Impulse radar micropowered for sensor apps

By Gail Robinson

LIVERMORE, Calif. -- A three-year-old technology rooted in a radar technique discovered roughly a century ago is rapidly amassing practical appllications as a low-cost, high-precision sensor. Developed here at Lawrence Livermore Laboratory, micropower impulse radar (MIR) harnesses the speed of light for precise motion control by combining digital logic with microwave techniques at a fraction of the cost of traditional equipment.

Conceived as a diagnostic system for Nova, Livermore's high-power laser, MIR technology has since been licensed to15 companies, and 20 licenses are said to be pending. "This is about using the speed of light with a low-cost gizmo," said MIR inventor Tom McEwan. "We're making radar available to the consumer for $10 or less."

The technology is based on pulse-echo radar, discovered around the turn of the century. Such radar leverages the speed of light as an integral component of its operation, measuring the echo that results when a pulse strikes an object to determine that object's distance from the pulse source.

Conventional radar systems transmit several thousand such pulses per second. MIR, by contrast, sends out over 1 million.

"In the past, people used radio waves, but nobody cared how fast they were going," McEwan said. "Now the consumer can use devices that actually clock the speed of light in the form of microwave propagation, allowing them to do things that they could never do before."

Using 1-micron CMOS circuitry and discrete surface-mount packaging, McEwan's design fuses high-speed sampling and time-based concepts to yield a low-cost system that offers precise response at close range (less than 200 feet). A 1.5-inch-square MIR circuit board can reportedly be built for $10 or $15 and can perform tasks that would conventionally require $40,000 equipment. The system is expected to be re-duced to a single silicon chip within a year.

Because it can run on low power, it is said to be ideal for implants; at 60 �A and 2.5 V, a complete radar unit, includ-ing the transmitter and receiver, can run for three years on a lithium AA battery. McEwan believes the power requirements can fairly easily be pared to 20 �A.

Licensee Data Systems Inc. has employed the technology in a battery-operated device that can be used to monitor rivers for flash flooding and to safeguard wastewater-treatment pools against overflow. And Amerigon Inc. (Monrovia, Calif.) plans to manufacture MIRE-based automobile-safety devices, such as parking aids and back-up warning systems, starting this year.

Data Systems' implementation -- an electronic dipstick, or what McEwan calls a "poor man's TDR [time-delay reflectometer]" -- could cost as littlle as $10 to make. That would bring the selling price considerably below the $10,000 to $50,000 of a conventional TDR system.

The system sends a 200-ps-wide impulse down a coaxial cable that goes to the top of the tank. A single wire transmission line or electromagnetic guide wire is positioned further down in the tank. In operation, a transmitted pulse hits the fluid level, reflects back and enters a circuit that measures the interval between transmit and receive.

A reflection is also generated at the top of the tank; the difference between that reflection and the reflection off the liquid level is the differential measurement.

"The differential measurement works wonders because the whole concept requires doing it dirt cheap and using off-the-shelf components," said McEwan.

Since temperature changes affect the timing of CMOS circuits, the absolute (as opposed to the differential) measurement would require correction. "The temperature would cause the measured distance to drift by several inches, so you would have a large error because of the temperature and the propagation delay in the gates," he said. With differential measurement, by contrast, drift is not a factor, "because the pulses coming back are time-locked to-gether; they are sequenced in one conductor."

Through sampling, a precise equivalent time measurement can be made at picosecond accuracy. Short-term jitter on the dipstick is kept to 50 fs as a result of averaging and of the CMOS technology's low noise. With accuracy within one-tenth of 1 percent, the system is workable in temperatures ranging from 55�C to 85�C and can be used for measuring fluid levels in gas, oil, chemicals and pharmaceuticals, with future uses envisioned in such consumer goods as washing machines and autos.

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