2008 IEEE MTT-S International Microwave Workshop Series on Art of Miniaturizing RF and Microwave Passive Components

A Band-Stop Filter with Far Spurious Stop BandsBao-Xin Wang, Qing-Yuan Wang, and Rong-Jun Liu

School of Physical Electronics, University of Electronic Science and Technology of China,Chengdu 610054, P. R. China, E-mail: * qywangguestc.edu.cn, Website: www.drwqy.com.

Abstract This paper presents a band-stop filters with farspurious stop bands. At first, a coaxial cavity with far spuriousstop band is proposed. Then several such coaxial cavities areconnected in series to form a band-stop filter. To illustrate thedesign process, a coaxial band-stop filter has been designed, witha stop-band centered at 5 GHz, a bandwidth of 2 GHz, and itsfirst spurious stop-band higher than 18 GHz. The sample filterhas been fabricated and tested. Tested results show goodagreement with simulated results.

The parameters of the band-stop filter are explicitly derivedfrom those of a standard filter, which is assumed as a reference. Ithas been proved in both the simulation and the measurement.

Index Terms-Band-stop filter, Spurious stop-bands, Coaxialcavity

I. INTRODUCTION

With the urgent requirements from military equipments andthe rapid development of the wireless communication systems,the demands for filters with good eliminating characteristic ismore an d m ore po pular t oday. As a p art of th e filters, band-stop filters are i mportant for m any app lications su ch ascommunication transceivers, radar and measuring instruments,especially in the area to s uppress the spurious outputs from ahigh power transmitter and harmonics from a nonlinear poweramplifier [1-2].

In the design of the band-stop filter, the undesired spuriousbands or harmonics prese nt a fu ndamental 1 imitation whi chcan seriously degrade the performance of the filter and can becritical in certain applications in the microwave and millimeterwave system s. T o reject t hese frequency parasites, manytechniques su ch as half wavelength sh ort circuit stub s, chipcapacitors or cascad ed rej ection band filters have b eenreported in literature[3]-[6].The disadvantage of the existing band-stop filter is t hat the

spurious stop-bands are too close to the working stop-band. Tosolve this problem, we will propose a coaxial cavity which canhave a working st op-band a nd very far a way sp urious stopbands.The rest o f the paper is a rranged as fo llows: In Section II,

The unique coaxial cav ity will b e proposed, to gether with itstransmitting ch aracteristics. Several co axial cav ities of th iskind are then used to construct a band-stop filter. CST is th enused to optimize the configuration parameters of the band-stopfilter. In Section III, the tested results from a sample filter with

the optimized configuration parameters will b e presented. Thelast section is a conclusion.

II. FILTER DESIGN

A. The PrototypeAbout sev enty years ago , H. Salin ger [7 ] repo rted filters

using circular cavities. Each cavity is a radial transmission linewith its outer port shorted. The filter is formed by such shortedradial t ransmission 1 ines co nnected by ci rcular waveguidesections. In ord er to decrease th e size o f t he filter, Saling eralso su ggested t o be nd t he shorted radial transmission 1 inesalong the axis of the structure.The coaxial cavity proposed in this paper is shown in Fig. 1.

It is similar to the bended radial transmission line proposed bySalinger, e xcept that our st ructure is a c oaxial one a nd acircular disk is added on the inner conductor. The circular diskhas two functions. First, it further minimizes the cavity and thesecond, it ca n push the spurious stop band far away from theworking stop band.

Fig. 1. Configuration of the coaxial cavity with far spurious stop-band

In order to demonstrate th e feasib ility o f th is typ e of thestructure, we use the software of the CST Microwave Studio tocalculate and simulate th e co axial cav ity filter. Acco rding t othe 5 0 Q stand ard im pedance, th e air-filled cav ity and bothcircular ports with SMA co axial tran sition, we can draw th ecalculated S- parameters sho wn i n Fi g. 2 and t he pa rametersshown in TABLE I.

978-1-4244-2877-9/08/$25.00 2008 IEEE 164 December 2008, Chengdu, China

TABLE I

PARAMETERS OF THE COAXIAL CAVITY

Parameters Initial Values Descriptions

a 3.00mm Inner diameter of the coaxialb 7.00mm Outer diameter of the coaxialdo 22.00mm Diameter of the circular diskdi 14.00mm Outer diameter of the cavityg 1.50mm Gap widths 3.00mm Length of the shorting stubt 1.50mm Thickness of the circular disk

freq, GHz

Fig. 2. Simulated S-parameters from the single circuit of the sample filter

From Fig. 2, it can be easily seen that the working stop bandis centered at 4.85 GHz. Please note that the first spurious stopband does not show up until 18 GHz.

Fig.3. Simulated S-parameters from the sample filter with the different disk

Working as a capacitance 1 oad, the ci rcular disk plays a nimportant part in the position of spurious stop bands, referringto th e Fig. 3, wh ere it shows S 21 param eter u nder di fferentsituations. To be sp ecified, th e position and size of circu lardisk significantly influence spurious stop bands.

