I think some of the confusion comes from what end of the line we are looking at and the nature of the imbalance. While the resistor may fix the near end, it will probably cause some termination problems at the far end. Reflections mostly, which on a short run analog line shouldn't be much of a problem. I haven't looked into this in detail. Also, if you look at the structure of a balanced transmission line, it should be really important to not have any imbalance out on the distributed part of the line, such as caused by having one wire of the pair having a different resistance, or by having a resistance anywhere but at the line termination - say 2000 feet out. If I interpret things correctly, this would give a line which has two termination resistances at which there is a peak of power transfer to the load, and neither of them would appear purely resistive, giving phase shift errors which make balancing the hybrid difficult or impossible, and degrading data transmission capability. Re the gain specs, I don't have a reference to them, but I suspect that there is stuff to be found on Google.
Brian D Heaton wrote:
Stephen,
Very interesting. I know I've seen all manner of messiness in the past when folks have monkeyed with balanced pairs. I'll take your word on the modeling data. I've not gone that far in depth with it.
You don't have the specs on gain adjustment handy do you? I've
probably got it buried in an old pub somewhere, but I don't have
anything in soft-copy.
THX/BDH
On Tue, 2003-11-04 at 18:34, Stephen R. Besch wrote:
I just finished modelling a standard 4-transformer hybrid coupled to a balanced RC transmission line. Cross talk was zero when the hybrid was balanced. Inserting a single resistor in series with tip or ring imbalanced the hybrid and cross talk appeared. This could be completely compensated with the proper RC on the opposite side of the hybrid, as predicted. It made absolutely no difference to the cancellation if the resistor was split. Since a balanced hybrid appears as a pure resistance (complex terms are 0) to the transmission line, placing a simple resistor in series with the hybrid (on either side) at the termination point will just look like 2 resistors in series and will properly terminate the line. There should be no effects at all from doing this other than the loss of some energy in the termination resistor, which can be made up for with a boost in Rx gain.
That's because the cores saturate on transformer based hybrids. This is not as likely to occur with active hybrids built with op-amps (which are found in almost all modern line cards), although it is possible if the gains are high enough. However the distortion from the clipping would be far worse than the echo.
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