50ohm and 100ohm impedence

[adroit_designer]

Hello all

Can anybody tell me why we use 50ohm impedence for single ended signals.
Why we use 100ohm/90ohm impedence for differential pairs.
how two types of impedence can work for a single board.
Can any body give me the answers in detail.

Thanks in advance.

 

[FvM]

The 50 ohms question has been probably answered a dozen of times at edaboard. You may want to search the forum for previous contributions, it's a true series topic.

There are several reasons of course, a simple one is sufficient to my opinion: These impedances can be best achieved in coaxial respectively differential pair cables. And yes, they can be easily implemented on a PC board, 100 ohms differential is most easy, 50 ohms single ended needs a somewhat unconvenient trace width on a standard two layer FR4 board, but still feasible.