RE: [SI-LIST] : LAN signal measurement with a loop back.

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From: Chris Hansen (chris.hansen@storlogic.com)
Date: Tue Nov 14 2000 - 05:04:12 PST


John,
    Depending on what type of DSO you are using, there are a couple of
options. If your DSO can do subtraction of 2 channels, then you can utilize
2 probes. Tie the grounds of each probe to a common point, keeping the
length of the ground lead as short as possible. Measure the + side of the
differential pair with one probe, and the - side of the differential pair
with the other probe. The result of the subtraction is the differential
measurement that you seek. The other option, the better option, is to get a
differential probe for your DSO. Tektronix makes several differential
probes for their scopes. There is also a lot of information on the
Tektronix web site for selecting the right scope/probe setup for the
particular application. One called, "ABCs of Probes" ... take a look. http:
//www.tektronix.com/Measurement/accessories/index.html
<http://www.tektronix.com/Measurement/accessories/index.html>
 
Chris Hansen
StorLogic, Inc.

-----Original Message-----
From: John@quantatw.com [mailto:John@quantatw.com]
Sent: Tuesday, November 14, 2000 12:45 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : LAN signal measurement with a loop back.

Dear SI gurus,

One question regarding how to measure Ethernet LAN differential signals.

I made a loop back by using 6 inch long CAT-5 UTP cable and a RJ-45 male
connector.
(Why 6"? , is there any standard for the length of the cable? )

I tied TX+ to RX+ and TX- to RX- at the end of the UTP cable. The other end
is connected to RJ-45. Then, I plug in the loop back to a LAN port.

With DSO, I measure the differential signal with one active probe. The
ground of the probe is tied to TX- and positive end of the probe is tied to
TX+.

Is it a right way to measure differential signals? Or shall I use two
probes?

Shall I put a 100 ohms resistor, as a terminator, to the end of cable?

Consider that the 100 ohms is used to terminated the differential signals
when TX+ and RX+ are not tied together.

However, when they are tied, the RX+/- shall work as a terminator because
cable's
impedance is equal to 100 ohms and tied to TX+/-. Am I right?

Thank you for your comments in advance.

John Lin
Senior SI Engineer , ARD4
Quanta Computer Inc.,Taiwan, R.O.C.
Email: John@quantatw.com
Tel: 886+3+3979000 ext. 5183

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