[time-nuts] 50 vs 75 ohm cables

Magnus Danielson cfmd at bredband.net
Thu May 10 08:53:27 EDT 2007


From: Peter Vince <pvince at theiet.org>
Subject: [time-nuts] 50 vs 75 ohm cables
Date: Thu, 10 May 2007 13:39:27 +0100
Message-ID: <v04210100c268c17d2911@[10.0.0.3]>

> I came across some telecom equipment the other day which had 
> reference outputs marked as 75 ohms.  I work in television, not 
> telecoms, and we use 75 ohm connections for video, but with most RF 
> stuff being (I believe) 50 ohms, and certainly all the HP and other 
> counters seem to have 50 ohm inputs, I rather assumed telecoms used 
> 50 ohms - obviously not!

No, it is 75 Ohm for unbalanced PDH signals such as E1, E2, E3 and E4 as well
as the american DS1, DS2, DS3 and DS4.

>  Can anyone tell me how and why the 50/75 
> ohm distinction came about?  Was it perhaps a VHS/Betamax type issue 
> with different manufacturers going their own ways, and then with so 
> much equipment out in the field,  neither side was willing to change?

There where something about damping etc.

Cheers,
Magnus




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