A switchable filter topology is proposed in the form of a self-scalable comb-line structure. This topology is capable of implementing a tunable frequency response with consistent performance over very large tuning ranges. A twopole prototype has been fabricated using 10 discrete PIN diodes and is capable of covering the frequency range of 290 to 600MHz in four steps (2-bit tuning). The in-band loss of the filter is better than 2 dB for all modes when a DC voltage of 5V is used for biasing the PIN diodes. The filter has a IIP3 of greater than 32dBm, and its RF structure occupies an area of 9 x 21 mm2.Index Terms-Tunable filter, comb-line filter, software defined radio. I. INTRODUCTIONBand-select filters are commonly used in multi-band radios to minimize the effects of out-of-band interferers. While this need can be addressed by hybrid integration of multiple filters in radios with a small number of bands, using filter banks in more sophisticated transceivers with a wide frequency range is costly and impractical. The task of filtering becomes particularly difficult in the case of software-defined radio (SDR) transceivers intended for operation in multi-decade range of frequencies. Filter banks in such applications can be formidably large and costly. Also, they require complex hierarchical switching networks that can give rise to a high overall insertion loss.In such circumstances, tunable and switchable bandpass filters offer obvious advantages, as they can replace the filter banks. Tunable and switchable filters, hence, have received great attentions in the past few years.Tunable filters are commonly designed by integrating solid-state/MEMS varactors or switches [1]-[6] within the structure of conventional lumped or distributed bandpass filters. Most commonly, a tunable response is attained by changing only the capacitors or inductors in the resonators of a bandpass filter. Although by choosing appropriate coupling schemes, this method can produce a consistently tunable frequency response over moderate tuning ranges, the uncontrolled variations of the coupling may result in a significant distortion in the frequency response for wideband tuning.In this paper we describe a switchable filter appropriate for tuning over up to one decade. The proposed method is based on a self-scalable comb-line topology.

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