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Advances in MMW Isolator Design Launch Manufacturers into Stratospheric Operating Frequencies


By Dr. Dave Rizzo


demand driven by 5G, 6G and be- yond, ultra-high definition video, au- tonomous driving cars, security ap- plications and the Internet of Things (IoT), the sky’s the limit for utilizing the higher ends of the electromagnet- ic spectrum. Meeting this demand requires


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products capable of capitalizing on the millimeter wave (MMW) bands that cover frequencies between 30 and 500 GHz. However, these higher frequencies present a significant problem that design engineers must address — that of standing waves.


t doesn’t take a crystal ball to know where the future of wireless is heading. With inexhaustible


Without control, these unwanted waves can attenuate power output, distort the digital information on the carrier and, in extreme cases, dam- age internal components. To counteract the problem of


standing waves at lower microwave frequencies, engineers rely on Faraday rotation isolators —


more commonly referred to simply as isolators. At their very basic level, an isolator is a two-port, input-and-out- put component that allows EM sig- nals to pass on in a direction, but ab- sorbs them in the opposite direction. Traditional isolators fall short at the higher frequencies required for next- gen wireless applications.


Challenges of Isolation A big part of the problem is that


the first isolators were designed more than a half century ago, with very few modifications, since the original concept. With recent ad- vancements, however, companies at the cutting edge of MMW technolo- gies are gaining the ability to launch products that operate optimally at stratospheric frequencies. “The new series of waveguide


isolators have been a key enabling technology for VDI, a large advance from what was previously available,” says Jeffrey Hesler, Ph.D., CTO of Virginia Diodes, Inc. VDI is a Virginia-based manu-


facturer of state-of-the-art test and measurement equipment, such as vector network analyzer, spectrum analyzer and signal generator exten- sion modules, for MMW and THz ap- plications. “The issue that MMW system de-


signers face is impedance mismatches and the resulting reflections between components,” says David Porterfield, founder and CEO of Micro Harmonics Corporation (MHC). Head quartered in Virginia, MHC specializes in design solutions for components used in MMW products. Under a two-phased NASA con-


tract awarded in 2015, the company successfully developed an advanced line of isolators for WR-15 through WR-3.4 (50 to 330 GHz) applications. “In MMW systems, the distance be- tween components is often more than a wavelength, putting reflected sig- nals out of phase,” says Porterfield. “The out-of-phase reflected sig-


nal can perturb the operating point of the upstream component. As you sweep frequencies,


the phase wire stripper MIRA 340 The programmable Mira 340 is designed for


universal use while maintaining maximum precision. It is perfect for processing wires and insulation material with demanding requirements. The Mira 340 is fitted with a rotary head with 4X-blades and offers unique functions designed to reduce production time and increase quality. Thanks to sequential processing, multi-conductor cables and multi-layer insulation material can be processed quickly, thereby saving time and increasing productivity.


changes and you get nulls, dips and degraded performance. However, when you insert an isolator between components, the reflected signal gets absorbed and the problem goes away. The highest possible isolation oc-


curs when the reverse wave is rotated exactly 45° into the plane of the isola- tor’s resistive layer. Isolation can de- grade by as much as 10 dB when the signal rotation is off by just 1°. While isolation is the namesake


of these components, the suppression of the reverse wave cannot come at the expense of attenuating the for- ward, input signal. Insertion loss is a measure of how much loss a signal incurs as it passes through the isola- tor in the forward direction. For traditional-style isolators,


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insertion loss is low in the microwave bands, but at MMW frequencies the loss becomes increasingly problemat- ic. For instance, in the WR-10 band (75 to 110 GHz) the insertion loss can exceed 3 dB, meaning half of the sig-


nal power is lost. In the WR-5.1 band (140 to 220


GHz) the loss climbs to more than 5 dB. Because of high losses, tradition- al isolators are often precluded for use in MMW systems. Faraday rotation isolators oper-


ate by using ferrite discs to rotate the signal. However, the traditional method to manufacture them has been to use ferrites that are substan- tially longer than the minimum re- quired length and then tune the mag- netic bias field to achieve optimal per- formance. This delivers good isolation, but at a much higher insertion loss.


Low Port Reflection A good isolator must also have


low port reflections. Voltage Stand- ing Wave Ratio (VSWR) is a measure of the reflections at the input and output ports. A good range at MMW frequencies is 1.5:1 or less, 1:1 equals no reflection. The importance of low port re-


flections is often overlooked. An iso- lator with high port reflections cre- ates an alternate set of standing waves. The adjacent components are still adversely impacted by out-of- phase signals reflected back into their ports. High isolation and low insertion loss are of little value if the port reflections are large. Power in the reverse traveling


signal is absorbed in the isolator, re- sulting in heat. The more heat it can handle, the higher the power rating. Historically, high heat was not an issue, as there was very little power available at MMW frequencies. As higher power sources become avail- able, the importance of power ratings increases. To handle the problem of high


heat loads, some newer isolators are already incorporating diamond heat sinks into their design. Diamond is the ultimate thermal conductor, ap- proaching 2,200 W/m-K, more than five times higher than copper. Dia- mond effectively channels heat from the resistive layer in the isolator to the metal waveguide block, thus low- ering operating temperatures for im- proved reliability. Finally, minimizing the size and


weight of MMW components is espe- cially important in today’s wireless applications. “A standard tradition- al-style isolator in the WR10 band is about 3 in. (7.6 cm) in diameter,” says Porterfield. “But, the newest de- sign shapes are rectangular and can be as small as 0.75 in. (2 cm) per side


and 0.45 in. (1.1 cm) thick.” Contact: Micro Harmonics, 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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