High Performance Helical Resonator Filters Ming Yu and Van Dokas COM DEV Ltd, 155 Sheldon Dr., Cambridge, Ontario, Canada, NIR 7H6 ming.yu@ieee.org Abstract - Complex filter functions are realized using cross-coupled helical resonators. Internal group delay equalization and multiple transmission zeros are implemented through the design, fabrication and test of a 7-1 and 8-2-2 filter. Excellent filter responses are achieved. Index Terms -helical resonators, cross coupled band pass filters, helix, group delay equalization I. INTRODUCnON Helical resonator filters [I, 21 are widely used in ground based UHF mobile communication systems. They exhibit reasonable Q and excellent in-band performance over a wide temperature range with less volume and mass compared to conventional coaxial cavity filters operating in this band. Helical resonator filters are best suited in applications where conventional lumped-element filters are very small but are too lossy (lower Q) and coaxial resonator filters (higher Q) are too big and unpractical. Compared to SAW filters, although much bigger, the helical approach is much simpler and requires no costly fabrication setup. Small quantities can he produced economically with quicker delivery. This type of filter becomes particularly attractive for satellite applications where low volume and mass coupled with high reliability and electical performance is a must. The most common filter functions realized with helical resonator technology are either Buttenvorth or Chebysbev without prescribed transmission zeros as extensively summarized in [l]. Low order (n=4) cross coupled filter, used in dual degenerate mode [3-51, gives further size and mass reduction with slightly lower Q and reduced spurious clean window. Further, cross-polarization stray couplings in dual-mode configurations prevents realizing complicated and high performance filter functions such as 8-pole with internal group delay equalization using many transmission zeros. Although a circulator coupled external equalizer can be used with a dual mode filter that often leads to unnecessary increase in mass and volume. In this paper, high order helical resonator filters are investigated to achieve very stringent performance required in communication satellites including sharp rejection response and group delay equalization. By utilizing the advances in the area of electromagnetic optimization [6] and computer aided tuning [7], a 7-1 and 8-2-2 filter were designed to meet satellite industry specifications. The filters were manufactured, tuned and tested. Results are presented in this paper. To the best our knowledge, this is the first time that such complex filter functions have been realized using helical resonators. 11. HELICAL RESONATOR The typical configuration of a helical resonator is shown in Figure 1 (inside a rectangular metal cavity). The resonator is mounted at 0.linch above the floor of a 12x0.7inch cavity. Using quarter-wavelength (U4457incb at 344MHz) as a starting point, it leads to a coil diameter of 0.65inch with 4.2 turns. The dimensions can be quickly verified using an EM eigen mode solver, as given in table 1. It is interesting to note that the manual calculation is as accurate as EM mainly because the frequency is relatively low. This also reveals that the effect of cavity loading on frequency is relatively small. The first spurious mode is projected to propagate just above lghz when the length of the wire reaches 3U4. Figure 1. EM model of a Helical Resonator The simulated Q is about 1250. A single copper cavity helical resonator has been measured. The measured Q is 1000 and first spurious response is around IGHz. 34" European Microwave Conference - Amsterdam, 2004 989
344.2 345.1 1032.7 1035.3 1721.1 1704.9 2 CBW = Ri BW = - n*gd Table 3. Normalized coupling matrix for 8-2-2 filter Wire diameter: 0.075inch I R1 =0.9622 1 R8=0.8262 The similar simulation was performed for another resonator at ahout 600MHz. The cavity size is 0.85*x0.6. A helical resonator with diameter of 0.65inch and 3 turns was selected. 111. FILTER DESIGN The first filter designed is a 7-pole with one transmission zero at low side (7-1). The center frequency is 307MHz and the bandwidth is 35 MHz. The normalized coupling matrix with input and output termination is shown in Table 2. R1 and R7 are magnetically coupled using a wire connected directly from U0 connector to the corresponding resonators. The coupling m3.5 is realized using g0.04 inch cross coupling probes. The rest of intercavity couplings are realized using 0.75inch wide irises with various depths. Table 2. Normalized coupling matrix for 7-1 filter R1= 1.1332 I R7= 1.1774 ml, 1 = 0.0123 ml,2 = 0.8896 m2,2 = 0.0019 m2,3 = 0.6150 m3,3 = -0.0607 m3,4 = 0.5678 m4,4 =0.1147 m4.5 = 0.5690 m5.5 = -0.1027 m5,6 = 0.6175 m6,6 = -0.0503 m6,7 = 0.8985 m7,7 = -0.0075 m3,5 = -0.1103 The second variant designed is a 8-pole filter with two transmission zeros and two real zeros used to equalize inband group delay. Within 60% of the pass band, the group delay and loss variation are designed to exhibit a very flat response. The center frequency (CF) is 598MHz and the bandwidth (BW) is 36MHz. The normalized coupling matrix is shown in Table 3. The inter-cavity couplings are realized using 0.58inch wide irises with various depths. The m1,8 iris width is 0.25inch. The m2,7 coupling is realized using a coupling wire (+0.02inch) connecting the 2"d and 7Ih resonators. The tap point, which determines the coupling bandwidth (CBW) and Ri, is derived using a well-known SI, group delay (GD) technique [SI: ml, 1 = 0.0207 mz, 2 = 0.0196 m3,3 = 0.0206 m4,4 = 0.0275 m5.5 =0.0026 m6,6 = 0.0065 m7,7 = 0.0163 m8, 8 = 0.0204 ml,2 = 0.8166 m2,3 = 0.5849 m3,4 = 0.5422 m4,5 = 0.5817 m5,6 = 0.5333 m6,7 = 0.5608 m7,8 = 0.749 m1.8 =0.0166 m2.7 =-0.0679 The inter-cavity coupling can be determined by solving the eigen value problem of a 2-cavity coupled model [9]: M..