The ability of receivers to reject signals outside the frequencies they are trying to receive is becoming more important. The recent LightSquared debacle was an excellent example of this. LightSquared was unable to use its mobile satellite spectrum for terrestrial broadband because GPS receivers couldn't handle the strong signals planned by LightSquared that would be close to the GPS band.
While it’s possible to design filters that work at one frequency, today's devices need to be able to operate on a wide range of frequencies. Using conventional filters and switches takes too much space and the switching required results in losses that reduce receiver sensitivity.
Dana Weinstein, an MIT assistant professor of electrical engineering and computer science, and Laura Popa, an MIT graduate student in physics, working with researchers at MIT's Microsystems Technology Laboratory (MTL) may have found a way to filter a wide range of frequencies in a small space. Their solution is miniature filters created using acoustic resonators. Using acoustic resonators created with techniques already common in the production of signal-processing chips, their filters improve performance while enabling 14 times as many of them to be crammed on a single chip.
Weinstein explained the new filter idea: “If I pluck a guitar string--that’s the easiest resonator to think of--it’s going to resonate at some frequency, and it’s going to die down due to losses. That loss is related to, basically, energy leaked away from that resonance mode into all other frequencies. Less loss means better frequency selectivity, and mechanical acoustic resonators have less loss than electrical resonators. Acoustic wavelengths are much smaller than electromagnetic wavelengths. So for a given frequency, my mechanical resonator is going to be much smaller.”
One of the problems with acoustic resonators is the capacitor used to convert electrical signals to mechanical vibrations and vice versa.
“The capacitors change the impedance that the antenna sees, so you may have unwanted reflections back into the antenna,” said Weinstein. “Each capacitor from each filter is going to affect the antenna, and that's no good. It means I can only have so many filters, and therefore so many frequencies that I can separate my signal into.”
Weinstein and Popa solved both the impedance matching problem and also the problem with losses in switches by using a gallium nitride transistor as part of the capacitor. The lower “plate” of the capacitor is a gallium nitride channel in its conductive state. The MIT News Release explains it this way. “Switching that channel to its non-conductive state is like removing the lower plate of the capacitor, which drastically reduces the capacitors’ effect on the quality of the radio signal. In experiments, the MTL researchers found that their resonators had only one-fourteenth the “capacitive load” of conventional resonators.” Weinstein noted, “The radio can now afford to have 14 times as many filters attached to the antenna so we can span more frequencies.”
Thomas Kazior, a principal engineering fellow at Raytheon was enthusiastic about Weinstein and Popa's filter.
“We’re talking about making filters that are directly integrated onto, say, a receiver chip, because the little resonator devices are literally the size of a transistor,” said Kazior. “These are all on a tiny scale. They can help with the cost problem because these resonator-type structures almost come for free. Building them is part of the semiconductor fabrication process, using pretty much the existing fabrication steps that you’re using to build the transistor and the rest of the circuits. You just may need to add one, or two at the most, additional steps--out of 100 or more steps.”
While I was able to find several papers by Dana Weinstein on the Web, none mentioned this work or provided detail on the frequency range the filters are able to handle. We may learn more after the work is presented in June at the International Conference on Solid-State Sensors, Actuators and Microsystems.
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