DARPA steers RF with micromachines

DARPA steers RF with micromachinesSteve Bush
Researchers at the Defence Advanced Research Projects Agency (DARPA) in Virginia are looking at micromachines on RF chips to replace conventional active components.
“The advantages are extremely low loss and good impedance matching,” said Elliot Brown of DARPA’s Electronic Technology Office:”The principle areas of interest are signal switching, filtering, phase shifting and impedance matching.”
The switches are akin to relays, with miniature switch contacts being operated, usually, by electric fields. Teledyne is developing something similar to replace the hundreds of relays found in automatic production test equipment.
There are several ways to filter with micromachines. Brown said: “Performing an electrical to mechanical transformation, in a similar way to a SAW device, allows signals to be filtered at HF frequencies. Mechanically tuned capacitors can be made, with a 10:1 range, that are viable at UHF. At higher frequencies, on-chip waveguides are so small that they can be moved by micromechanical structures for tuning purposes. You could envisage a multi-pole Chebyshev filter made like this.”
Phase shifting is a case of adding delays, in the form of lengths of transmission line, together. “The transmission loss in the switches is a small fraction of a dB over a wide frequency range form DC up to tens of gigahertz. We have data up to 94GHz. The bandwidth of the switch is the bandwidth of the line because the switch is so small and they are broad band and phase linear,” said Brown.
Impedance matching is especially needed in high frequency power amplifiers. Brown said: “The output impedance of an amplifier can be low, but you want it high to match into loads. Micromachined switches and capacitors look suitable for integrating alongside the power transistors to adjust impedance without distortion or intermodulation. Possibly adaptively in real time.” Ship shape… An 8in. wafer based phased array antenna is close to a usable size for missiles.
One conceivable use is as a multiple phase shifter and phased array antenna. This application would use a whole wafer and works like this. RF power is fed onto the surface of the wafer from a microwave source. An array of antennas on the wafer surface receives this energy. Eachantenna has its own micromechanically switched phase shifter which adjusts the phase of the signal to the appropriate phase for beam forming later in the system. A via carries the shifted signal through the wafer and out to one of many transmission antennas in an array on the far side of the wafer. This second array forms the beam in space. “Eight inch wafers are starting to allow a reasonable sized antenna to be made. A 12 inch silicon wafer would be better though.”


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