December, 2020


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Wheeler. “The challenge was to lower the weight and size, without losing any performance.” Wheeler says that for any antenna system, if


the antenna aperture shrinks, gain (G) drops by a logarithmic amount, which is in contrast to the goal. But if the noise temperature (T) is lowered, then the gain that was lost can be saved. “Our solution was to cryogenically cool the low noise amplifier,” says Wheeler. “We can get down to less than 100 Kelvins


with commercially available cryo-coolers,” he says. “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.” Micro Harmonics specializes in design solu-


tions for components used in mmWave products. Under a NASA contract awarded in 2015, the com- pany successfully developed an advanced line of isolators for 50 to 330 GHz applications. That successful project led NASA to award the compa- ny a subsequent grant to address the issue of isolators at cryogenic temper- atures. “Low-noise integrated circuit


amplifiers work, because of the nature of a Schottky diode or a FET transistor, in that as it gets cooler, it has lower noise,” says Wheeler. “However, cryogenic low-noise ampli- fiers are not cheap. With ferrite isola- tors you get more bang for the buck: a better gain over noise figure at room temperatures, and even more so at cryogenic temps.” There are numerous material


issues that must be addressed to ensure that an isolator is able to withstand the rigors of thermal cycling. The substantial temperature dependence of the ferrite magnetiza- tion is also a challenge. Ferrite mag- netization follows a modified Bloch law, increasing by more than 20 per- cent when cooled from room tempera- ture down to 4 K. As the temperature decreases there is less thermal ener- gy and it is easier to align magnetic dipoles in the ferrite. The design used by Micro


Harmonics compensates for the change. It also uses magnetic arma- tures designed to achieve a focused, uniform bias field in the ferrite. This strong magnetic saturation allows the shortest possible length of ferrite — hence the small footprint — while achieving a low insertion loss of less than 1 dB at 75 to 110 GHz and only 2 dB at 220 to 330 GHz.


Proven in Research and Practice While manufacturers are now realizing the


benefits of isolators for cryo’ applications, on the research side, Anferov and his team at the University of Chicago are on a mission to see just how low they can go. “Our lab does experiments at 1 Kelvin, and


there are components that can function at temper- atures close to absolute zero,” he says. “However, at the extremely high frequencies demanded by today’s applications, it takes a specialized ferrite isolator to perform consistently under such extremes; a ferrite that won’t over-rotate the field and create unwanted issues.” It is essential for any mmWave application that


each isolator is tested over the full frequency band on a vector network analyzer to ensure compliance. This includes reliability testing (Belcore) and cryogenic cycling tests. Comprehensive VNA test data should


Page 51 Cryogenic-Capable Isolators Improve Performance


back up every component, since there are often signa- tures in the data that can be missed. “Knowing that isolators would now perform


in the mmWave bands at single-digit Kelvin tem- perature was good news for us because that was one less component we had to worry about,” says Anferov. For Wheeler’s mil-spec work, the cryo- genic isolators will help ensure the reliability of Plymouth Rock’s technology and products. “In harsh environments, the contaminates on the radome of the antenna can really add to the system noise figure, due to reflections (VSWR),” says Wheeler. “By integrating a cryogenic isolator in front of your low-noise receiver you will realize a reduction in the noise and increase the gain ratio.” Contact: Micro Harmonics Corp., 20 S


Roanoke Street, Suite 202, Fincastle, VA 24090 % 540-473-9983 E-mail: sales@mhc1.com Web: www.microharmonics.com r


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