Microwave Filters for Communication Systems: Fundamentals, Design and Applications

CHAPTER 10

SYNTHESIS AND APPLICATION OF EXTRACTED POLE AND TRISECTION ELEMENTS

Although the folded and transversal array coupling matrices form bases from which a wide variety of useful filter configurations may be derived, there are certain sections that are difficult or impossible to synthesize by coupling matrix operations alone. In this Chapter, we shall follow the circuit-based method for synthesizing the “extracted pole” section, and its direct application in bandpass and bandstop filter design. Then the role of the extracted pole section in the formation of another useful section able to realize a single transmission zero, the “trisection”, will be examined. The trisection may be realized directly or used as part of a cascade with other sections. However it also finds application in the synthesis of more complex networks, where firstly a cascade of trisections is synthesized and then further reconfiguration is carried out using coupling matrix transforms. The techniques used to transform a cascade of trisections to form cascaded quartets, quintets and sextets are outlined. Finally the synthesis of the “box section” from a trisection is described, and its derivative, the “extended box” configuration.

10.1 EXTRACTED POLE FILTER SYNTHESIS

The low-loss requirement for high-power filters usually means that the designs are low-degree, with no in-band equalization provision (group delay equalization increases the insertion loss). Other requirements are a relatively steep out-of-band ...