If LAN Testers can estimate shorts Why use a TDR? - Page 1
Forum section: electronics / general

author: TheNewLab
author_rank: Frequent Contributor
timestamp: December 20, 2014, 01:54:50 am
content: I have been looking at how to locate breaks (shorts or open) on electrical and audio XLR type cables. I have a LAN tester that can measure length of Cat 5/6 cables and approximate location of a short or open. I keep finding the solution is a TDR with function generator and a scope.Why can't I use capacitance and the ohm resistance profile of cable,then use the Cap function of a DMM or use an LCR meter? I already know the Capacitance and resistance per meter of cable. electrical I would know the AWG, for XLR, I can find that spec. Do I really need the scope and TDR meter?

author: johansen
author_rank: Super Contributor
timestamp: December 20, 2014, 01:59:18 am
content: If you have known good conductors you can do a four wire ohms check and find the short to within 10% of the length pretty easily.i guess its the same amount of work as setting up a TDR.i can't figure out how a capacitance meter would give you the location of a short, but you could probably find the location of an open to within 10%

author: westfw
author_rank: Super Contributor
timestamp: December 20, 2014, 02:08:58 am
content: I can't imagine why you'd need a special TDR setup to test LANs. Don't most ethernet chips essentially include their own TDR logic? (They did back in the LANCE/i82586/Thicknet days, and they worked! Of course, the whole twisted pair world is a different kettle of fish; but I can't see how a built-in TDR would be that much worse than a fancier TDR setup.)

author: LukeW
author_rank: Frequent Contributor
timestamp: December 20, 2014, 03:15:51 am
content: Some Ethernet chipsets do have built-in TDR, the Marvell gigabit chipset in one of my PCs does, and has an accompanying Windows program for cable diagnostics and length measurement. But not sure if all of them do.

author: westfw
author_rank: Super Contributor
timestamp: December 20, 2014, 08:39:31 am
content: I may have misunderstood the original question. As far as I know, ethernet cable testers ARE likely to use TDR to find cable length and breaks/shorts. That sort of thing is pretty easy to do when you have digital data with 20MHz+ clocking already there.It's an interesting thought, whether you could use a ethernet testing on audio cables. Or design a low-cost tester that would do something similar...

author: Muttley Snickers
author_rank: Unknown
timestamp: December 20, 2014, 11:52:15 am
content: To TheNewLab,We use a variety of testing devices including, but not limited to the Tektronix Telescout TS100 & 200, Riser Bond 1205cx, AEGIS CZ-3000 MK2, and although these are now getting a bit old in the tooth, they are still valid and valued tools. These TDR's in our line of work are able to distinguish faults more than just short or open, they can indicate branches, tee's, saturated cables, distance to anomaly's, and so on, and the skill involved of making a diagnosis on a cable run based on a displayed wave form really is an art unto itself IMO.We also use run of the mill LAN map tester's for day to day stuff, but if it's not one of our cable run's or it's an existing site then we don't muck around, the heavy gear comes out. This may or may not be relevant to what you do, but I thought that it was worth a mention.Best RegardsMuttley