A miniaturization method of parallel coupled-line filters using lumped capacitors and grounding

This article proposes a new miniaturization method for a parallel coupled-line filter using lumped capacitors and grounding. The proposed method can dramatically reduce the size of the resonators in the parallel coupled-line filter. In addition to the size reduction, an excellent suppression of spurious harmonic responses, which are inherent in this type of filter, is achieved with improved skirt characteristics. To show the validity of the proposed method, a filter that has a center frequency of 900 MHz and a fractional bandwidth (FBW) of 10 percent is scaled down to half its original dimension. The measured results show good agreement with the theory.