Compact, low-intermodulation multiplexer employing interdigital filters

Abstract

An improved multiplexer comprises a housing, a transmission line transformer section within the housing extending in a first direction, and at least two interdigital transmission line filters adjacently disposed within the housing along the first direction. The transformer section has a first end electrically coupled to an antenna circuit connector, and a second end electrically coupled to the housing (i.e. grounded). Each filter includes several resonating elements. At least one resonating element of each filter is located in proximity with a portion of the transformer such that electrical coupling between the filter and the transformer is established. The length of each coupling portion of the transformer is approximately one-quarter electrical wavelength at the center frequency of the associated filter. The dimensions of the filter cavities are preferably selected to be as small as possible without unacceptably degrading the filter insertion loss. Further, the filters, and the associated coupling portions of the transformer, are disposed in close proximity. The filter cavity size and spacing are such that the transformer coupling portions of adjacent filters are no more than about one-eighth wavelength apart (measured along the first direction). This provides a significant reduction in the physical volume required for prior art multiplexers employing interdigital filters. An adjustable-frequency resonating element formed as a longitudinally extending body portion having an deformable leaf adjustment member at one end. Deformation of the adjustment member modifies the effective electrical length, and thereby, the resonant frequency, of the resonator.