{"id":452,"date":"2011-04-14T13:44:15","date_gmt":"2011-04-14T11:44:15","guid":{"rendered":"http:\/\/www.anyma.ch\/blogs\/research\/?p=452"},"modified":"2011-04-14T13:44:15","modified_gmt":"2011-04-14T11:44:15","slug":"designing-75ohm-traces","status":"publish","type":"post","link":"https:\/\/www.anyma.ch\/blogs\/research\/2011\/04\/14\/designing-75ohm-traces\/","title":{"rendered":"Designing 75Ohm traces"},"content":{"rendered":"

Found this interesting application note “Microstrip and Stripline Design” http:\/\/www.analog.com\/static\/imported-files\/tutorials\/MT-094.pdf<\/a><\/p>\n

with a formula to calculate PCB traces to have a 75 Ohm impedance:<\/p>\n

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For a given PCB laminate and copper weight, note that all parameters will be predetermined except for W, the width of the signal trace. Eq. 3 can then be used to design a PCB trace to match the impedance required by the circuit. For the signal trace of width W and thickness T, separated by distance H from a ground (or power) plane by a PCB dielectric with dielectric constant \u03b5r, the characteristic impedance is:<\/p>\n

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Note that in these expressions, measurements are in common dimensions (mils).
\nThese transmission lines will have not only a characteristic impedance, but also capacitance. This can be calculated in terms of pF\/in as shown in Eq. 4.<\/p>\n

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As an example including these calculations, a 2-layer board might use 20-mil wide (W), 1 ounce (T=1.4) copper traces separated by 10-mil (H) FR-4 (\u03b5r = 4.0) dielectric material. The resulting impedance for this microstrip would be about 50 \u03a9. For other standard impedances, for example the 75-\u03a9 video standard, adjust “W” to about 8.3 mils.<\/p><\/blockquote>\n

 <\/p>\n

I’m using PCB prototyping material from Bungard <\/a>with 1.5mm thickness and 35\u00b5m copper, FR4, so that translates to<\/p>\n