The 40 GHz band duplexer with E-plane planar circuit Toshihisa Kamei a), Yozo Utsumi, and Nguyen Thanh Department of Communications Engineering, National Defense Academy, 1 10 20 Hashirimizu, Yokosuka, Kanagawa 239 8686, Japan a) kamei@nda.ac.jp Abstract: We report on the design and construction of a prototype band pass filter using inductive-strips (E-plane planar circuits) based on electromagnetic-field simulations. We also report on the construction and characteristics of a prototype duplexer consisting of two band-pass filters-one each for the uplink and downlink-and a Y-shaped branch circuit. As a result of the experiment of duplexer, an insertion loss of about 3.1 db in a bandwidth of 195 MHz for the uplink filter and an out-of-band attenuation of 50 db at 41.47 GHz could be achieved. And an insertion loss of about 3.0 db in a bandwidth of 763 MHz for the downlink filter and an out-of-band attenuation for the downlink filter of 59 db at 42.09 GHz could be achieved. Keywords: E-plane planar circuit, inductive strip, band pass filter, duplexer Classification: Microwave and millimeter wave devices, circuits, and systems References [1] T. Kamei, Y. Utsumi, and K. Chino, The Duplexer for Cable Television in 40 GHz Band, Proc. of the 2006 IEICE Society Conf., Nagoya, Japan, C-2-67, p. 86, Sept. 2006. [2] T. Kamei, Y. Utsumi, K. Chino, and H. Suzuki, Duplexer for Cable Television in 40 GHz Band, IEICE Technical Report, MW2006-137, pp. 35 38, Nov. 2006. [3] Y. Konishi and K. Uenakada, The Design of a Band pass Filter with Inductive Strip-Planar Circuit, IEEE Trans. MTT, vol. MTT-22, no. 10, pp. 869 873, Oct. 1974. [4] G. L Matthaei, L. Young, and E. M. T. Jones, Low-pass prototype filters obtained by network synthesis methods in Microwave filters, impedancematching networks, and coupling structures, Chapter 4, pp. 83 162, McGRAW-HILL, New York, 1964. 1 Introduction This paper reports on the design and construction of a prototype transmit/receive band pass filter for bidirectional communications in the 40-GHz 549
band using flat inductive strips (E-plane planar circuit) that can be mounted in a waveguide, and on the construction of a bidirectional 40-GHz duplexer for urban cable television (CATV) [1][2]. The use of an E-plane planar circuit enables a 40-GHz functional device to be integrated into the waveguide structure making for a low-cost, low-loss, and compact structure conducive to mass production. Band pass filters using inductive strips were first reported by Konishi et al. for the 12-GHz band [3]. This paper introduces three new achievements in this area as follows. (1) For a 2-step band pass filter of various bandwidths, we performed electromagnetic-field simulations to determine strip widths W 0, W 1 and resonator length l 0 so as to achieve maximally flat transmission characteristics. Then, using an expression relating bandwidth to coupling-coefficient k, we prepared a design chart showing the relation between coupling-coefficient k 01 and W 0 and that between couplingcoefficient k ij and W i and enabling the design of an n-step band pass filter using an E-plane planar circuit. (2) Using this design chart, we designed and constructed a prototype 40-GHz band pass filter specifically for use in an urban CATV bidirectional transmission system, and showed that filter transmission characteristics obtained by experiment agreed well with those of electromagnetic-field simulations. (3) We constructed a 40-GHz duplexer for urban CATV using prototype filters in a Y-branch circuit. 2 Design of a band pass filter using inductive strips (E-plane planar circuit) Figure 1 shows a structure of E-plane planar circuit Band pass filter with inductive strip and a design chart. Figure 1 (a) shows a structure of a 2-step E-plane planar circuit Band pass filter. Filter characteristics are achieved by a structure that divides a waveguide with an a b cross section at the center of its H-plane and inserts inductive strips of thickness t. The strength of inductive coupling changes according to the width (W 0, W 1 ) of an inserted inductive-strip. Here, the strength of coupling is expressed by coupling-coefficient k 12 and the strength of coupling with the outside by coupling-coefficient k 01. An inserted strip with a large width corresponds to weak magnetic-field coupling and small k, while an inserted strip with a small width corresponds to strong magnetic-field coupling and large k. And resonator length l 0 determines the center frequency of the filter. Band pass filter characteristics can be determined by adjusting the strength k ij of coupling between resonators by inductive-strip width W i. We prepared a design chart for the 40-GHz band, which is now being studied for use in urban-catv bidirectional transmission systems. Supposing uplink and downlink band pass filters with center frequencies of 42.1125 GHz and 41.12 GHz, respectively, we performed simulations using the Microwave Studio electromagnetic-field simulator to determine W 0, W 1 and l 0 for each of these filters such that the transmission characteristics of 2-step band pass filters having various bandwidths (where 3 db-down bandwidth is taken to be Δf) become maximally flat. For the electromagnetic field simulations, we employed 3D electromagc IEICE 2007 550
