5 ms·
The pair of parallel conductors (wet string) forms a balanced transmission line[1] just like the copper pair in traditional phone line. The ADSL transceivers a
by topspin 9y ago
The pair of parallel conductors (wet string) forms a balanced transmission line[1] just like the copper pair in traditional phone line. The ADSL transceivers at each end automatically adapt to the available transmission line; you'll note that instrument reports that this 2 meter line looks like 4.5km of copper; it is a very poor conductor, so the ADSL transceivers fall back to lower frequency operation, which is apparently sufficient for low speed communication.
I do wonder about impedance matching here. Traditional phone lines apparently have a characteristic impedance of either 600 or 900 ohms. That pair of wet strings look to be separated by about the right distance to produce impedance in that neighborhood, or maybe the line length is tuned to eliminate reactance. It isn't mentioned but I suspect that RevK understood this.
[1] https://en.wikipedia.org/wiki/Transmission_line https://en.wikipedia.org/wiki/Transmission_line
- 0x47df 9y agoWhilst the impedance of the line under yesterday's test conditions wasn't measured, it was replicated again this morning, and from memory it was around the 900 ohm mark. I'll be testing again tomorrow (trying to run bonded data across two of them) so I'll try and get a recorded measurement of the line's impedance at some point rather than just something from memory.
- topspin 9y agoGreat! That will be very interesting. It was just guesstimation on my part but the diameter and separation of the strings looked similar to high impedance lines used in SW amateur equipment typically in the 400-600 ohm range. A couple sites I've read state that ASDL actually experiences about 100 ohms impedance as opposed to voice (600 ohms) due to the large frequency difference. If all that is true then bringing the strings closer together will lower impedance which should improve the match.