=- CF - f,'-f2 "' BW f,'+ f,' Where f and f, represent the resonant frequencies assuming symmetric plane to be perfect electric conductor (PEC) and magnetic conductor (PMC). Figure 2 and 3 show the construction of the 7-1 and 8-2- 2 filters. The dimensions of the copper filters are about 4.4x2.3x1.4inch and 4.2x2.1xl.Zinch respectively. IV. MEASURED AND SIMULTED DATA Figure 4 shows the measuredkimulated retum loss and insertion loss of the 7-1 filter. The "noisy" trace is the measured data (typical with all plots in this paper). The measured spurious response at lghz shown in figure 5 is caused by the 3N4 resonance. It implies that another higher order mode helical filter can also be realized at lghz (the EM simulation predicted a similar Q as U4 resonator at 307MHz). Figure 6 shows the in-band loss variation of the 7-1 filter. The measured insertion loss of the copper filter is about 0.5dB. which represents a Q of approximately 800. All measured data agrees very well with the simulations. Figure 7 shows the measuredsimulated r em loss and insertion loss of the 8-2-2 filter. Figure 8 shows the inband loss variation. The measured insertion loss from a copper filter is about 0.9dB, which represents a Q of 1100. The measured and simulated group delay performance is illustrated in Figure 9. Again all measured data agrees very well with the simulated data. 990 34" European Microwave Conference - Amsterdam, 2004
Both filters were later fabricated using silver plated aluminum cavities meeting satellite hardware standards. The frequency drift over a 75 C delta is about 4ppm using 8ppm coil material. VII. CONCLUSION High performance cross-coupled filters using helical resonators are presented in this paper. A 7-1 and 8-2-2 prototype was designed, built and tested using fullelectromagnetic simulation techniques. To the best our knowledge, this is the fust time that such complex filter functions have been realized using helical resonators. Excellent in-band loss variation, group delay and out-ofband rejection have been demonstrated to meet the stringent specifications of satellite communication systems. Measured data correlates very well with simulated data. VII. AKNOWLEDGEMENT The authors wish to acknowledge the useful discussion with Mr. Peter Vizmuller during the concept design stage of one of the filters. REFERENCES [I] Peter Vizmuller, Filters with Helical and Folded Helical Resonators, Artech Hose, Inc. Norwood, MA, 1987, [2] Everard, J.K.A.; Cheng, K.K.M.; Dallas, P.A.; High- Q helical resonator for oscillators and filters in mobile communications systems, Electronics Letters, Volume: 25, Issue: 24, 23 Nov. 1989 Pages:1648-1650 Fiedziuszko, S.J.; Kwok, R.S.; Novel helical resonator filter structures, Microwave Symposium Digest, 1998 IEEE MTT-S International, Volume: 3, 7-12 June 1998, Pages:1323-1326 vo1.3 Kwok, R.S.; Fiedziuszko, S.J.; Dual-mode helical resonators, Microwave Theory and Techniques, IEEE Transactions on, Volume: 48, Issue: 3, March 2000 Pages:474-477 R. Levy and K. Andersen An optimal low loss HF diulexer usine helical resonators. Microwave Symposium -Digest, 1992., IEEE MTT-S International, 1-5 June 1992, Pages:l187-1190 vo1.3 M. A. Ismail, D. Smith, A. Panariello, Y. Wang and M. Yu. EM Based Design Of Larre-Scale Dielectric Resonator Filters And MultipGxers By Space Mapping, IEEE Transactions On Microwave Theory And Techniques Special Issue on Electromagnetics- Based Optimization of Microwave Components and Circuits, Vo1.52, Jan. 2004, pp386-392 Ming Yu, (Invited) SimulatiodDesign Techniques for Microwave Filters - An Engineering Perspective, Workshop WSA: State-of-the-Art Filter Design using EM and Circuit Simulation Techniques, International Symposium of IEEE Microwave Theory and Tech, May 2001, Phoenix AZ J.B. Ness, A unified approach to the design, measurement, and tuning of coupled-resonator filters, IEEE Transactions on Microwave Theory and Techniques, Vo1.46, Apr 1998. pp. 343-351 M. E. Sabbagh, K. Zaki, and Ming Yu, Full-Wave Analysis of Coupling between Combline Resonators and its Application to Combline Filters with Canonical Configurations IEEE Transactions on Microwave Theory and Techniques, Vol. 49, No.12, December, 2001, pp2384-2393. Figure 2. Photo of a 7-1 helical filter Figure 3. Photo of a 8-2-2 helical filter 341h European Microwave Conference - Amsterdam, 2004 991
Figure 4. Measured and Simulated Response of a 7-1 helical filter Figure 5. Out-Of-band of a 7-l Figure 6. Measured and Simulated insertion loss of a 7-1 filter Fnwenc~tGrW Figure 7. Measured and Simulated Response of a 8-2-2 filter '1, 4m ; ~110., ; -, 1, 1 Figure 8. Measured and simulated insertion loss of a 8-2-2 filter "J8 0.185 059 0595 Gb om 06, o m R=W).<Mhl Figure 9. Measured and simulated group delay of a 8-2-2 filtei 992 34" European Microwave Conference - Amsterdam, 2004
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