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MAY 2018 I DESIGN007 MAGAZINE 35 not only removes the heat as a threat to the reliability of the transistors, but helps minimize thermally inducted PA performance variations. Thermal conductivity of 0.5 W/mK or higher is considered good for a PCB material. TCDk is a circuit material parameter that indicates how that material's Dk is affected by variations in temperature. Ideally, a mate- rial would have a TCDk of 0 ppm/ºC for no change in Dk with temperature. But practical circuit materials exhibit some changes in Dk with temperature and a TCDk of 50 ppm/ºC is considered good for a circuit material, resulting in only small changes in Dk with temperature. For the amplifiers and other circuits in 5G sys- tems that will rely on fine quarter-wavelength circuit structures, circuit materials with low TCDk values will help minimize performance variations. The smaller wavelengths and circuit features needed for millimeter-wave PAs and circuits in general will require thinner substrate materials compared to lower-frequency microwave PAs and circuits, and maintaining a tight tolerance in that thickness is just as critical as for thicker materials. Those thinner circuit materials can also be more sensitive than thicker circuit materials to the effects of other circuit material characteristics, such as copper surface rough- ness. Copper surface roughness can result in such circuit effects as transmission-line loss and phase variations, so copper surface roughness should be minimized in any circuit materials specified for the smaller-wavelength, higher- frequency circuits in both 5G microwave and millimeter-wave PAs. As an example, Rogers offers a variety of materials with different thicknesses and other characteristics needed for the two different fre- quency ranges. For 5G PAs at 6 GHz and below, 20- and 30-mil-thick ceramic-based RO4385 circuit laminates are low-cost circuit materials that maintain consistent performance across wide temperature ranges. They have a Dk of 3.48 in the z-axis at 10 GHz, tightly controlled within ±0.05. They are ideal for competitive applications and can be fabricated with stan- dard epoxy/glass (FR-4) processes. For 5G PAs at millimeter-wave frequencies, 5- and 10-mil-thick RO3003 laminates consist of PTFE with ceramic filler. They feature a Dk of 3.0 in the z-axis at 10 GHz tightly controlled within ±0.04. They feature extremely low loss at higher frequencies that helps get the most gain from the active devices in an amplifier cir- cuit, even at the various millimeter-wave bands expected to serve the many backhaul links of future 5G wireless networks. DESIGN007 This article originally appeared in February 2018 as a blog in Microwave Journal. John Coonrod is technical marketing manager for Rogers Corporation. For years, manufacturers have offered computers with increasing amounts of memory packed into smaller devices. But semiconductor companies can't reduce the size of memory components as quickly as they used to, and current designs are not energy-efficient. One promising version of magnetic device relies on the magnetoelectric effect. Existing devices, however, tend to require large magnetic and electric fields that are dif - ficult to produce and contain. One potential solution for this problem is a new switch- ing element made from chromia (Cr2O3), which, one day, may be used in computer memory and flash drives. "The device has better potential for scaling, so it could be made smaller, and would use less energy once it's suitably refined," said Randall Victora, a researcher at the Univer - sity of Minnesota and an author on the paper. Next, Victora and Ahmed aim to collaborate with col- leagues who work with chromia to build and test the device. If successfully fabricated, then the new device could potentially replace dynamic RAM in computers. Design for Magnetoelectric Device May Improve Your Memory