Reconfigurable Microwave Filters

Abstract

Reconfigurable microwave filters make microwave transceivers adaptable to multiple bands of operation using a single filter, which is highly desirable in today’s communications with evermore growing wireless applications. Tunable filters can replace the necessity of switching between several filters to have more than one filter response by introducing tuning elements embedded into a filter topology. Microwave tunable filters can be divided in two groups, filters with discrete tuning, and filters with continuous tuning. Filter topologies presenting a discrete tuning generally use PIN diodes or MEMS switches. On the other hand, filter topologies using varactor diodes, MEMS capacitors, ferroelectric materials or ferromagnetic materials are frequently used to obtain a continuous tuning device. Filter topologies can mix continuous and discrete tuning by combining tuning elements as well, e.g. the use of switches and varactors on a filter topology can form part of a discrete and continuous tuned device. Center frequency is the most common filter parameter to reconfigure. Fewer designs reconfigure other parameters, such as the bandwidth or selectivity. When deciding which technology is adequate for a given application, the designer must consider the following issues: cost, power consumption, size, performance and operating frequency. This chapter intends to provide a broad view of the microwave reconfigurable filters field, where different technologies used to reconfigure filters are discussed through different chapter sections, and finally an overall view of the field is given in the conclusions section at the end of the chapter. This chapter starts discussing filters that use active devices as tuning elements in section 2; these include the PIN diode, the varactor diode, the transistor and Monolithic Microwave Integrated Circuit (MMIC) implementation. Section 3 discusses the use of Micro Electro Mechanical Systems (MEMS) as tuning elements on filter topologies; the section discusses the use of MEMS switches and MEMS varactors. Section 4 contains tunable filters using ferroelectric materials, where devices using the most common ferroelectrics are discussed. Section 5 contains filters that use ferromagnetic materials as tuning elements, the section discusses circuits using Yttrium-Iron-Garnet (YIG) films and other ferromagnetic tuning mechanisms. Section 6 describes devices that combine some of the technologies discussed in previous sections to achieve reconfigurable filter parameters. Section 7 contains a discussion of