... I don't see anyone making a stab at a reply here, so I've got some ideas to share ... When presented with problems like this - there are two ways to proceed. A) Secure bona-fide 120 Ohm cable or B) fake it. When I say 'fake it' I'm not being flippant even though it may look so. The options under this category range from 'winding your own 120 Ohm twisted pairs to taking some 100 Ohm pairs and massaging these to 'up' the impedance. Upping the impedance could take one one of two possible forms - 1) loosening up the twisted pairs would up it - or 2) adding a suitable R (resistor or resistor network) would be the other. The test that they seem to be specifying the 120 Ohm cable for (in conjuction with the usual 100 Ohm cable) looks to be one in which 'reflections' from the resulting line mismatch that would occur are meant to test your circuit - testing it to see how it deals with any possible 'jitter' from the resulting 'reflected' signal - or maybe the cable pair to cable pair cross (crosstalk) coupling - in which case you'll either need a *real* piece of 120 Ohm cable or be able to create your own ... In any case - you should have a TDR or perhaps a reflection/transmission test set (network analyzer, baluns, etc) to actually set up and measure what you've created. The scope of that discussion would require me to begin charging a fee (one has to eat!) - so I'll leave that to someone else to discuss. Jim ----- Original Message ----- From: "Matt Bennett" To: Sent: Sunday, September 02, 2001 8:36 PM Subject: [OT]: Help! need 120 ohm cat5 cable > I've been searching for a very specific type of cable for Ethernet > testing. In my daytime alter-ego, I work for [a very large computer > manufacturer in Texas], where I design and test Ethernet- testing to the > IEEE specifications (the specifications for Ethernet are free 6 months > after they are issued, by the way; check the IEEE website: > ). The latest thing is Gigabit > Ethernet. In the Gigabit Ethernet testing specification, one of the > tests calls for a very specific cable (section 40.6.1.1.1) to do jitter > testing with- a cable made up of a few sections of 100 ohm and 120 ohm > cat-5 cable. According to ISO/IEC 11801, Category 5 cable (100 MHz) can > be 100, 120, and 150 Ohm (I think) impedance. Pretty much everybody > uses 100 Ohm cable- which makes the 120 Ohm cable virtually impossible > to find. > > I've heard things to the effect that 120 Ohm cable is more commonly used > outside the US, but in my searching, I can't find any sources. Alcatel > used to make some, but they don't make it anymore, and they don't have > any alternate sources. According to a very terse email I got from > Belden, they only deal in 100 ohm Cat5 cable. Every other company I've > contacted about it has come up as a dead end. The total amount I need is > under 3 meters, but heck, I'd buy a 1000' spool if I had to. > > Since this list has members from all over, maybe someone out there could > help me. In particular, I need 120 Ohm impedance, 4 pairs of > conductors, shielded or unshielded Category 5 cable. Help? > > Matt Bennett > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body