in cases where the lower power-handling capability
of silicon-germanium might necessitate a design change,
such as adding more antenna elements to generate
the same output, "we're potentially saving so much
money that we can make tradeoffs in the design that
get around those limitations," he added. "If our
elements are two or three orders of magnitude cheaper,
and we only need twice as many, we still come out
way ahead in terms of cost."
Another consideration that may be more of a design
challenge than a drawback is that SiGe-based radar's
lower per-element power equates to a larger antenna
for greater sensitivity - perhaps tens of meters
in size, depending on the application.
researchers such as senior research engineer Tracy
Wallace are exploring ways to make these larger
systems "tactically transportable." The
work is being supported by the U.S. Missile Defense
"They can be much thinner and they can be folded
up onto themselves," Wallace explained. "We have
sketches, models and drawings of how that can be
Depending on the radar's destination, or if the
fabrication cost of folding the radar is too high,
the antenna and its supporting systems may simply
be fashioned in a manner that facilitates final assembly
on site, says Wallace, noting that some types of
radar are already constructed that way.
Designers are also investigating ways to measure
and compensate for deformities caused by the effect
of gravity on a large aperture. One aspect of that
is knowing the exact locations of all radiating elements
to within a fraction of a wavelength, according to
Wallace. One approach favored by Wallace and his
team involves photogrammetry, which provides information
about physical objects by interpreting patterns of
electromagnetic radiant energy and multiple digital
photographs taken from different locations.
consideration arising from larger antenna arrays
is the increased amount of data they collect, "so
more computer resources are needed," Wallace said. "But
as technology advances, that comes pretty cheap."
In another major government contract, GEDC researchers
are developing silicon-germanium technology for electronic
systems for NASA to use in lunar and Martian exploration,
and interplanetary space probes.
Besides the advantages of low cost, high integration
capability and high speed, SiGe chips are ideally
suited for space because of the material's natural
radiation hardness, a key concern for all space electronics,
particular interest to NASA is that silicon-germanium
circuits also perform well in space's cryogenic
temperatures - close to absolute zero, according
to Cressler. Most electronic components do not
work well in a very cool environment such as space.
At present, spacecraft, probes and planetary rovers
must be fitted with electronic "warm boxes," which
add significant bulk, weight and cost to missions.
"If you want your electronics to operate in the
shadows of craters on the lunar landscape, for example,
you're talking about an extremely frigid environment
- minus 230 degrees Celsius or 43 Kelvins above absolute
zero," Cressler noted. "Silicon-germanium electronics
can operate at temperatures approaching absolute
zero, and thus are ideally suited for such applications.
It would be a huge advantage from a space-mission
perspective to be able to simply let your electronics
operate at those cold temperatures, and thus NASA
is very interested in our SiGe research."
The first silicon-germanium transistors were demonstrated
in the late 1980s, but only in the past five years
or so has the field attracted widespread attention
from the private sector, Cressler says.
With more than 20 scientists and graduate students
involved in silicon-germanium research, Cressler's
GEDC group is the largest university team in the
world devoted to device and circuit research in SiGe.
"Anybody involved in high-speed communications circuits
cares about SiGe," he said. "This new technology
is an enabler for rethinking the way business-as-usual
is done across a wide array of electronics applications,
and that makes it really exciting to work on."