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Agilent AN 1287-8 Simplified Filter Tuning Using Time Domain Application Note Table of Contents 3 Introduction 3 Difficulties of filter tuning 4 Ideal tuning method 5 Basic characteristics of bandpass filters 6 Time-domain response of simulated filters 7 Effect of tuning resonators 8 Effect of tuning coupling apertures 10 Practical examples of tuning filters 10 Setting up the network analyzer 11 Example 1: Tuning resonators only 13 Example 2: Tuning to a "golden filter" 16 Example 3: Using simulated results for a template 17 Effects of loss in filters 18 More complex filters 18 Cross-coupled filters 19 Duplexers 20 Conclusion 21 References 22 Summary: Hints for time-domain filter tuning 23 Appendix A: Understanding basic bandpass filter design 25 Appendix B: Using time-domain in the network analyzer for filter tuning 2 Introduction The increase in wireless communications services Difficulties of filter tuning is forcing more and more channels into less fre- The interactive nature of coupled-resonator filters quency spectrum. To avoid interference, very strin- makes it difficult to determine which resonator or gent filtering requirements are being placed on all coupling element needs to be tuned. Although some systems. These systems usually employ coupled tuning methods can achieve an approximately cor- resonator filters to handle the power levels and rect filter response, final tuning often requires the provide the needed isolation. The difficulty of tun- seemingly random adjustment of each element ing these filters quickly and accurately often limits until the final desired filter shape is obtained. manufacturers from increasing their production Experienced tuners can develop a feel for the volumes and reducing manufacturing cost. proper adjustments, but months are often required before a novice can be proficient at tuning complex In a coupled-resonator cavity-tuned filter, the cen- filters. The time and associated cost of tuning, and ter frequency of each resonator must be precisely the difficulty and cost in training new personnel tuned. The couplings between resonators must also can limit a company's growth and responsiveness be precisely set to achieve the proper passband to changing customer needs. response, low return loss (reflection), and small passband ripple. Setting coupling coefficients and Some companies have attempted to automate the tuning the resonators are as much art as science; tuning process, using robotics to engage and turn often a trial-and-error adjustment process. Until the tuning screws, and an algorithmic process to now, there has been no alternative. accomplish the tuning. The