Path: utzoo!utgpu!watserv1!watmath!att!rutgers!cs.utexas.edu!samsung!munnari.oz.au!uhccux!querubin
From: querubin@uhccux.uhcc.Hawaii.Edu (Antonio Querubin)
Newsgroups: comp.dcom.lans
Subject: Re: 50-75 ohm impedance matching
Message-ID: <8601@uhccux.uhcc.Hawaii.Edu>
Date: 14 Jul 90 03:19:33 GMT
References: <1990Jul13.210239.17750@ux1.cso.uiuc.edu>
Reply-To: querubin@uhccux.UUCP (Antonio Querubin)
Organization: University of Hawaii
Lines: 31

Thanks to all those who responded to my base note.  Almost everyone pointed
out a problem which I hadn't considered - the DC collision detection process
would be affected by a normal transformer.  A normal transformer would pass
the AC signal but not the DC.  After pondering this new twist for a few minutes
I realized that an 'auto-transformer' would pass the DC as well as AC.

For those of you who don't know what an auto-transformer is, in it's simplest
form it is a single coil with the secondary as just a tap on the primary (or
vice versa):
             (----------------   secondary out
             (
-------------)
  primary    )
             )
             )
             )
-------------)----------------   common

Eg. An equivalent turns ratio of 3:4 would mean the primary is attached
3/4 up the coil while a 2:1 ratio would mean the secondary is a tap at
the half-way point of the coil.  Theoretically this *should* work :-).

But someone also suggested using the 75 ohm cable directly but with 75 ohm
terminators instead.  This makes more sense since all that's needed is a 
point-to-point link.  Connection to a 50 ohm segment would require the 
impedance matcher above (or a repeater) between the two cable types.

Further comments on the above two schemes are welcome!

Antonio Querubin, Jr.
querubin@uhccux.uhcc.hawaii.edu