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54 DESIGN007 MAGAZINE I AUGUST 2022 Several factors that will influence the imped- ance of a circuit are: • Cross-sectional area of the transmission line (height x width) • e height of the dielectric material between the transmission line and the reference plane • e dielectric constant of the dielectric material • Spacing between the transmission lines (differential pairs) e devil is in the mathematical details when it comes to controlling impedance. Document- ing impedance requirements properly is more onerous than most people realize. Simply stat- ing target impedance, trace requirements, and material tolerances oen leaves knowledge gaps for the manufacturer and increases the likelihood of producing boards that do not make the cut during testing. e math is the math when it comes to con- trolled impedance and it must be done at some point. Why not during the design phase? Using an impedance calculator will help designers build the right tolerances into their designs and avoid issues like signal mismatch. Imped- ance testing then becomes a double-check instead of the tool relied upon to determine if impedance documentation is correct. When it comes to the math of controlling impedance, even simple equations really aren't that simple. The following equation, taken from IPC-2141, is used for a single ended microstrip structures. It is a good demonstra- tion of the influences on impedance. is is the simplest mathematical representation of impedance, and it can present up to a 7% error in calculations at high speeds. W = Width of trace T = Copper thickness H = Dielectric ickness = Dielectric Constant (D k ) Lack of specificity in the design notes can create risk during the transition from design to manufacture. e components of this equation all come with a +/- tolerance during manufac- turing and raw material creation. For a PCB to have the desired result of matching or con- trolled impedance for critical structures, it is imperative that the PCB manufacturer under- stand these factors. Manufacturing doesn't happen in a vacuum and lessons learned from past experience will help reduce risk associated with controlling impedance. PCB manufacturers should be col- lecting data on every board produced, and they can leverage that in predictive models that will provide a good starting point to fill in documen- tation gaps and achieve the desired outcome. A good partner for producing boards that reliably handle high-speed signals uses sound methods for process control and data col- lection. ey fundamentally understand the influence of impedance and scrutinize signal transmission. But it is a two-way street. e designer should consider impedance factors in advance of manufacture. Matching the mate- rial characteristics (D k ), targeted layer spac- ing, and impedance needs during the design phase will make the process go more smoothly and help avoid costly rework. DESIGN007 Matt Stevenson is the VP of sales and marketing at Sunstone Circuits. To read past columns, click here. Figure 3: There are a variety of free online con- trolled-impedance calculators available to make this math easy for designers.