Feature: MMW
2. Low insertion loss Whilst isolation is the name of these components’ game, 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 (75-110GHz) the insertion loss can exceed 3dB, meaning half the signal power is lost. In the WR-5.1 band (140-220GHz) the loss climbs to over 5dB. Because of high losses, traditional isolators are often precluded from use in MMW systems. “A designer’s main fear is that the isolator will significantly
degrade the strength of the final output,” said Porterfield. “It can be frustrating for engineers to try to tune the standing waves out of each system, usually with limited success. Many of the alternate methods used are narrow-band in nature, so that the solution may work well only over an insufficiently narrow band of frequencies.” Faraday rotation isolators operate by using ferrite discs to
rotate the signal. However, the traditional method to make them has been to use ferrites that are substantially longer than the minimum required length, and then tune the magnetic bias field to achieve optimal performance. This delivers good isolation, but the expense of much higher insertion loss. Porterfield points out a two-fold problem with this
1. High isolation Isolation is a measure of how much of the signal travelling in the reverse direction gets through the isolator. Because isolators are intended to prevent this from happening, the higher the isolation, the better. “Te issue that MMW system designers face is impedance mismatch
and the resulting reflections between components,” said David Porterfield, Founder and CEO of Micro Harmonics Corporation (MHC), which specialises 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 (50-330GHz) applications. “In MMW systems, the distance between components is oſten more
than a wavelength, putting reflected signals out of phase, effectively perturbing the operating point of the upstream component,” said Porterfield. “As you sweep frequencies, the phase changes and you get nulls, dips and degraded performance. However, insert an isolator between components, and the reflected signal gets absorbed and the problem goes away.” Te 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 10dB when the signal rotation is off by just 1°. “Te only way to confirm such precision is to fully characterise each
isolator on a vector network analyser,” said Porterfield. “Tis validates total compliance, as opposed to just spot-checking at a couple of frequencies in the band.”
Isolator sample set
workaround. First, there is more of the lossy ferrite in the signal path and, second, the ferrite loss parameter increases at lower magnetisation levels. To minimise loss, it is essential that the ferrite length is as
short as possible. The design developed for NASA saturates the ferrite with a strong magnetic bias field, which allows for the shortest possible length of ferrite to achieve the ideal 45° rotation. This lowers the insertion loss to less than 1dB at 75-110GHz and only 2dB at 220-330GHz. “The extension of isolator technology above 220GHz is an
impressive technical feat and a key technology that enables us to deliver accurate measurements with higher sensitivity than ever before,” said VDI’s Hesler.
www.electronicsworld.co.uk September/October 2020 21
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