# An RF-MEMS-Based Digitally Tunable SIW Filter in X-Band for Communication Satellite Applications
## 1 Introduction  
Tunable filters have been attracting attention over recent years as they are one of the key building blocks for reconfigurable radio front ends, multiband and subsystems. Frequency tunability in planar technology has been widely investigated in the literature [1]. To bypass the Q-factor limitations of planar technology, an alternative solution consists in using substrate integrated waveguide (SIW) technology [2]. Due to a higher quality factor and good integrating abilities with planar circuits, SIW is considered a good candidate for many microwave applications. In particular, SIW seems a very good candidate to cope with constraints induced by new trends in the development of communication satellites. In that context, there is a real need for innovative tuning devices that can address several frequency plans. Recent scientific literature has described several technological options to achieve the tuning of SIW filters [3]. Amongst them, solutions based on piezoelectrical or mechanical technologies [4; 5] are not suitable as they cannot match all the constraints induced by our spatial application (size, weight, speed, power consumption, etc.). Thus, a choice was made to design a solution based on electrically-controlled microelectronic devices (diodes, transistors, radio frequency micro-electrical-mechanical systems (RF-MEMS), etc.) implemented on the top of an SIW structure. In order to make an SIW filter tunable, the authors of [6] proposed a new concept using vertical metallic posts inserted into the cavity. A 5% frequency shift around 10 GHz was achieved by either connectingor disconnecting the metallic posts. Depending on the purpose of the desired tuning device, digital or analog tuning is required. In this study, the need to switch between frequency plans is addressed by a focus on digital tuning. In [7], the digital tunability of cavities using packaged RF-MEMS switches from the Omron Corporation resulted in a filter with a 28% tuning range, from 1.2 to 1.6 GHz. The relative bandwidth was between 3.2% and 4.2% over the tuning range, and the use of bias and MEMS switches was demonstrated to tightly prescribe the tuning frequency range. However, the use of numerous bias currents for both coarse and fine tuning implies the degradation of the Q-factor. A different switching device was used in [8], with tunability achieved using PIN diodes implemented upon cavities. The filter presented a 25% tuning range from 1.55 to 2 GHz, with 2.3% to 3% relative bandwidth. To satisfy high-frequency applications, another tunability technique was applied on evanescent-mode cavities, as described in [9]. It was performed by using open-ended microstrip stubs, whose lengths can be adjusted by RF-MEMS switches. A frequency shift of 4% was realized. The fractional bandwidth and the minimum insertion losses were approximately 3.6% and 4.57 dB, respectively. Analysis of previous results indicates that RF-MEMS appears to be a good choice for tuning, especially when high-frequency filters are needed.  
In this paper, the emphasis is on experimental results from the design of a single fourth-order tunable filter with two contiguous states, able to address four contiguous frequency plans. The single filter is intended to replace a switched bank of four independent filters. However, the final aim of this study is to demonstrate the mastery of frequency shift using additional wire bondings.  
Tunability was achieved using a combination of metallic posts, additional wire bondings and commercial off-the-shelf RF-MEMS. The position of the metallic posts defined the potential achievable frequency range whereas the additional wire bondings determined the accessible frequency shift within this potential achievable range. Finally, the RF-MEMS played the role of a switching device which enables the selection of the desired states according to specifications. The filter was designed in X-band. The proposed filter and biasing network were built on the same alumina substrate plate.