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Filter Design

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Technical FeatureDesign of a Compact Bandpass Filter with Capacity Loaded Ridged WaveguideA K-Band bandpass flter, using a single ridge waveguide and capacity-loading posts, is presented in this article. It is based on transmission line and ridged waveguide theories. In contrast to the traditional evanescent-mode ridged waveguide flters, the width of both ridge and capacitor-loading post of the proposed new type of the ridged waveguide bandpass flter are the same. A flter is designed, simulated and fabricated at 22.3 GHz, demonstrating a fractional bandwidth of 42.2 percent, from 17.6 to 27 GHz, a VSWR less than 1.5 and an out-of-band rejection of60 dB at 15 and 30 GHz, respectively. Based upon these good performances, the proposed ridged waveguide flter can be used in microwave and millimeter wave communication systems.Waveguide bandpass flters are play-ing an increasingly important role in modern communication systems and other microwave felds. With the rapid devel-opment of modern microwave and millimeter wave communication systems, the requirement for flters with high performance, wide band-width and small size is growing. To make the waveguide flter more compact, a traditional resonator is modifed in order to obtain multiple resonant frequencies. Hence, one resonator can be treated as the equivalent of multiple resonators. The dual-mode flter is one of the most widely used in multiple-mode flters that have been developed.1-3 However, the limitation for such a flter is that the width of a dual-mode flter is greater than that of tra-ditional rectangular waveguide flters.Evanescent-mode flters have been pro-posed by Graven to reduce the width of the waveguide flter and the technology had been applied for years.3-6 Besides, the evanescent- mode flters perform well in out-of-band rejec-tion. And they have small size and light weight, making such flters more widely used in micro-wave and millimeter wave systems.Evanescent-mode ridged waveguide band-pass flters have been developed7-9 where various confgurations of ridge are used and wide spurious-free out-of-band responses are achieved. However, the disadvantage of this technology is that the different cross sections of the cavity make the fabrication diffcult for building a wideband flter.Cohn has reported the ridge waveguide and analyzed its properties.10 Because of its high power capacity, low insertion loss and good frequency selectiveness, the ridged waveguide flter has attracted attention, followed by many reports on the subject, to reduce the size of Xianrong Zhang, Qingyuan Wang, Jinhua Cao and AnJ PengUniversity of Electronic Science and Technology of China, Chengdu, China124 MICROWAVE JOURNAL OCTOBER 2012__10M32 FINAL.indd 124 10/1/12 1:52 PMMITEQ ad 579 7.8125x10.75.qxd:ima ad 9/19/11 1:24 PM Page 1MWMJITEQ1011.indd 125 9/27/12 11:19 AM126 MICROWAVE JOURNAL OCTOBER 2012Technical Feature

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1]11C (2)x 1xcosh1 x1 x2 ln4x1 xd12122 2where x represents b2/b1. Hence, the total capacitance per unit length can be expressed asC ab 2C (3)11 22d= +The equivalent inductance per unit length is expressed asL a a2 b (4)11 1 21( )= where 1 is the permeability of the medium. The cutoff frequency of the ridged waveguide f'c can be expressed using Equations 3 and 4 as follows:f 12 L2 C(5)1ab2Ca a b'c111 1 22d11 2 1( )= = + The cut off wavelength of the ridged waveguide can be expressed asthe lters.11-12 Compared to rectan-gular waveguides, ridged waveguides have the advantages of wide funda-mental-mode operating bandwidth, low cutoff frequency and low wave impedance.10-11 Fundamental-mode operating bandwidth of four to one or more is easily obtainable with ridged waveguides. The low cutoff frequency yields a small cross section and hence, a compact size ridged waveguide com-ponent can be achieved. The low wave impedance allows an easy transition to planar transmission lines such as strip lines or microstrip lines. Therefore, ridged waveguide bandpass lters are the ideal components for these appli-cations.In this article, a new type of 10-or-der, wideband bandpass lter, employ-ing ridged waveguide and capacitor-loading is proposed. It is noticed that the widths of the ridge and posts are identical, making the manufacturing process convenient. The design speci-cations for the lter are a 22.3 GHz center frequency, demonstrating a fractional bandwidth of 42.2 percent, from 17.6 to 27 GHz, with a VSWR less than 1.5, an out-of-band rejection at 15 and 30 GHz less than 60 dB, respectively. The bandpass lter was fabricated and