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Embedded ultrathin radios let walls listen, talk

Posted: 23 Aug 2013 ?? ?Print Version ?Bookmark and Share

Keywords:embedded ultrathin radio? invisible communications system? smart building?

To make their low-temperature circuits, Sturm and Wagner used amorphous silicon in transistors rather than crystalline silicon. The amorphous silicon does not require the high temperature of the crystal form, but it also lacks the crystal's highly ordered inner structure. For electrons, that switch is like changing from a smooth superhighway to a bumpy gravel road.

"These transistors do not perform nearly as well as the ones that Intel would make on one of its chips," Sturm said. "In fact, if you tried to make the radio with a circuit that Intel would use, but with our transistors on plastic, it simply would not work."

It was a puzzling challenge. Super-efficient transistors are so fundamental to modern circuitry that working around them seemed anachronistic, like putting a crank starter on a new sports car.

"You could say we are taking a page out of history," Sturm said. "The genesis of the idea is, how do you make something when you cannot rely on high-performance transistors?"

In a modern radio, the frequency depends on the movement of electrons across a transistor: the faster the movement, the higher the frequency. The challenge for the researchers was to find a way to speed up the electrons despite the low-performing amorphous silicon transistors.

One way was to revisit ideas used before the development of the transistor revolutionized electronics. Before then, electronics depended on vacuum tubes, which were slower and less responsive than transistors.

"Many of these circuits went out years ago with vacuum tube technology," Wagner said. "But I call Naveen the Mozart of circuits. He just knows all of them, and the ways they function, and the ways that we might be able to take advantage of them."

Verma and electrical engineering graduate students Liechao Huang, Warren Rieutort-Louis, Yingzhe Hu and Josue Sanz Robinson hit upon the idea of using a circuit developed in 1922 by the father of FM radio, Edwin Armstrong. Called a super-regenerative circuit, the setup could use other components to increase the radio's frequency and bypass the relative poor performance of the amorphous silicon transistors.

The super-regenerative circuit bounces electrons between a capacitor and an inductor, causing energy to be stored and discharged from each. The energy changes caused by the rapid bouncing create the radio's frequency. And because the speed of the bouncing depends on the super-regenerative circuit's capacitor and inductorand not the transistorsit can allow the radio to operate at a relatively high frequency even if the transistors are poor quality.

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