PORTLAND, Ore. A research team from Michigan-based universities says it has succeeded in integrating the last two components needed to create a one-chip wireless transceiver.
"Our research group picked up the challenge to integrate the last two off-chip components onto a wireless transceiver," said Michael Flynn, head of the wireless-interface group at the Wireless Integrated Microsystems Engineering Research Center (WIMS ERC) at the University of Michigan (Ann Arbor).
"Thanks to Kamal Sarabandi, we have demonstrated a Zigbee [2.4-GHz] wireless link using our 1-centimeter-square slot antenna and thanks to probably the world's foremost expert on RF MEMS [microelectromechanical systems for radio frequencies], Clark Nguyen, we have also developed a
wineglass-like resonator to replace the off-chip quartz crystal.
"Now all the wireless components can be on one chip enabling everything from hearing aide-sized cell phones to smart dust," said Flynn. Kamal Sarabandi, a member of the WIMS ERC, is director of the Radiation Laboratory at the Electrical Engineering Computer Science (EECS) College of Engineering at the University of Michigan. Clark Nguyen, who developed the wineglass resonator, is an EECS associate professor.
"Sarabandi's group has been talking to Intel about commercializing the antenna design in wireless laptop computers, and others have been showing interest in the wineglass resonator," said Flynn.
The so-called wineglass resonator replaces the quartz-crystal clock chips that keep electronic watches and nearly every other digital chip (including transceivers) lock-synched to a rock-solid time base. It is based on the same principle as and is named for the cocktail party trick of flicking a fingernail against the edge of a wineglass, then listening to how much it resonates, to measure how fine the crystal is. (The electrically resonating quartz crystal in a clock chip is made from glass that is pure crystalline silicon dioxide without the lead that's added to wineglasses or the iron that makes sand yellow.)
A tiny MEMS flicker pings a 32-micron-radius disk, creating a measurable resonance at just the frequency (60 MHz) and within the specifications for the Zigbee ISO-802.15.4 wireless band. That enables an on-chip reference oscillator with low enough phase noise to replace the ordinary quartz crystal reference chip in a wireless transceiver.
"The frequency stability was 34 ppm over 0 deg.C to 70 deg. C, matching the stability you expect from a quartz-crystal-based chip," said Flynn.
The wineglass MEMS resonator is fabricated on a 100-micron2 piece of silicon with an integral transresistance-sustaining amplifier, all implemented in 0.35-micron CMOS. The measured phase-noise density of "100 dBc/Hz at 1 kHz offset from the carrier and "130 dBc/Hz at far-from-carrier offsets exceeds Zigbee requirements.
"We've already demonstrated a Zigbee wireless link where this RF MEMS frequency reference from professor Nguyen's group replaces the traditional quartz crystal. Now we want to put both the RF MEMS and the slot antenna on a wireless-transceiver chip," said Flynn.
The slot antenna the common in-the-field antenna, made by cutting a slot in a discarded section of waveguide was downsized for the on-chip version, resulting in a spiraling slot cut into a 1-cm2 metal layer on the top of the chip. By tailoring the antenna's shape, the researchers were able to match the antenna's impedance to the rest of the transceiver without requiring a lossy passive matching network.
The researchers expect the technology developed with Defense Advanced Research Projects Agency and National Science Foundation funding to be applied in remote wireless environmental sensors, cell phones, laptops and two-way radio watches. "We think we can make sensor nodes that are almost invisible. With this technology, you could just sprinkle them around," said Flynn.