hydrogen in nitrogen (1000 ppm hydrogen con-
centration). The typical capacitance change for
this test was about 20 fF. This cantilever system
has also been used to detect hydrogen levels
down to approximately 100 ppm. Lower levels are
practical with thinner cantilevers and coatings.
0.12                               35
002  -Pd coated
Sacono. Prom,,Stan
Fig. 6. Multichannel hydrogen data
RF TELEMETRY
A wireless network for data reporting is needed to
field arrays of distributed sensors. For this, we
have developed an RF-telemetry chip with on-
chip spread-spectrum encoding and modulation
circuitry to improve the robustness and security
of sensor data in typical interference- and mul-
tipath-impaired enirnens"'  We have also
provided for a selection of distinct spreading
codes to serve groups of sensors in a common
environment by the application of code-division
multiple-access techniques. Our initial intended
operation is for use in the 915-MHz Industrial,
Scientific, and Medical (ISM) band.
The 'Wirtxl' chip, shown in Fig. 7, is comprised
of a 10-bit analog/digital converter (ADC) with
four input multiplexer, a 63-bit digital spreading--
code generator, a state-machine controller that
allows the chip to act as an unattended data ac-
quisition system, and the radio-frequency modu-
lator and transmitter. The entire chip operates on
3.3V and is mounted on a printed-circuit board on
the bottom side of the battery pack shown in
Fig. 3. The chip was fabricated in a 0.5-m bulk
CMOS process.

Fig. 7. Wirtxl wireless data acquisition chip
SUMMARY
We have developed a sensor-readout-telemetry
system that is battery-operated, utilizes multiple
microcantilever sensors that allow mixtures of
vapors to be measured with a high sensitivity.
This research was sponsored by the U. S. Dept.
of Energy and performed at Oak Ridge National
Laboratory, managed by Lockheed Martin Energy
Research, Inc. for the U. S. Dept. of Energy un-
der Contract No. DE-AC05-960R22464.
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