Low-temperature cofired ceramic LC filters for RF applications [Applications Notes]

Over the past few years, a great amount of effort has been spent on LTCC filters or related research [21]¿[28]. Some of these components have more advanced functionalities, such as a combination of filter and balun, or a complete front-end module with balun, filter, and matching network. At the same time, advanced methodologies have also been developed to design these components [29]. All the enjoyment of miniaturized RF components or modules is due to the promising three-dimensional design flexibility of LTCC technology. It is expected that a variety of functionally complex components will be continuously available on the market such that the cost and size of the overall wireless terminal can be further reduced. Various aspects of designing miniaturized LTCC LC filters have been briefly discussed here. Conventional coupled LC resonators are commonly used in LTCC filters for RF applications because of their compact size advantage. The shunted parallel resonator in this type of filter can be realized directly as a lumped-element or distributed quarter- wavelength stripline with or without loaded capacitors. Filters with distributed resonators in general have better passband insertion loss performance. Moreover, there are many ways to improve the rejection performance in an LTCC filter design; for example, by modifying the resonators or admittance inverters so as to introduce transmission zeros at the stopband. It was demonstrated, through a design example, that by making use of the flexibility of LTCC technology, a compact LC bandpass filter can be realized for various RF applications.