RADIO FREQUENCY & MICROWAVES FEATURE


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 manufacturer 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 at a much higher insertion loss. Porterfield points out a two-fold problem with this workaround. First, there is more of the lossy ferrite in the signal path, and second, the ferrite loss parameter increases at lower magnetization levels. To minimize loss, it is essential that


the ferrite length be reduced as much 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° of rotation. This lowers the insertion loss to less than 1 dB at 75-110 GHz and only 2 dB at 220-330 GHz. “The extension of isolator technology above 220 GHz is an impressive technical feat, and a key technology that enables us to deliver accurate measurements with higher sensitivity than we were previously able to achieve,” notes VDI’s Hesler.


3. LOW PORT REFLECTION A good isolator must also have low port reflections. Voltage Standing 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 reflections is often overlooked. An isolator with high port reflections creates 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.


4. HIGH POWER RATING Power in the reverse traveling signal


is absorbed in the isolator, resulting 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. However, as higher power sources become available, 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, approaching 2200 W/m•K (watts per meter-Kelvin), more than five times higher than copper. Diamond effectively channels heat from the resistive layer in the isolator to the metal waveguide block, and thus lowers operating temperatures for improved reliability.


5. SMALL FOOTPRINT Minimizing the size and weight of MMW components is especially important in today’s wireless applications. “A standard traditional-style isolator in the WR10 band is about 3 inches long, with a cylindrical section in the center that’s about 1.3 inches in diameter,” observes Porterfield. “But the newest design shapes are rectangular and can be as small as 0.75 inches per side and 0.45 inches thick.” The same technology used to reduce insertion loss – utilizing the shortest possible length of ferrite – also partially accounts for the reduction in footprint. In addition to


the five critical characteristics, other properties of modern isolators


improve their utility at MMW frequencies; for instance, wide bandwidth. Standard waveguide bands typically extend to 40 per cent on either


side of the center frequency. Newer, high-performing isolators operate over extended bandwidths exceeding 50 per cent from center frequency, giving designers greater freedom to build more bandwidth into their systems. Additional advances include isolators


that operate in cryogenic conditions, which is critical because a traditional isolator designed for room-temperature operation will perform poorly when cooled.


Micro Harmonics www.MicroHarmonics.com sales@mhc1.com


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