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Agilent Network Analysis Solutions Advanced Filter Tuning Using Time Domain Transforms Application Note 1287-10 Introduction The level of experience and The basic technique has been This application note reviews expertise required to accurately comprehensively covered in these time domain tuning tune coupled-resonator cavity Agilent application note 1287-8, techniques, and extends the filters, cross-coupled filters, and which also describes how technique for use in tuning duplexers effectively precludes coupled-resonator band-pass filters with cross-coupled these devices from mass produc- filters can be easily and deter- resonators that produce tion at high speed. Ironically, ministically tuned. To achieve transmission zeros near the these same filters are increasing- the proper passband response, filter passband, as well as ly needed in large quantities, as and to achieve good return loss duplexer filters that have a a result of the spectral density and passband ripple, the center common (antenna) port, an resulting from the runaway suc- frequency of each resonator is upper passband (transmit) port, cess of wireless communications precisely tuned, and each and a lower passband (receive) services. The time required to coupling between resonators port. tune these filters accurately lim- precisely set. The method is its manufacturers from increas- based on the time-domain Together with application note ing their production volumes response of a filter's return 1287-8, this application note and reducing manufacturing loss, in which the time-domain provides a comprehensive cost. Fortunately, it is possible to response is obtained by a special compilation to filter tuning in the dramatically reduce both the type of discrete inverse Fourier time domain, including theory, time required to tune these transform of the frequency application, set-up, and tuning types of filters, as well as the response. Readers are encouraged procedures. experience and expertise to review the material contained required. The method removes in application note 1287-8 for filter tuning from the realm of information about the basic art, and makes the process pre- technique and how it is applied dictable and repeatable. Even to tuning coupled-resonator relatively inexperienced filter cavity filters. tuners can tune multiple-pole filters with great success with a minimal amount of training. 2 The technique defined A five-pole coupled resonator fil- ter with four coupling structures will be used to illustrate the basic tuning technique