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New for January 2010! MEMS phase shifters has been an "active" area of research for many years. That was a joke, get it? MEMS phase shifters are passive!
MEMS phase shifters always brag about low loss; indeed, they have lower loss than comparable MMIC technology. But MEMS technology in general has a lot of problems, which are described here.
MEMS are processed on many different substrates, such as alumina, glass, high-resistivity silicon. But the "magic" of creating that wonderful, movable strap, the MEMS process teams usually omit two key elements of microwave thin-film circuitry: thin-film capacitors, and thin-film resistors. Without capacitors for matching networks, MEMS phase shifter designers are stuck using crude architectures that ensure wide phase variation with frequency, and undesirable characteristic for a phased array.
The switching time of a MEMS phase shifter is on the order of 1 microsecond, 1000 times slower than most MMIC phase shifters.
More to come!