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RF MEMS clears way for software radio on-a-chip

Posted: 19 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:RF MEMS? software radio? CMOS?

An aspiring fabless semiconductor house, WiSpry Inc., recently laid claim to sampling the world's first integrated RF MEMS. The chip!a digital capacitor array on a CMOS die!can match a cellphone's antenna impedance dynamically, rather having it set at the factory, saving dropped calls and extending battery life. And that's just the start, says WiSpry. Its forthcoming line of RF-MEMS devices, according to the company, will eventually yield the holy grail of RF: software radio!an UWB communications channel that can be tuned to different bands anywhere in the spectrum.

'Beginning at the beginning'
"In the long run, we want to offer a completely tunable RF-MEMS front end!almost a software radio on-a-chip," said Jeffrey Hilbert, co-founder of WiSpry. "But we are beginning at the beginning!that is, with matching antenna impedance with our tunable capacitor arrays."

Keeping the antennas' impedance matched to the RF front end in a cellphone has become increasingly difficult, as a result of the trend to make antennas internal. Internal antennas tend to couple the RF signal to the user's body, so that just picking up the handset can change the antenna's impedance. Today, electrical engineers design a passive impedance-matching network that can handle the difference between holding the handset and lying it flat on a table, but WiSpry claims its active network can adapt to even unforeseen impedance-matching situations, such operating a handset wearing gloves.

"It sometimes takes a design engineer four weeks to design an impedance-matching network for the antenna on a new cellphone model," said Hilbert. "But with our tunable capacitor array, the impedance matching can be done dynamically, shortening handset design time and improving performance."

WiSpry, a five-year old startup, just decided to start talking about its RF-MEMS development work because it has begun sampling its first chip!an array of 80 digital capacitors on a die, yielding a programmable 10-fold dynamic range in capacitance. The chip won't be ready for customers until next year, but one cellphone handset maker has already signed up for the dynamically matching antenna-impedance application. By adding the tunable digital capacitor array to its cellphone!which can dynamically adjust its impedance in 10?s!a cleaner signal gets through, resulting in fewer dropped calls, extending the range of the mobile wireless service, and lowering the transmission power required, thus extending battery life of the handset.

"It turned out that, 90 percent of the time, the antenna was not perfectly matched to the 50Ω input impedance of the RF front end," said Hilbert. "But with our tunable capacitor arrays, the impedance can adapt in real-time to always be matched."

WiSpry was spun-off from Coventor Inc.!the MEMS design-software maker!in 2002. The company began by pursuing a MEMS version of the venerable reed relay, which is the standard way of switching RF (for instance, to switch an antenna between two different radio-frequency front ends). After solving the MEMS-design problems for RF switches, the company turned to more challenging problems, such as tunable RF filters and eventually designed the array of digital capacitors on a MEMS chip that the company is currently sampling.

MEMS specialist
Along the road to its first chip, WiSpry has accumulated a portfolio of MEMS technology, including integration with CMOS, SiGe and GaAs technologies used in various types of RF front ends. The company has also perfected a wafer-scale encapsulation technique that isolates its MEMS structures from the environment, and permits standard CMOS-fabrication equipment to handle MEMS wafers as cheaply as are ASICs. The company has filed multiple patent claims for its MEMS techniques!including contact welding for long-term reliability and a triple-layer beam structure that avoids static charging effects. The company is also retaining its most difficult-to-duplicate technologies as trade secrets. So far, WiSpry has had two rounds of financing, netting more than $8 million in venture capital, which it has invested in the RF-MEMS infrastructure for its chips.

WiSpry's first RF-MEMS chip, the digital capacitor array, is being fabricated at Jazz Semiconductor in 0.18?m CMOS on 200mm wafers. The tiny picofarad capacitors change their value 10-fold!either between 0.1pF and 1pF or between 0.2pF and 2pF!in response to a voltage that changes the distance between capacitor plates to adjust their value. The digital capacitors can change their values in increments as small as five femtofarads. Besides tuning the radio better, the variable capacitor array can add 10-20 percent to a cellphone battery's lifetime, since a matched antenna does not require as much broadcasting power to go the same distance.

WiSpry's first customer, the unnamed cellphone handset maker, plans to have digital capacitors doing dynamic antenna impedance matching in its handsets by 2008. In addition to tuning antennas, WiSpry's digital capacitor arrays can be used for tunable RF filters, power amplifier tuning and other programmable impedance-matching applications, according to the company.

- R. Colin Johnson
EE Times




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