FEATURE RADIO FREQUENCY & MICROWAVES
ADVANCES IN MMW ISOLATOR DESIGN LAUNCH MANUFACTURERS INTO STRATOSPHERIC OPERATING FREQUENCIES
the resulting reflections between components,” states David Porterfield, Founder and CEO of Micro Harmonics Corporation (MHC).
Improvements in the five critical characteristics of isolators benefit electronics manufacturers in the new path towards next- gen wireless
I
t doesn’t take a crystal ball to know where the future of wireless is heading. With inexhaustible demand driven by 5G, 6G and beyond, ultra-high definition video, autonomous driving cars, security applications and IoT, the sky’s the limit for utilizing the higher ends of the electromagnetic (EM) spectrum. Meeting this demand requires products capable of capitalizing on the millimeter wave (MMW) bands which presently cover the frequencies between 30 GHz to 500 GHz. However, these higher frequencies present a significant problem that design engineers must address: that of standing waves. Without control, these unwanted waves can attenuate power output, distort the digital information on the carrier and, in extreme cases, damage 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 output, component that allows EM signals to pass in one direction but absorbs them in the opposite direction. However, traditional isolators fall short at the higher frequencies required for next- gen wireless applications. 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 advancements, however,
30 JUNE 2020 | ELECTRONICS
companies at the cutting edge of MMW technologies 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, and a large advance from what was previously available,” says Jeffrey Hesler, PhD, CTO of Virginia Diodes, Inc. VDI is a Virginia-based manufacturer of
state-of-the-art test and measurement equipment - such as vector network analyzer, spectrum analyzer and signal generator extension modules - for MMW and THz applications. “The compact size, extremely low insertion loss, and the wide bandwidth have allowed us to use isolators in a wider variety of our systems than was previously possible, and have led to significant improvements in key system performance metrics such as source power and sensitivity,” says Hesler. By understanding these advancements in each of the five properties of isolator functionality, designers can better harness isolators to improve their MMW products.
1. HIGH ISOLATION
Isolation is a measure of how much of the signal traveling in the reverse direction passes back through the isolator. Because isolators are intended to prevent, or minimize this from happening, the higher the isolation, the better. “The issue that MMW system designers face is impedance mismatches and
Headquartered in Virginia, MHC (www.
MicroHarmonics.com) specializes in design solutions for components used in MMW products. Under a two-phased NASA contract awarded in 2015, the company successfully developed an advanced line of isolators for WR-15 through WR-3.4 (50 GHz to 330 GHz) applications. “In MMW systems, the distance between components is often more than a wavelength, putting reflected signals out of phase,” continues Porterfield. “The out-of-phase reflected signal can perturb the operating point of the upstream component. As you sweep frequencies, the phase 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 occurs when the reverse wave is rotated exactly 45° into the plane of the isolator’s resistive layer. Isolation can degrade by as much as 10 dB when the signal rotation is off by just 1°. “The only way to confirm such precision
is to fully characterize each isolator on a vector network analyzer,” says Porterfield. “This validates total compliance, as opposed to just spot-checking at a couple of frequencies in the band.”
2. LOW INSERTION LOSS
While isolation is the namesake of these components, the suppression of the reverse wave can’t come at the expense of attenuating the forward, input signal. Insertion loss is a measure of how much loss a signal incurs as it passes through the isolator in the forward direction. For traditional style isolators, insertion loss is low in the microwave bands, but at MMW frequencies the loss becomes increasingly problematic. For instance, in the WR-10 band (75-110 GHz) the insertion loss can exceed 3 dB, meaning half of the signal power is lost. In the WR-5.1 band (140 -220 GHz) the loss climbs to more than 5 dB. Because of high losses, traditional isolators are often precluded for use in MMW systems. “A designer’s main fear is that the isolator will significantly degrade the strength of the final output,” continues Porterfield. “It can be frustrating for engineers to try and tune the standing waves out of each system, usually with limited success. Many of the alternate methods used are narrow band in
/ ELECTRONICS
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