measured. Both the simulated and measured results are presented.THEORY AND ANALYSISFor the cross section of the single ridged waveguide shown in Figure 1, the width and height of the single ridged waveguide are a1 and b1. The width of the ridge is a2 and b2 is the distance from the top of the ridge to the top of the waveguide. The equiva-lent circuit is also shown, where Cs is the electrostatic capacitance originat-ing between the ridge and ground, while Cd denotes the capacitance formed from the discontinuity of the ridge. L is the equivalent inductance of the waveguide from the two sides of the ridge. The capacitance Cs is given by:C ab (1)s1 22= where 1 is the dielectric constant of the medium. The capacitance Cd depends on the b2/b1 ratio. Using a conformal transformation, Cd can be expressed as follows13 Fig. 1 Single ridged waveguide (a) cross section, (b) equivalent circuit.a2a1CdL C LCs Cd(a)(b)b1b2__10M32 FINAL.indd 126 9/28/12 11:41 AMwww.sei-device.com Ra d a r , Wi r e l e s s C o mmu n i c a t i o n s , S AT C OM, a n d S p a c e High-Reliability GaN Power Where You Need ItFrom tactical communications to the challenges of space, count on the leader in GaN technology for higher reliability, more power, and more performance.GaN for Mobile UHF to 3.5 GHz 10/30/90/100WSPDT Switch VHF to 3.5 GHz 30WVHF to 2.5 GHz 60WTo learn more, contact usacomm@ sei-device.comGaN for Radar S-Band 2.9 to 3.3 GHz 600W Discrete & Pallet3.1 to 3.5 GHz 600W Discrete & PalletX-Band9.2 to 10 GHz 60W Discrete120W DiscreteTo learn more, contact usaradar@ sei-device.comGaN for Space UHF to 2.5 GHz 4/8/20/50/100WTo learn more, contact usaspace@ sei-device.comGaN for SATCOM 5.8 to 6.7 GHz 100W7.7 to 8.5 GHz 100W13.75 to 14.5 GHz 50WMWJSUMITOMO1012.indd 127 9/27/12 11:19 AM128 MICROWAVE JOURNAL OCTOBER 2012Technical FeatureLowNoiseAST54SASL03SFreq.(GHz)Part No.DC~6Gain(dB)0.50NF(dB)34OIP3(dBm)GainBlockAWB207AWB209AWB389ASW320ASW314Freq.(GHz)Part No.DC~4DC~4DC~4DC~3DC~316.019.916.318.014.5Gain(dB)2.702.103.102.303.00NF(dB)3836384143OIP3(dBm)CATVASL390ASL31CASL590ASL331ASL551Bias(V/mA)Part No.5/1205/1058/1605/2206/43323.323.523.419.719.6Gain(dB)989598101106Pout(dBuV)64/7260/6964/7064/6060/64CSO/CTB(dBc)@ 2 GHz @ 2 GHzIFASF240ASF250ASF255Freq.(GHz)Part No.DC~1DC~1DC~126.316.922.4Gain(dB)3.002.502.20NF(dB)414342OIP3(dBm) @ 150 MHz @ 500 MHzASL552 8/240 11.2 105 76/7312.8ASL52D6ASL21X DC~5DC~4DC~521.030.00.650.600.80331022 ASL30G 21.5DC~4 0.90 35 13.0ASL882 12/520 21.6 110 60/66Here, the 'c10 is the fundamental-mode cutoff wavelength of the ridged waveguide. Thus, 'c10 = 3.8 4.5 = 17.1 mm, far longer than the cutoff wave-length of the traditional waveguide.It is noted that the conventional single ridged waveguide structure shown in Figure 2 can be regarded as a specic case of transmission line. From transmission line theory, the capacity-loaded ridged waveguide equivalent circuit is also shown. The conventional ridged waveguide trans-mission line has the impedance Z0and electrical length L, while its cor-responding capacity-loaded transmis-sion line is made up of two symmetric sections of transmission lines with the impedance Z1, electrical length 1l and capacitance represented by the susceptance j. Discussed from the point of transmission line, the network functions are:( ) = = + = + 1f (6)ab2Ca a bThen, can be given by2ab2C ba 1 aa (7)'c 'c 1 122d11 2 1'cc'cc22d111211f (6)ab2Ca a bThen, can be given by2ab2C ba 1 aa (7)'c 'c 1 122d11 2 1'cc'cc22d11121( ) = = + = + where c is the cutoff wavelength of the dominant mode of the rectangular waveguide. Here a = 4.5 mm, thenc 2a 9 mm (8)Substituting b = b = 2.9 mm yields,a a F ba 0.45 3.8(9)111'c10 c1010 = == + = Fig. 2 Ridge waveguide and equivalent circuit, (a) single ridge, (b) capacity loaded ridge.(a)Z0,LZ1,L Z1,Lj(b) Fig. 3 Schematic of a compact capacitor loaded ridged waveguide lter.L6hh0L5lL4h6lp lp1 lp0h5 h4 h3 h2 h1L3 L2 L14-R0.5L0aw1w2.51.1__10M32 FINAL.indd 128 9/28/12 11:41 AMEmersonConnectivity.com Toll free: 800-247-8256Phone: 507-833-8822EMERSON. CONSIDER IT SOLVED.Emerson Network Power and the Emerson Network Power logo are trademarks and service marks of Emerson Electric Co. 2011 Emerson Electric Co.IF CABLE INTEGRITY IS CRITICAL AND YOUNEED PRODUCTS TO WITHSTAND HARSH ENVIRONMENTS...FAILURE IS NOT AN OPTION.THATS MISSION CRITICAL CONTINUITY. Emerson Connectivity Solutions RF connectors and cable assemblies are designed to offer reliable connectivity solutions for RF & Microwave, Military and Security/Defense applications.MWJEMERSON1012.indd 129 9/27/12 11:19 AM130 MICROWAVE JOURNAL OCTOBER 2012Technical FeatureThe generalized scattering matrix of the discontinuity is obtained using the mode-match