Hui Wu, professor of electrical and computer engineering, who is a pioneer in a circuit design called an "injection locked frequency divider," or ILFD, has overcome the last hurdle to making the new method work. Wireless chip manufacturers have been aware of ILFD and its ability to ensure accurate data transfer using much less energy than traditional digital methods, but the technique had two fatal flaws: It could not handle a wide range of frequencies and could not ensure a fine enough resolution within that range. Wu, together with Ali Hajimiri, associate professor of electrical engineering at California Institute of Technology, surmounted the first problem in 2001 and has now found a solution for the latter.
The Achilles' heel of ILFDs has always been their inability to efficiently and reliably divide the frequency by anything but two--a serious drawback to achieving fine frequency resolution, which is a must for modern communication systems.
When a cell phone or a laptop using wi-fi or Bluetooth communicates wirelessly, the data is transmitted at very specific frequencies. One person can talk on a cell phone at a frequency of 2.0001 gigahertz, and someone else nearby can talk at 2.0002 gigahertz, and neither one will pick up the other's conversation. In order to make sure a device is both receiving and sending information on exactly the right frequency at all times, it must maintain a very accurate and stable clock, which is generated by a special circuit called "phase-locked loop." This circuit consumes a dramatic portion of the battery usage on wireless devices.
This is where Wu's new design makes the practical application of ILFDs possible. He has introduced a new topology into this circuitry--instead of the old three-transistor design, his has five transistors--creating what he calls "differential mixing." The new circuitry topology allows the ILFD to divide by three as well as two.
This tiny change has huge ramifications. By varying how many clock pulses are divided by two or by three, any frequency can be selected, making the power-saving ILFD method viable for the first time.
Wu's group has designed and fabricated several prototype chips, and their test results have successfully demonstrated his concepts. One prototype, an 18 gigahertz divide-by-3 ILFD, was recently presented at this year's International Solid-State Circuits Conference, the premiere technical conference in semiconductor industries. Wu also is working on other power-saving aspects of chip design that he hopes can be used to stretch the battery life of wireless devices even further.
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Longer battery life, thanks to chip design
niversity researchers have broken a barrier in wireless chip design that may mean longer battery life for devices such as cell phones and wi-fi laptops. The new design uses a 10th as much battery power as current designs and has the potential to use much less in emerging wireless devices of the future.
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