We have presented an RF MEMS tuneable lowpass filter. Both distributed transmission lines and RF MEMS capacitances were used to replace the lumped elements. The use of RF MEMS capacitances gives the flexibility of tuning the cutoff frequency of the lowpass filter. We have designed a low-pass filter at 9–12？GHz cutoff frequency using the theory of stepped impedance transmission lines. A prototype of the filter has been fabricated using parallel plate capacitances. The variable shunt capacitances are formed by a combination of a number of parallel plate RF MEMS capacitances. The cutoff frequency is tuned from C to X band by actuating different combinations of parallel capacitive bridges. The measurement results agree well with the simulation result. 1. Introduction Radio frequency microelectromechanical systems (RF MEMSs) technology offers an attractive capability for RF systems, particularly in support of switching and tuning functions. One such component is a microelectromechanical voltage tuneable capacitor, which can enable a wide tuning range and high-quality (Q) factor. In this paper, we present a design of a tuneable low-pass filter combining RF MEMS capacitors and high impedance transmission lines. The low-pass filter has been designed by means of the theory of stepped-impedance transmission line filters. The filter can be developed with surface micromachining without the need for any external inductors. This will give flexibility in integration and easy fabrication processing. Compared with solid-state varactors, MEMS tuneable capacitors have the advantage of lower loss and a potentially larger tuning range [1]. The interconnection loss and noise can also be reduced compared with the use of off-chip solid-state RF components. Among all the MEMS capacitors developed to date, the parallel plate configuration is the most commonly used. The actuation mechanism is mostly electrostatic as it consumes very little power during actuation. In standard filter design at low frequency, the MEMS varactor is used as a lumped element with off-chip inductors. At high frequency, some low pass filter designs were reported with integrated transmission lines and MEMS switches (see Cai et al. [2] and Fang et al. [3]). In [2] the tuning of the filter is obtained by changing the transmission line length using MEMS switches. In [3] planar spiral inductor and shunt capacitor are used but without any tuning capability. Also, low-pass and band-pass filters integrating capacitive MEMS switches and short inductive transmission lines have been realised above 10？GHz [4–6]. In
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