application of evanescent mode waveguide to filter design,” IEEE Transactions on Microwave Theory Techniques, vol. 25, pp. 1013–1021, 1977. M. Morelli, I. Hunter, R. Parry, "Stop-band improvement of rectangular waveguide filters using different width resonators: selection of resonator widths,"IEEE MTT-S international. Microwave Symposium Digest, May. 2001, pp. 1623-1627. G. Craven, “Waveguide hand-pass filters using evanescent modes,” Elecfron. Lett., vol. 2, pp. 251-252, 1966.. Vlad Lenive ,and John Ness, “Direct-coupled filter utilizing ridgeloaded cavities,” European Microwave Conference, 33rd , pp. 1251̽ 1254, 7-9 Oct. 2003. C. Rauscher, and S.W. Kirchoefer, “Miniature ridge-waveguide filter module employing moldable dielectric material,” Microwave Theory and Techniques, IEEE Transactions, Vol. 54, Issue 3, pp.1190̽1196, Mar. 2006. G. L. Matthaei, L. Young, and E. M. T. Jones, Microwave Filters, Impedance-Matching Networks, and Coupling Structures. NewYork:McGraw-Hill,1964
