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This question already has an answer here:

Why is the trace impedance of a single-ended trace on a PCB fixed to 50 ohms or 75 ohms (for video signals)? Why can't it be fixed to any other impedance values?

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marked as duplicate by embedded.kyle, Dave Tweed, Daniel Grillo, Chetan Bhargava, PeterJ Feb 5 '14 at 20:35

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    \$\begingroup\$ If you look at most of the related questions, you'll get your answer. \$\endgroup\$ – scld Feb 5 '14 at 13:06
  • \$\begingroup\$ I read few of them. But I still couldn't figure out why exactly 50 ohms or 75 ohms value is being used. \$\endgroup\$ – Avin Feb 5 '14 at 13:09
  • \$\begingroup\$ @embedded.kyle Thanks for pointing out that link also. It didn't show up while I was searching earlier. \$\endgroup\$ – Avin Feb 5 '14 at 14:20
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Short answer: In transmission line theory, the lowest loss is at ~75 Ohms. The highest power handling is at ~30 Ohms. 50 Ohms is just a tradeoff between the two. You can make the impedance whatever you want, as long as they match (but sticking to 50/75 is most common).

Higher impedances are used many times in audio applications or measurement equipment, for example. The 50/75 Ohm systems are usually for signal transmission of some sort (A/V, radar,communications, etc.)

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