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46 DESIGN007 MAGAZINE I JUNE 2020 dependency for the effects that roughened cop- per has on high-frequency performance. If the circuit application is operating at a low frequency, the copper surface may not have any influence on the RF performance. Howev- er, when frequency increases to the point of the skin depth having the same dimension as the copper surface roughness, the surface rough- ness can certainly impact insertion loss and phase response. As the frequency continues to increase, which causes a thinner skin depth, the roughened surface of the copper will have an increased influence on RF performance. Continuing with more detail on the copper foil surface roughness, the substrate thickness can exaggerate the RF effects of copper surface roughness. Here's a simple analogy: When the copper planes are close together, which is the case for a circuit using a thin substrate, the cop- per surface roughness effects will be more sig- nificant. As opposed to using a thick substrate and the copper planes being farther apart, the copper surface roughness will have less impact on the RF performance of the circuit. Another example of a high-frequency cir- cuit material app note is related to space ap- plications. The circuit material testing require- ments for space are well-defined through agen- cies like NASA and other similar organizations. Rogers has an app note which gives a summa- ry of specific high-frequency circuit materials tested per NASA requirements. There are typically many different circuit ap- plications for one space system, and each cir- cuit application may have different material property needs; however, the overriding speci- fication may be the NASA data, for example. Given the list of high-frequency circuit mate- rials that will meet the NASA requirements, some materials may be good for power ampli- fier applications, and other materials may be good for antenna radiating elements. Typically, the designer will need to perform a trade-off analysis with the list of materials that are ac- ceptable to the NASA requirements and con- sider the different properties of the materials for their space application. Another app note that Rogers offers is relat- ed to laminates which have resistive foil em- bedded in the laminate. The resistive foil lami- nates can be used for a wide range of PCB ap- plications, and the app note cannot cover all aspects of these applications. The app note can, however, cover the criti- cal topics associated with these types of lami- nates, and the topics may or may not be ap- plicable to a specific application. For this app note, the main issues related to resistive foil laminates are discussed. In general, these is- sues are a normal variation of the visual ap- pearance of the resistive layer and the resis- tor tolerance which can be achieved by using these laminates to make planar resistors. While app notes are a very good tool for the designer to use, the app notes which a high- frequency circuit material supplier will issue are typically not like an app note that a com- ponent supplier would issue. The app note for a material supplier will usu- ally address the different nuances of the mate- rial, which may or may not be critical to a par- ticular PCB application. It is the designer who will need to understand how the app note ap- plies to their design, but it is always suggest- ed that the designer consults with the material supplier if there is any confusion on the app notes or related datasheets. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coonrod, click here. When the copper planes are close together, which is the case for a circuit using a thin substrate, the copper surface roughness effects will be more significant.

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