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RE: [RFI] Ethernet RFI

To: "RFI Reflector" <rfi@contesting.com>
Subject: RE: [RFI] Ethernet RFI
From: "Jim Brown" <jim@audiosystemsgroup.com>
Reply-to: Jim Brown <jim@audiosystemsgroup.com>
Date: Sun, 23 Jan 2005 15:15:19 -0600
List-post: <mailto:rfi@contesting.com>
On Sun, 23 Jan 2005 13:18:08 -0600, Ed -K0iL wrote:

>But only one of those would place the "twisted pairs" into the 
>correct EMC signal configuration. 

Not only is correct wiring critical for EMC, it is also quite 
critical for performance as an Ethernet cable. 

Understand this basic concept. A CAT5 cable (or CATx, for that 
matter) consists of FOUR TWISTED PAIRS.  EACH TWISTED PAIR IS A 
100 OHM TRANSMISSION LINE.  (CAPS FOR EMPHASIS). Further, each of 
the pairs is assigned a different twist ratio (twists per ft, 
called the "lay") to further reduce crosstalk between the pairs. 
The standards that define Ethernet systems define which colors are 
assigned to which ratios, and to which pins on the connector. 

When you connect Ethernet equipment with this cable, you are 
connecting four 100 ohm transmission lines between that gear. Not 
all of those pairs are used by most equipment, and by all 
transmission standards. 

Now, if you connect the wires to the wrong pins (let's say the 
blue/white where the orange/white should go), several things are 
wrong. First, it's no longer a twisted pair, it's two wires, so 
the noise rejection (and noise radiation) go up. Second, the 
impedance is no longer 100 ohms, so the data transmission gets 
unreliable, causing errors, retransmissions, and slow speeds. 
Third, the crosstalk between pairs goes sky-high, causing more 
errors and data problems. 

Bottom line -- you cannot test data cables with an ohmeter, you 
MUST test them with a specialized data-transmission test set that 
measures end to end loss, crosstalk, and related parameters. These 
test sets cost big bucks. 

Jim Brown  K9YC


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