hans@umd5.umd.edu (Hans Breitenlohner) (08/20/87)
In article <617@bbking.PRC.Unisys.COM> rmarks@bbking.PRC.Unisys.COM (richard marks) writes: >Question: I've read for years about the 50 vs 53 ohm impedance difference. >Everyone says that the difference may cause transmission errors. Now I've >got a couple of thousand feet (several nets) of RG58A/U (53 ohm) and everything >works well. > >The question is: Is this 50 vs 53 ohm difference a real problem or is everyone >just giving lip service to it? Has anyone has problems that were fixed by >going to true 50 ohm thin cable? > Whenever you have a discontinuity in cable impedance, some of the signal travelling across that discontinuity will be reflected. This effect is cumulative, and if you have several discontinuities in close proximity the problem gets worse because original signal, reflection, and reflections from adjacent splices may interfere with each other. I don't have a formula for the amount of reflection. I used to have a good reference in a TI book (Line drivers and receivers, or Linear and Interface, or something similar), unfortunately I can't find it any more. In any case the effect should be of the order of a few percent at most. It may reduce the 600 ft. maximum for the thin Ethernet segment proportionally. You need to avoid the case of alternating 50 and 53 ohm segments, as would happen if you have a backbone of one type of cable, and used the other type for splicing in new drops. I would expect that six transitions on one cable segment would not cause an appreciable problem. If you are concerned about the well-being of your Ethernet, a time domain reflectometer is the tool to answer all your questions. Lacking such a device, a simple pulse generator and oscilloscope is quite suitable to point out any problems due to cable mismatches, bad splices, bad terminators, kinked cables, or almost anything else which might be affecting your Ethernet. If your system consists entirely of 53 ohm cable, I can see no possible problem being caused by that. You might consider adjusting the terminators, if you can find suitable 53 ohm resistors.