why impedance tx line always 50 & 60 ohms

[Rame]

60 ohms

Hi Friends,


Whenever we refer to SI books or application notes, the impedance of the
transmission line is mentioned as 50-ohms or 60-ohms. What is the speciffic
reason behind this value? why can't it be something different than this? OR Why
this 50-ohms conceptualized from the driver's perspective? (evolution of
transmission line theory for Digital Design)
Please flood me some info on this.



Note:This was an interesting query made in some forum...

Regards,
Ramesh

 

[haerifar]

i think this value came from Coaxial Cable, we mostly use COAXIAL cable for transmission and as you know the specific coax cables is 50 ohm ampedance,
maybe ...

 

[melc]

Which forum, the pcblibraries ?
50 ohm is not the single impedance line used on PCB. Differential lines have 100 ohms or 200 ohms. Some video amplifiers are requesting 75 ohms.
So it's not a general rule, but it seems the most common impedance line is 50 ohm because it offers good propagation performances.
BTW, a 50 ohm computed impedance line could easily fly between 40 and 60 ohms in the real world.

greetings

 

[abhi002]

hope this helps!!