Microwave and RF filters are common components of communication devices. Both transmitters and receivers use filters for rejection of signals in the unwanted frequency bands. A major application of such filters is in the cellular/PCS phones. The most commonly used filter for cellular/PCS application is the coaxial ceramic type in which several coaxial ceramic resonators with very high relative dielectric constants are coupled to each other. These filters often are installed on top of circuit board and substantially increase height to the board thickness. As a result the filters are one of the components that restrict the implementation of a thin cell/PCS phones. Multi-layer circuit board with several layers of dielectric material and plated through blind via holes have become a common technology used in the cellular telephone handsets.
With the advent of Monolithic Microwave/millimeter wave Integrated Circuits (MMIC, MmmwIC) the needs for implementing high performance/space efficient filters have been increasing. The semiconductor substrate real estate especially material suitable for microwave/millimeter wave applications (e.g., GaAs) is costly and restrictive. Filters are often implemented off the chip. There is a great demand for means providing size reduction leading to cost efficient on chip implementation of filters.
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