It is a common misconception that isolators designed to work at room temperature will work reasonably well at cryogenic temperatures. The problem is that the ferrite materials have a strong temperature dependence that impacts the signal rotation. This can cause significant over-rotation of the signal and severely degrade performance at cryogenic temperatures.

“We tried using regular isolators from one vendor. We cooled them down and assumed they would work, but they weren’t behaving right.”

Alexander Anferov, GRA
Shuster Lab, University of Chicago

“We can get down to less than 100 Kelvins with commercially available cryo-coolers… Our biggest challenge was finding an isolator that could perform at those temps. Fortunately for us, a company called Micro Harmonics had just designed some specifically for NASA.”

Dana Wheeler, CEO
Plymouth Rock Technologies

At Micro Harmonics we are developing a line of isolators designed for optimal performance at cryogenic temperatures. The ferrite is biased in magnetic saturation for minimal insertion loss and the length of the ferrite rod is optimized to achieve the desired rotation at cryogenic temperatures.

Sophisticated models are constructed to simulate the thermal stress levels throughout the isolator as it is cooled. Materials are chosen that reduce thermal stress. Reliability is verified through repeated thermal cycling in a liquid nitrogen bath. Our isolators are built to withstand the rigors of repeated cryogenic cycling.

Our cryogenic isolators are routinely tested at 77 K and 25 K in our cryostat. We use a resistive thin film for isolation that is not in the class of super conductors. The performance of our cryogenic isolators has been verified down to 1 K.

Cryogenic models are now available in bands from WR-15 through WR 5.1. A model at WR-28 (FR280C) has also been released. Models at WR-4.3 and WR 3.4 will be added pending customer demand.

Model Flange
Band
(GHz)
Insertion Loss
(dB, typ @ 25 K)
Isolation
(dB, typ @ 25 K)
FR280C WR-28 26 – 40 0.5 25
FR148C WR-15 50 – 75 0.5 28
FR122C WR-12 60 – 90 0.7 25
FR100C WR-10 75 – 110 0.8 30
FR90C WR-9 82 – 122 0.5 30
FR80C WR-8 90 – 140 0.7 27
FR65C WR-6.5 110 – 170 0.9 23
FR51C WR-5.1 140 – 220 1.2 23
FR43C WR-4.3 170 – 260 Pending
FR34C WR-3.4 220 – 330 Pending

Cryogenic Faraday Rotation Isolators



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