Estimating line R in a twisted pair telephone line at home - Page 1
Forum section: electronics / general

author: Zucca
author_rank: Unknown
timestamp: November 02, 2018, 06:43:51 pm
content: Inspired by this italian post below (don't worry if you can't read italian, it's not necessary):http://spazioinwind.libero.it/elettroidee/linea.htmlI decided to test my telephone line, hoping to investigate/understand more my 15,9MBit Real / 20MBit Ideal ADSL.The idea is so simple...Basically we consider what is outside the home as a VDC generator V_line with a R_line in series... and we do just two VDC measurement at the telephone socket:1) Nothing plugged in, no voltage drop on R_line: what we measure is just V_line2) Punt a Rload (1KR 1/2W) connected as a phone and measure again VDC at the socket and we got a reading V_sl (V socket load)Then:R_line = (V_line/V_sl-1)*Rloadin my home I got:V_line= 53,23 VRload= 998,32V_sl = 24,45 V--> R_line =1175Now according to the guy above there is a R=500placed at the beginning of the line....so I need to take it off, result my line has 675... which seems to me pretty high.Am I right?

author: nctnico
author_rank: Super Contributor
timestamp: November 02, 2018, 07:10:00 pm
content: It doesn't work that way. The AC impedance is supposed to be 600 Ohms. Say between 300Hz and 3600Hz. Near DC you are looking at the DC feed resistance of the telephone lines and the DC resistance of the wiring. A better way is to use a function generator set to 1kHz and inject the output into the telephone line through a 600 Ohm series resistance. If you measure the AC voltage across the resistor and then across the phone line you can calculate the impedance. The voltage across the resistor tells you the current and using Ohm's law you can calculate the line impedance using the voltage across the line and the current. You'll need a DMM with an accurate true-RMS at 1kHz for this measurement. A standard DMM likely won't do but check the specs for the AC range.

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2018, 07:28:56 pm
content: As nctnico says, DC resistance is one thing, line impedance is something else.Twisted pair telephone wires usually have a signal impedance of 100...120 ohms. The source and terminal impedance is country dependent for historical reasons and also influenced by tip/ring/offhook signaling.

author: Zucca
author_rank: Unknown
timestamp: November 02, 2018, 07:45:13 pm
content: Thanks very interesting...can we say that DC resistance < line impedance? I mean if there is a weak point/high attenuation on the line, if I can detect it with the R DC method, I will expect it also in the f domain.What worries me, the R/impedance souce in Italy should be 500 Ohm so I suppose my line is not wired properly... 675 ohm is too much.... I am about 2km away from the cabinet.I don't have a f gen in Italy with me at the moment... definitely I will do it in the future.

author: SeanB
author_rank: Super Contributor
timestamp: November 02, 2018, 07:59:52 pm
content: With phone lines and DSL the important things are to keep the twisted pairs as intact as possible, and not to have any stubs before you have the DSL filter. After the filter you have POTS and it does not worry the DSL signal at all, you can have almost anything there with minimal effect, but before you want to have the twisted pair, good connections and no large open loops, and the cheap flat cable to the DSL modem and the filter must be as short as possible, and use good connections.As you are getting very close to the peak for the service you have, the line is good signal wise, and any improvements will be done by cleaning all the connections from the actual cable to the DSL modem up to the point you meet the actual phone company line side, after which you are stuck with having Telephonica Italia come out and check the line with a TDR analyser, and finding which of their joints is causing a reflection and fixing that side.

author: Zucca
author_rank: Unknown
timestamp: November 02, 2018, 08:11:00 pm
content: Thanks,I fixed the twisted pair cable line a lot today:1) a corrosion spot at junction box out side my home, copper line were exposed to the elements and nasty green... fixed by cutting the cable, isolating the junction box and soldering them back with heat shrink. I got 0,1Mbit more for that.2) I found an unused socket behind the closet that was between the DSL moden and out side: isolated/patched that, now the wire go straight from outside to the DSL modem. I got 0,1Mbit more for that.Then I was thinking what could be outside my home, that's why I was investigating the R line in DC.Sure I am pretty happy with 15Mbit, but I think when the VDSL FTTC will be here next year I will see how poor my line is. I have 2 line interruptions (nicely patched today, but still) from the street to themodem...2020 the FTTH... and I can then forget about the copper drama....Then again I have (I could be wrong) 675 R in DC line... fishy

author: tggzzz
author_rank: Super Contributor
timestamp: November 02, 2018, 08:14:20 pm
content: Quote from: zucca on November 02, 2018, 06:43:51 pmNo - and be careful of doing experiments with a piece of wire that is not under your control: there might be a human being connected to it somewhere else!Try repeating the experiment with a setup that is known and under your control. I suggest you go out and buy 1m of 75ohm coaxial cable of the type used for TV aerials. Now repeat your experiments and try to measure 75 ohms.Once that practical example is out of the way, in order to progress you will need to understand the theory. The key search phrase is "transmission line impedance".

author: Zucca
author_rank: Unknown
timestamp: November 02, 2018, 08:21:42 pm
content: Quote from: tggzzz on November 02, 2018, 08:14:20 pmFrom the cabinet to my home there is one line. I am pretty sure nobody cares if I play with my line. The cabinet should be failure (short) resistant anyway.Quote from: tggzzz on November 02, 2018, 08:14:20 pmDon't think is a good idea, the 75 ohm cable will generate a reflection with the 600 Ohm impedance telephone line.I just wanted to check the R line in DC, as a stupid and simple sanity check.

author: Zucca
author_rank: Unknown
timestamp: November 02, 2018, 08:52:34 pm
content: https://www.etsi.org/deliver/etsi_es/201900_201999/201970/01.01.01_60/es_201970v010101p.pdfQuoteThis is the sanity check I was looking for.

author: Red Squirrel
author_rank: Super Contributor
timestamp: November 02, 2018, 09:36:21 pm
content: I don't know too much about impedence, was never really able to wrap my head around it no matter how much I read about it, but I think resistence and capacitance and inductance play a role, if you know some of those values think you can calculate impedance?If you're curious call the phone company and ask them to do a line test, they can tell you the resistance and capacitance tip to ring and tip to ground. This tests from the CO to your house. The phones you have plugged in will affect capacitance, but so will distance.I would not try to test this yourself from home as you'll get weird results as the line is connected to the line card/equipment at the other end. It is also supplying 54vdc so you probably don't want to be putting an ohm meter on that or you'll let the magic smoke out.

author: macboy
author_rank: Super Contributor
timestamp: November 03, 2018, 02:11:33 pm
content: 22 awg wire has 0.053 ohm/m.You have ~ 4000 m (2km x 2 conductors). Do the math.