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

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From: L· (John@quantatw.com)
Date: Mon Nov 13 2000 - 21:44:31 PST


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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