# Why Characteristic Impedance must be 50 ohms?

Where does this number come from?

For single ended it must be 50 ohm and for differential pairs 100 ohm. Why?

For PCB with controlled-impedance these are that common numbers. Outside of the PCB you can find others numbers for characteristic impedance. But what is the reason of use these numbers for the PCB tracks?

• While I can guess where this question is coming from, you might want to expand upon your presupposition, providing basis and context. 50 Ohms for which type of situation? You'll find if you search a little, that characteristic impedance or nominal impedance values of 600, 450, 300, and 75 ohms, or 600, 75 or 50 are very common depending on the transmission line context. This question, as it stands, is not representative of actual facts. May 10, 2013 at 8:32
• See also this article, which covers the math for 50 Ohm and 75 Ohm characteristic impedance, and the justification for standardizing on these. Basically, 50 Ohms is a compromise between optimal power handling and lowest loss for air-dielectric coaxial lines. May 10, 2013 at 8:37
• @Sorry. I talked about PCB Tracks. I'll take a look that article. May 10, 2013 at 8:42
• Yup, hence I gave the question an upvote after your clarifying edit. May 10, 2013 at 8:53
• You can design PCB tracks to be any impedance you want within reason May 10, 2013 at 10:35