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DESIGN OF WAVEGUIDE BANDPASS FILTER IN THE X-FREQUENCY BAND

January 18, 2008
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The design of passive microwave functions, and especially filters, is a study domain of the most important interest. Indeed, with the widespread applications of communication systems, the bandpass filter is an essential component.

Thus, the quality of filter is extremely important. The development of bandpass filters has emphasized high electrical performances in terms of insertion losses and selectivity. Within this context, designers are faced to several problems linked to the design control, in keeping with the modeling accuracy. The design of such filters requires specific techniques which cover a wide spectrum of knowledge. Students must be introduced to that necessary knowledge that will make them immediately productive upon graduation.

The present paper deals with filter design techniques taught to students in second year of engineering school. During their courses, students get acquainted with filter synthesis techniques, microwave transmission lines analyses, electromagnetic model techniques as well as simulation software, either circuit (Agilent-ADS) or electromagnetic (Ansoft-HFSS) frameworks.

The present study recount practical works that gather this entire knowledge and allow students their application in concrete case.

Development of filter design rests on three steps. Application of Tchebycheff theory is first performed. So as to meet the filter specifications, the filter order is determined as well as its lumped element circuit prototype. In expectation of the filter implementation into waveguide technology, that limits the achievable impedance range, synthesis techniques based on impedance inverters and series resonators are then introduced.

Implementation into waveguide technology requires equivalent network for impedance inverters. From circuit analyze point of view, impedance inverter are modeled as inductance set in parallel. Implementation of the whole cascaded elements of the filter is then performed on Matlab.

The circuit analyze results evidenced the difference between lumped- (Tchebycheff theory) and semi-lumped- (Richards transformation) elements representation. Hence, so as to meet the expected specifications, further analyses were carried out to define the appropriate filter order.

 

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