The Design of Miniature Multilayer Bandpass Filters With Mixed Couplings

This paper introduces a new design process for miniature coupled resonator filters with mixed couplings, which can better preserve wideband response matching with coupling matrix prototype. A special type of matrix transformation is introduced in this paper to transform the coupling matrix into a configuration that consists of an equal number of capacitive and inductive main couplings. Based on this, lumped-element circuit models can be produced. Then, by using π to T transformations for some parts of the circuit, all the elements have realizable values. A miniature bandpass filter with a fractional bandwidth of 20% of this type based on N×N coupling matrix is demonstrated with both simulated and measured response. The filter is implemented using multilayer liquid crystal polymer circuit technology and has a compact size of 0.0294λ_g × 0.0303λ_g ×0.00052λ_g, where λ_g is the guided wavelength at the center frequency of 250 MHz.