Miniature Distributed Filters for Software Re-configurable Radio Applications

Variable preselection filters will be an important component of any software re-configurable radio design. To be of practical use, such a filter should occupy a minimum of board area in any transceiver layout. Slow-wave filters are introduced and their essential electrical characteristics and miniaturisation advantages are outlined. It is shown that this filter architecture is easily tuneable. MEMS switches can be used to switch line lengths and thus cause the centre frequency of the filter to be altered. MEMS switching is a preferred tuning method because it eliminates the non-linearity introduced by conventional tuning elements. A technique for designing a capacitively loaded, split ring filter is presented and some simulation and measurement results are given. The possibility of pushing the second resonant frequency beyond twice the fundamental frequency is highlighted as a feature of slow-wave filters. This feature is shown to be of limited advantage with the architecture under discussion.