Filters are key components in RF front-end circuits and usually occupy much of the volume of the systems. Thus, reducing the size of filters is the main challenge in making RF systems compact. In addition to compact size, high performance is also desired. The presence of harmonics or spurious passbands is a fundamental limitation of microwave circuits, which can seriously degrade their performance and can be critical in certain applications.
In this thesis, a simple and effective filter design method is proposed based on the structure of parallel short-ended coupled-line with capacitive loading for size reduction and ultra-broad rejection of the spurious passbands. It is achieved by adding lumped capacitors to the conventional coupled-line section such that the required length of the coupled-line can be largely reduced while maintaining approximately the same characteristics around the center frequency, and meanwhile the spurious passbands are excellently suppressed. In addition, the introduction of a cross-coupling capacitor into the miniaturized couple-line can create a transmission zero at the 2nd harmonic frequency for enhanced frequency selectivity and attenuation level. The aperture compensation technique is also applied to achieve a strong coupling in the coupled-line section.
In order to examine the feasibility of the proposed structure, such a compact two-stage bandpass filter operating at 2.3 GHz with a fractional bandwidth of 10% was designed and realized with LTCC technology. Measured results are also provided, from which attractive features are observed experimentally as to size reduction and multi-harmonic suppression.