[SI-LIST] Re: Why 50 ohms

I was not around 'back then', but a book I have 
provides one explanation why 50 ohms was originally chosen:

        AUTHOR  Thomas S. Laverghetta
        TITLE           Practical Microwaves
        ISBN            0-13-186875-6
        DATE            1996
        PUBLISHER       Prentice-Hall

It describes how attention in coaxial cable environments
is minimized at 77 ohms, and how power handling capability
is maximized at 30 ohms.  Between these two values, 50
ohms was chosen as a compromise.

Whatever the historical reason is, the reason I personally
(try to) stick with 50.0 ohms is to be compatable with
our test equipment.  Nearly every piece of equipment in our
lab (particularly anything above 500 MHz) is designed
with 50 ohm terminations and cables.  

For sensitive equipment like network analyzers and
time domain reflectometers, the farther away from 50
ohms your device is, the less accurate the
measurement will be.  If you're interested in creating
an accurate model of your device (active or passive),
it is much easier to do if its impedance is around
50 ohms.

Also, because most of our signal generators and
bit error rate testers have 50 ohm source and
characteristic impedance, the signal integrity of
the test signals is optimized with our device
having the same 50 ohm impedance.  The farther we
stray from 50 ohms in our device, the worse the signals
from our test equipment looks.

I'm happy to say I'm not quite old enough to know
exactly why 50 ohms was originally chosen, but
I believe there is good reason to continue using
it.

Pat



> Hello SIexperts,=0D=0A         I am a student, doing 
> Engineering in Electri=
> cal.=0D=0AWhile discussing with my friend, he told me that 
> the PCB trace im=
> pedance should be maintained as 50 Ohms (for differential=3D 
> 100 and for so=
> me RDRAM buses 27ohms).=0D=0AI tried to find the answer for 
> this in some of=
>  the standard books but i could find the answer for this. 
> Books also saying=
>  that we should maintain 50 ohms. But i couldn't find the 
> reason for that.=
> =0D=0A=0D=0ACould any body please through a light on me on 
> this topic.This =
> may be a newbie question. But i thought this group is the 
> correct place to =
> get my answer.=0D=0A=0D=0A=0D=0Athanks in 
> advance,=0D=0ASiMan=0D=0A =0A
> 
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