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18 The PCB Design Magazine • May 2015 In this column, we will once again be focus- ing on controlled impedance structures, both from the layout side and the simulation side. I will break them down into the sub-categories of the models they represent and the important points to remember when using the various models. I will also be asking questions such as, "Why would a fabricator ask for a larger imped- ance tolerance?" and "Where does the fabrica- tor draw the line for controlling various struc- tures?" Later, I will break down my Top 10 do's and don'ts of signal routing. A Few Rules of Thumb Let's start with single-ended structures, both co-planar and those in "free space," i.e., not coupled to any adjacent copper pour. For external single-ended structures start- ing with quarter or half-ounce copper, the trace width is typically about twice the dielectric needed between the impedance signal and its reference plane. Example: A 4.25 mil trace needs about a .0026"–.0028" dielectric to be a reference plane for 50 ohms on half-ounce starting copper (1.5 ounces after plating). by Mark Thompson, C.I.D. PrOTOTrOn CIrCuITs THE BARE (BOARD) TRuTH The Do's and Don'ts of Signal Routing for Controlled Impedance feature coulmn

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