RE: Line impedance measuring & matching -- techniques

Hi all,
I haven’t got any Ethernet cable hanging around but I do have this 46.5’ length of Thick DeviceNet cable rated for CAN bus and 120 Ohm  impedance.
Driven at one end by an Agilent 33220A Arbitrary Waveform generator set for 5V out and 120 Ohm impedance along with 1uS pulse every 5uS.
Scope at the other end is Tek MSO3034 with probe impedance set to 1M
Here’s the result without termination at the scope end.
http://www.autoartisans.com/CAN/NoTerminationThickDeviceNet.jpg
 
And here’s the result with termination at the scope end.
http://www.autoartisans.com/CAN/TerminationThickDeviceNet.jpg
 
And just for general interest the next photo is from a 36m long cable where there are 50 nodes are on the last 10m and the other end has one CAN node.  It’s also Thick DeviceNet terminated at both ends with 120 Ohms.  Each node is tapped of the main cable about 19cm apart with a stub length of 30cm.
 
It’s in this photo you can see the reason why there are bus length limitations on the controller area network bus.  The scope triggers on the ACK, which follows the CRC Delimiter, but is delayed by 300nS due to the to/from distance over the length of the cable.  
 
That little bump in each bit (about 200ns into the bit) is described perfectly in Steve Corrigan’s article on node to node spacing.  And the shifting of the bus levels after the ACK is due to ground bounce as the load goes from minimal current draw to rated cable current draw over the length of the bus on the power wires which share the ground with CAN signals.
 
http://www.autoartisans.com/CAN/SmplACKDel36m-RGB-2-Photo3.jpg
 
The green trace is the differential sum of the CAN_H and CAN_L.  I only had a two trace scope at that time.  The TTL side of the node receiver would have been more interesting.
 
John Dammeyer
 
 
From: [email protected] [mailto:[email protected]] On Behalf Of Al Thomason
Sent: February-09-16 8:34 AM
To: [email protected]
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-