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www.us-tech.com
December, 2020
Cryogenic-Capable Isolators Improve Performance of mmWave Systems
By Dr. Dave Rizzo
cations. While recent improvements in isolator designs are solving many of these problems, one critical chal- lenge remains — finding isolators that operate optimal- ly under cryogenic conditions. For manufacturers of ultra-high-frequency wire-
S
less applications, such as 5G and 6G communications, stand-off security scanning and military defense prod- ucts, the issue of mmWave and cryogenics is relatively new. In fact, some system designers may still be unaware that an isolator built to operate at room tem- peratures will fail to operate optimally when tempera- tures are reduced to cryogenic levels. “That happened to us,” says Alexander Anferov, a
graduate research assistant in the Schuster Lab at the University of Chicago. “We tried using regular isolators from one vendor. We cooled them down and assumed they would work, but they weren’t behaving right.” Anferov, a recent Caltech graduate, looked to
NASA and its Jet Propulsion Laboratory just outside Los Angeles for a solution. “It turned out they had just commissioned a grant for a company to design isolators specifically for cryogenics,” he says. “After talking with them it became obvious from shared experiences that we were actually causing the problem in our setup by utilizing isolators that could not stand up to extremely cold conditions.” Due to the fact that there is no industry standard,
mmWave manufacturers often, though unintentionally, make components out of metals that when cooled to cryogenic levels start to superconduct.
Setup to characterize the quantum properties of 100 GHz.
ilence is golden when it comes to filtering out unwanted reflected noise, especially in extremely high frequency, millimeter wave (mmWave) appli-
“That completely changes the device properties for
the worse,” says Anferov. “The real issue is that the results are unpredictable. Surprise resonances and new leakage paths can crop up, and power that used to be absorbed can be reflected instead.” A Universal Challenge
Antenna designers are very familiar with the con-
stant battle of standing waves. Without control, these unwanted waves reflect back into the transmitter to attenuate power output while raising unwanted noise input. Especially in the mmWave bands, which cover the frequencies between 30 to 500 GHz, the reduction of transmitted signal strength jeopardizes the battle — almost literally in military applications. To reduce the voltage standing wave ratio (VSWR)
and help increase the signal-to-noise (S/N) ratio, microwave engineers typically rely on isolators (Faraday rotation isolators). These discrete components allow electromagnetic signals to pass in one direction but absorb them in the opposite direction, reducing noise.
However, Dana Wheeler, CEO of Massachusetts-
based Plymouth Rock Technologies, explains how stan- dard isolators often become problematic with next-gen electronics that require components that must with- stand more extreme environments. “We received an SBIR grant from the U.S. Navy to
decrease the size of the large satcom antenna systems on aircraft carriers in order to put them higher up onto the ship’s superstructures because the jet-blast from the new fighter planes was damaging the radomes,” says
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