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34 DESIGN007 MAGAZINE I MAY 2018 such as the 6 GHz and below used with 5G systems, than for millimeter-wave frequencies, such as 30 GHz and above, as will be used for short-range backhaul links in 5G wireless net- works. Selecting an optimum circuit material for each band of frequencies requires under- standing which Dk value best supports each of the two different frequency ranges. Then it is a matter of finding circuit materials that possess those Dk values along with as many as pos- sible of the other circuit material attributes that help make a good, high-performance, high-fre- quency PA. Whether for microwave or millimeter-wave frequencies, circuit materials for high-fre- quency PAs must be capable of supporting circuitry that achieves the impedance match to the power transistors in those PAs. Such impedance matching is also necessary for the active devices in lower-power amplifiers, such as driver amplifiers and even in low-noise amplifiers (LNAs). Suitable circuit materials for such imped- ance-matching networks must be capable of keeping circuit impedance variations to a mini- mum, and this is typically done through tight control of the substrate thickness, with no vari- ations in thickness; tight control of conductor widths, such as microstrip transmission lines, to maintain the same impedance; tight control of the copper thickness on circuit laminates; and tight control of the circuit material's Dk, especially with temperature. Although using a circuit material with tight control of Dk, such as 3.50 ± 0.05, can help maintain the impedance of high-frequency transmission lines within a narrow window as might be needed for imped- ance matching within a PA circuitry, variations in the substrate thickness can have even more impact on maintaining consistent impedance of high-frequency transmission lines. A circuit material with a Dk tolerance of ±0.05 or lower is considered to have a tightly controlled Dk value. With increasing frequencies, signal wave- lengths are decreasing, requiring ever-smaller circuit features. Many of the PA circuit configu- rations used at both microwave and millimeter- wave frequencies, such as Doherty amplifiers, are dependent upon quarter-wavelength trans- mission-line circuit structures and the dimen- sions of these structures are a function of the substrate thickness. If that circuit substrate thickness is not tightly controlled, it is easy to understand how the impedance of extremely fine transmission-line and circuit structures can vary with those variations in substrate thickness. In general, a substrate thickness variation of ±10% or better is a sign of tightly controlled circuit material thickness. Feeling the Heat Whether at microwave or at millimeter-wave frequencies, PA circuits are subject to perfor- mance variations brought about by changes in temperature, from both the operating environ- ment and from the PA's own active devices, such as power transistors or ICs. In the search for circuit materials for both microwave and millimeter-wave PAs for 5G applications, find- ing circuit materials capable of effective ther- mal management is critical to minimizing a PA's performance variations as a result of the thermal rise brought about by its own active devices. Two circuit material parameters are of particular interest when assessing a mate- rial's thermal behavior: thermal conductivity and thermal coefficient of dielectric constant (TCDk). High thermal conductivity allows for effi- cient flow of heat away from any heat-generat- ing active devices mounted on a PCB, such as a PA's power transistors. Consistent heat flow Selecting an optimum circuit material for each band of frequencies requires under- standing which Dk value best supports each of the two different frequency ranges.

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