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Layer Stackup Planning for RF Circuit Boards Designers Notebook Feature Column by Vern Solberg When designing multiple layer circuits requiring impedance control, the circuit board designer will work closely with an engineer- ing specialist cognizant of RF printed circuit board design and layout, including mixed-sig- nal applications. e RF engineer will be able to furnish the circuit board designer guidance for "best practices" when planning the RF cir- cuit board and manufacturing guidelines that may apply to particular components, recom- mend qualified circuit board fabricators, and specify suitable base materials and copper foil materials for the application. Controlled impedance is the characteris- tic impedance of a transmission conductor's associated reference planes. Impedance con- trol is most relevant when high-frequency sig- nals cross over transmission lines on adjacent layers for RF circuits; a uniform controlled impedance will be imperative in achieving signal integrity and provide signals with mini- mal distortion. e RF engineer will guide the circuit board designer in establishing the thickness and location in the layering scheme for the copper-clad, glass-reinforced core materials and the thickness of the prepreg material selected for joining layers during the lami- nation process. A key concern is establishing the dielectric core mat- erial's thickness. e dielectric thickness will determine the physical separation of the conduc- tors on the signal layer from the "reference plane" layer. e following furnishes a num- ber of "best practices" from RF engineering specialists. 14 DESIGN007 MAGAZINE I APRIL 2025