netic simulation software called MW Studio (from CST) and used the finite integration technique (an algorithm for solving Maxwell equations in integral form) and the perfect boundary approximation technique (an expanded algorithm using a cubic mesh that can perform analysis while preserving complex shape information including surfaces) as analysis techniques. The waveguide used here was the WRJ-400 (a=5.69 mm, b=2.85 mm) and inductive-strip thickness was t=100 μm. The relation between coupling-coefficient and filter center frequency and bandwidth is given by Eq. (1) (6)[4]: λ 0 (1) λ g = 1 ( λ 0 2a )2 w = Δf f 0 (2) w λ = ( λ g0 ) 2 w (3) λ 0 πwλ k 01 = (4) 2g 0 g 1 πw λ k j,j+1 j=1ton 1 = 2 (5) g j g j+1 πwλ k n,n+1 = (6) 2g n g n+1 Here, n is the number of filter steps, k j,k j+1 is a value obtained by normalizing the inverter s characteristic impedance by Z 0, Z 0 is characteristic impedance of the waveguide, w is relative bandwidth, λ g0 is the guide wavelength at filter center frequency f 0, λ 0 is free-space wavelength at f 0,andg 0, g 1, g n+1 are equivalent circuit constants of the filter under design. We now determined the relation between k 01 and W 0 and that between k ij and W i from the relation between W 0 and W 1 that achieves maximally flat characteristics having various Δf as determined from electromagnetic-field simulations and from the relation between Δf and k 01, k ij in Eq. (1) (6). Figures 1 (b) and 1 (c) show these results as design charts. Next, taking out-of-band attenuation into account, we decided to design a 3-step maximally flat filter (bandwidth 200 MHz) for the uplink and a 5- step Tchebyscheff filter (bandwidth 800 MHz, in-band ripple 0.5 db) for the downlink. In the design of the uplink filter, we determined k 01 and k 12 from Eq. (1) (6) given that f 0 =42.1125 GHz, Δf=200 MHz, and n=3. We obtained k 01 = 0.098 and k 12 =0.007 giving us W 0 =1.98 mm, W 1 =5.62 mm, l 0 =3.22 mm, and l 1 = 3.221 mm. Similarly, in the design of the downlink filter, we set f 0 =41.12 GHz, Δf=800 MHz, and n=5 and obtained k 01 =0.182, k 12 =0.039, k 23 =0.032 giving W 0 =1.204 mm, W 1 =3.320 mm, W 2 =3.589 mm, l 0 = 3.500 mm, l 1 =3.506 mm, and l 2 =3.506 mm. As shown by this example, an n-stepbandpassfiltercanbedesignedusingeq.(1) (6) and the design charts shown in Figs. 1 (b) and 1 (c) given filter center frequency, bandwidth, and number of steps. 551
Fig. 1. The band pass filter with E-plane planar circuit. (a)structure. (b)design chart of W 0 vs. k 01. (c)design chart of W 1 vs. k 12. 3 Experimental results Figure 2 shows the characteristics of the prototype filters that we constructed based on the design parameters determined in the previous section. The solid line represents S 21 transmission characteristics and the broken line S 11 reflectivity characteristics. Figure 2 (a) shows the experimental results for the 3-step maximally flat filter used for the uplink. The bold line represents experimental values and the thin line simulation values. Simulation calculations include loss. Figure 2 (b) shows the experimental results for the 5-step Tchebyscheff filter used for the downlink. As in Fig. 2 (a), the bold line represents experimental values and the thin line simulation values. In both cases, experimental results agree well with simulation results demonstrating the validity of the proposed design method. For the uplink, we constructed a 3-step maximally flat filter and obtained a center frequency of 42.1125 GHz, a 3 db bandwidth of 196 MHz (design value: 200 MHz), and an insertion loss of 3 db. This corresponds to an unloaded Q value of Q 0 =1860. For the downlink, we constructed a 5- step Tchebycheff filter and obtained a center frequency of 41.12 GHz, a 3 db bandwidth of 798 MHz (design value: 800 MHz), an insertion loss of 1.25 db, and an in-band ripple of about 1 db (design value: 0.5 db). This corresponds to an unloaded Q value of Q 0 =1780. The ability to design low-loss band pass filters with an unloaded Q value of about 1800 means that filter characteristics far superior to those of ordinary planar-circuit filters such as microstrips can be achieved. Figure 3 shows experimental results for a duplexer consisting of prototype filters using E-plane planar circuits and a Y-branch circuit. The insertion loss 552
of the Y-branch circuit used here is about 0.3 db. The solid line represents S 21 transmission characteristics and the broken line S 11 reflectivity characteristics. Although transmission loss here is slightly inferior to that measured for a single filter,these results show a bandwidth of 195 MHz and an insertion loss of about 3.1 db for the uplink filter and a bandwidth of 763 MHz and an insertion loss of about 3.0 db for the downlink filter. It was found that an out-of-band attenuation for the uplink filter (the attenuation from the flat section at the upper frequency limit used in the downlink) of 50 db at 41.47 GHz could be achieved, as could an out-of-band attenuation for the downlink filter (the attenuation from the flat section at the lower frequency limit used in the uplink) of 59 db at 42.09 GHz. Fig. 2. The characteristics of the prototype band pass filters. (a)the 3-step maximally flat filter for up link. (b)the 5-step Tchebyscheff filter for down link. Fig. 3. The prototype duplexer. (a)external view of the duplexer. (b)the frequency characteristics of the prototype duplexer. 553
4 Conclusion We investigated the design of band pass filters using inductive strips (Eplane planar circuit) by simulating a 2-step maximally flat filter having various bandwidths using the electromagnetic-field simulator. We determined inductive-strip width and resonator length for that filter and prepared a design chart for an n-step band pass filter. Using this design chart, we designed a 40-GHz band pass filter, which is now being studied for use in urban- CATV bidirectional transmission systems, and achieved a high-performance filter with an unloaded Q value of about 1800. Experimental results were found to agree well with simulation results demonstrating the validity of the proposed design method. We also constructed a duplexer using prototype filters based on this method and obtained good characteristics with out-of-band attenuation better than 50 db. 554