[SI-LIST] Re: Transmission line impedance
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 20 Dec 2006 09:47:23 -0500
Sam,
There are different answers to the question of "why 50 ohms?" depending on
the applications in mind, and one's background. 50 ohms has long been used
as ONE of the standard impedances for both RF and for test equipment, and
its roots go back to the compromises made in the early days of coaxial cable
(balancing acts between cable loss and maximum power handling and such, plus
an attempt to match certain antennas).
As for PC boards, the reasons are different, except for the historical
perspective. You ask, "why can't it be something different than this?" It
can. Single-ended trace impedances in that range (50-80 ohms or so) tend to
be relatively easy to manufacture (not too thin, not too wide), and are a
good compromise when considering typical output stage drive strengths and
power dissipation, capacitive loading, plus other factors. Plus a 50 ohm
line or output is fairly easy to connect to a 50 ohm 'scope input, which is
why you sometimes see it being used as a test load on data sheets and
application notes for high speed parts.
You can find various articles written in magazine back issues and on the
internet about "why 50 ohms?" Not all of them are accurate, however.
Regards,
Andy
> 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? ...
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