Fig.4 sho ws t he S-p arameter of two su ch co axial cav itiesconnected back to back. The correspondi ng parameters aregiven i n T ABLE I I. E xcept t hat t he bandwidth of t he s top-band is a little to o narrow, th e filter alm ost satisfies ou rrequirement.

-1 0-

-60-2 4 6 8 10 12 14 16 1E

freq, GHz

Fig. 4. Simulated S-parameters from 2 coaxial cavities connected back to back

TABLE II

PARAMETERS OF TWO COAXIALS CONNECTED BACK TO BACK

Parameters Initial Values Optimaized Valuesa 3.00mm 3.00mmb 7.00mm 7.00mmdo 22.00mm 23.26mmdl 14.00mm 14.72mmd2 14.00mm 15.1 OmmgI 1.50mm 1.66mmg2 1.50mm 1.54mmsl 3.00mm 3.28mms2 3.00mm 2.62mmt 1.50mm 1.00mm

B. Simulation ofMulti- CavitiesIn order to expand the bandwidth of the stop band to 2 GHz,

20 coaxial cavities are co nnected in series in such a way thatthe fi rst a nd seco nd ca vities, an d eac h s ubsequent o ddnumbered ca vity and a djacent e ven numbered ca vity areconnected back to back.

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The S-p arameters of the order-20 b and-stop filter is shownin Fi g. 5. It can be seen that the st op band i s cent ered at 5. 0GHz, wi th a bandwidth of 2.2 G Hz a nd elimination hi gherthan 46 dB. The reflection in all the pass-bands is lower than -10 dB.

-10-

-20

-30- i

-40-

50-~ ~ ~ ~~-S21 SIil-60-

2 4 6 8 10 12 14 16

freq, GHz

Fig. 5. Simulated S-parameters from an order-20 band-stop filter withcoaxial cavities

III. TESTED RESULTS

Fig. 6 shows t he photo of a sam ple fi lter fa bricatedaccording to the parameters ob tained from s imulations. Thetotal volume of the filter is 180 mm X 30 mm X 30 mm. Fine-tuning scre ws ha ve bee n u sed t o c ompensate fo r t hefabrication errors.

Fig. 7 sh ows t he measured S- parameters of the samplefilter. The centre frequency of the stop band is 5 GHz, with 2GHz bandwidth. At the same time, the filter is free of spuriousstop band up to 1 7.9 GHz. The reflection in the pass band islower than -10 dB, the stop-band rejection from 4.0 GHz to 4.2GHz is better than -40 dB, and the stop band rejection from 4.2GHz to 6.0 GHz is even better than -63 dB.

Fig. 6. Photo of a sample filter fabricated

freq, GHz

Fig. 7. Tested S-parameters from the sample filter

IV. CONCLUSIONS

In this paper, a band-stop filter with far spurious stop bandshas been reported. At first, a co axial cav ity with far sp uriousstop band was proposed. Then two such co axial cav ities areconnected in series to form a b and-stop filter. In o rder toexpand the bandwidth of the stop band, an o rder-20 filter h asbeen designed, fabricated and tested. To illu strate the designprocess, a co axial b and-stop filter has been presented, with astop-band centered at 5 GHz, a ban dwidth of 2 GHz, and itsfirst spurious stop-band higher than 18 GHz. The sample filterhas the spurious response more than 3.6 times far away fromthe centre frequency, with re flection lower than -10 dB in th epass ba nd. It s st op-band re jection has al so been i mprovedconsiderably. Tested resu lts show good agr eementw ithsimulated results.

REFERENCES

1] L. Young, G. L. Matthaei, and E. M. T. Jones, " Microwave band-stopfilters with narrow stop bands," IRE Trans. on Microwave Theory andTechniques, vol. M TT- 10, pp. 416-427, November 1962.

[2] B. M Schiffman and G. L M atthaei, "E xact design of bandstopmicrowave filters," IEEE Trans. ors Microwave Theory and Techniques,vol. MTT-12, pp. 6-15, January 1964.

[3] Minakova L B, Rud L A, " W aveguide Band-Stop Filters Based onPartially Fill ed H-plane Stubs," Microwaves Rad ar and Wi relessCommunications,2002,1(20):77-80.

[4] Smain A mari,Uwe Rosenber g, "Dir ect Sy nthesis of a New Class ofBandstop Filters. " IEEE Transactions onMicro wave Theory an dTechniques, Vol.52,pp.607-616,February 2004.

[5] Richard J. Cameron,Ming Yu, Ying W ang, "Dir ect-Coupled M icrowaveFilters With Sing le and Dual St opbands, " IEE E Transactions onMicrowave Theory and T echniques,Vol.53, pp. 3288-3296, Novem ber2005.

[6] Snyder, R.V ., " Quasi-elliptic compact highpowernotch filters using amixed lu mped anddistr ibuted cir cuit, " Microwave T heory andTechniques, IEEE Transactions, Vol. 47, pp.518 - 522 , April 1999.

[7] Salinger H, "A C oaxial Filte r for Vestigial-Sideband T ransmission inTelevision," Proceedings of the IRE, 1941,3(29): 115-120.

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