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46 The PCB Design Magazine • April 2017 There are other concerns with glass weave ef- fect. To keep the explanation simple, it will be assumed that the laminate has one glass weave layer and that the circuit is a simple microstrip transmission line. Glass weave effect is often a concern where PCBs are used in large volume and potentially causing a circuit-to-circuit per- formance difference. A basic way to conceptual- ize the issue is to assume that most circuits will have a critical conductor randomly aligned to the glass weave pattern and the wave associated with that conductor is experiencing an averag- ing effect of the glass bundles and open areas. Then, in perhaps one circuit out of 100, the criti- cal conductor is aligned perfectly, directly on top of an area of glass bundles and the wave will ex- perience a higher Dk than the previously men- tioned circuits. The higher Dk brings multiple effects: The impedance can be lower , the phase angle shifted, and the wave velocity slowed, all of which can impact the circuit performance. A different concern for glass weave effect is the issue of circuits which have coupled features or use differential pair technology. When a pair of conductors in a circuit design has a well-de- fined relationship, each conductor must have the same wave propagation medium. If one conductor of the pair has a different medium, the coupled pair will not perform as expected by the designer. In RF applications, coupled con- ductors are used in filter and directional coupler designs. In high-speed digital application, cou- pled conductors are used in differential pair de- signs. For RF applications, if one conductor ex- periences a different Dk than its pair, the phase angle between the pairs will not be as designed and a shift in performance can occur. The high- speed digital application often has timing issues where the signals from each conductor of the pair will need to arrive at a point in the circuit at the same time. When the signals arrive at dif- ferent times, that is knows as skew, which may be due to glass weave effect slowing the wave velocity of one conductor more than the other. Skew can be very problematic with very high- speed digital circuitry. Many laminates formulated for high-fre- quency applications avoid issues with glass weave effect by utilizing a filled resin system. In that case, the open areas between the glass bundles do not have an abrupt difference in Dk from the glass bundles, as is the case with an unfilled resin system. The filler is typically a dif- ferent Dk value than the resin system, which is also different than the glass bundles. The added filler helps to average the Dk differences in the isolated areas, and there is less of a discrete Dk difference between the glass bundles and open areas. Finally, some high-frequency laminates have no woven glass fabric; these materials are often used at millimeter-wave frequencies. Don't forget that it is always a good idea to contact your materials supplier if you have questions about glass weave effect. PCBDESIGN John Coonrod is technical marketing manager for Rogers Corporation. To contact him or read past columns, click here. WOVEN GLASS WEAVE EFFECT: ELECTRICAL CONCERNS AND REMEDIES Researchers have shown how to modify com- mercially available silicon wafers into a structure that efficiently absorbs solar energy and with- stands the high temperatures needed for "concentrated solar power" plants that might run up to 24 hours a day. The research advances global efforts to design hybrid systems that combine solar photovoltaic cells, which convert visible and ultraviolet light into electricity. "The key point is that to capture sunlight as efficiently as possible you have to do two things that compete with each other: one is to absorb as much power from the sun as possible, but secondly, not reradiate that power," said Peter Bermel, an assistant professor in Purdue University's School of Electrical and Computer Engineering. Low-Cost 'Solar Absorber' Promising for Future Power Plants

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