Re: Line impedance measuring & matching -- techniques

Hi, 
so the use case for this is that we need to have both CAN (dual busses) and Ethernet installed. We have industrial Ethernet cable that have 4 wires and quite large cross section. We try to optimize production costs and BOM. 

The Ethernet was listed with 100ohm at 100Mhz. I got the same at 1Mhz :) 

I could prove that it was quite bang on 100Mhz. If I put 150ohm the signal is all distorted. 
We went ahead and installed the cable on a full system for next level of testing. It worked fine there also. We still use 120ohm for termination. To not mess with the transceivers. 

One improvement still would be to change to split terminators, but we have not went ahead with that yet. 

Br, 
Bertil 

----- Original Message -----

From: "Al Thomason" <[email protected]> 
To: [email protected] 
Sent: Tuesday, February 9, 2016 6:34:29 PM 
Subject: RE: [CANLIST] Line impedance measuring & matching -- techniques 



From: [email protected] [mailto:[email protected]] On Behalf Of Bertil Bäck: 

“… So one need to find out what the Ethernet cable impedance is at 1Mhz. I just did as a article on the Internet said. I put a trim potentiometer in one end of the cable and a Frequency generator in the other end and put in on 1Mhz and square wave. I have a scope on both ends of the cable and trim the potentiometer so I get the best square signal. Then I measure the resistance of the trim potentiometer. This is a very crude way of getting the impedance of the cable.” 

Am wondering what the results were / what conclusion you came to with regard of using Ethernet type cable? 
(And was it CAT-5, or something else???) 

-al-