Path: utzoo!attcan!uunet!dino!ux1.cso.uiuc.edu!thucydides.cs.uiuc.edu!morrison
From: morrison@thucydides.cs.uiuc.edu (Vance Morrison)
Newsgroups: comp.dcom.lans
Subject: Re: 50-75 ohm impedance matching
Message-ID: <1990Jul13.210239.17750@ux1.cso.uiuc.edu>
Date: 13 Jul 90 21:02:39 GMT
Sender: usenet@ux1.cso.uiuc.edu (News)
Reply-To: morrison@thucydides.cs.uiuc.edu.UUCP (Vance Morrison)
Organization: University of Illinois, Urbana-Champaign
Lines: 50

Hello,

If you are interested, National Semiconductor puts out a LAN chip
book that describes the 8350 (I think) ethernet controler.  In
this book it describes the changes that need to be made for
use on a 75ohm cable.

Note that for a point to point link you should be able to get
away with simply connecting up the (Thin) cards with a T connector
and placing a 75ohm resistor at both ends.

There will be no reflections, because a ethernet card looks like
an open (after all, if it didn't when you connect a card to the
middle of a net, there would be reflections at the card). 

Practically speaking I have seen ethernet set up normally (except
for the 75ohm cable), and seen it work.  This undoubtedly has some
effect on length, and node count restrictions, but with a point
to point setup like yours, you have a lot of slop to spare.

Vance

---------------------------------------------------------
P. S. 

The transformer idea in its simple form may not work.  The problem
is that Ethernet uses a DC signal to detect collisions.  (When
a card is transmitting, it transmits a AC signal imposed on a DC
bias if two cards try to transmit, then twice the DC voltage will
be seen, and a colision can be detected.  

Now in reality you still might get away with it, since you could design
the transformer to pass signals effectively down to say 1000Hhz.
Since collisions last quite a bit less than 1ms, the transformer will
pass the DC signal 'long enough' for the colision to be detected.
It depends on exactly how the tranceiver detects colisions.

To answer you direct question, the number of turns of a transformer
is not a critical design factor.  It is only lower bounded by your 
desire to have a lot of coupling per winding resistance at frequencies 
above 1000Hhz.  At such high frequecies, even 100 turns should be 
sufficent (on a good, high frequency toroid core).   

All of the above is simply for academic interest.  I really think
you can get away with treating the 75ohm cable as if it was a 50 
ohm cable (execpt for the different resistance values in the terminating 
resistors).  


Vance