Testing ADSL lines with an oscilloscope.  - Page 1
Forum section: electronics / general

author: G7PSK
author_rank: Super Contributor
timestamp: July 10, 2012, 05:41:02 pm
content: I have a need to test the working of an ADSL router modem due to no connection showing. I was wondering if that could be done with an oscilloscope as a proper ADSL tester costs between £500 and £2500 The line will not work with the internet dispite the phone working. BT say that the problem is at the house end (mother in laws) and want to charge £150 for a visit.. Now seems very unlikely to me that several modems are all not working in exactly the same manner with one just out of the box. It is a problem that I have had myself in the past with the ISP sending out engineers who all say the problem is BT's end and BT saying otherwise with my problem BT even rewired the phone lines in the house and still no internet so finally conceding and fixing the exchange. I have fixed up a patch lead to connect the scope, I was wondering do I need a capacitor in line with the probe and what should the signal look like etc.

author: SeanB
author_rank: Super Contributor
timestamp: July 10, 2012, 06:23:23 pm
content: Should look like a giant load of hash sitting on a -48VDC level. If you AC couple then it will be around 1V of basically noise.Best is to go to the house, disconnect everything other than the master socket, plug in a telephone and check it will dial out and give good voice. Then disconnect the phone, and plug in the ADSL modem direct into the master socket and check there is no signal.Open the socket and clean and retighten the contacts on the BT side, and if you still have nada there ADSL wise then tell BT it is their fault. If you plug in any decent ADSL modem it will tell you connect speeds and attenuation figures on the line if it is able to sync. More than roughly 40dB attenuation and most will not work well.If you have a spectrum analyser that will go down to 100kHz or less then it will look like there are multiple carriers on the line, each evenly spaced and extending in frequency from around 50kHz to a couple of MHz. These are the different data carriers.Most of the time the problem is caused by house wiring that is poor, and add ADSL filters to all phones and other telco equipment other than the ADSL modem. I have often seen the plugs misconnected, and then there is no ADSL with phone working more or less. If you suspect a filter change it, you will have plenty around most likely, as parts of the kit.

author: G7PSK
author_rank: Super Contributor
timestamp: July 10, 2012, 08:10:12 pm
content: I have already done all that, the modem will just not sync and the problem has only been there since BT were working on the lines last week but they just don't wont to believe that. I will just have to keep on at them to get it fixed.

author: Monkeh
author_rank: Super Contributor
timestamp: July 10, 2012, 08:17:44 pm
content: Typical BT attitude. There really is nothing you can do except keep at them until they send an engineer out. Anddo notpay them unless they can prove in front of you that the line is working!

author: grumpydoc
author_rank: Super Contributor
timestamp: July 10, 2012, 08:55:14 pm
content: The £150 is only if they find the fault in your wiring or there is no fault, if you connect a known good ADSL modem to the test socket (that's the one behind the master socket faceplate and it won't sync then you've eliminated your wiring as the cause and there is obviously a fault so you shouldn't have to pay - you might have to stand your ground though.Is BT your m-i-l's ISP? For broadband faults you need to go via the ISP, you can't deal with BT wholesale direct, don't try to report a broadband fault via the main BT customer service.If, however, thereisa voice fault report that to BT and get them to fix it first. Quite often that fixes the broadband issue as well. Don't mention the BB when you report the voice fault, they have been known to blame the modem.

author: G7PSK
author_rank: Super Contributor
timestamp: July 10, 2012, 09:13:32 pm
content: BT is the mother in laws isp, trouble is BT internet is separate from BT wholesale or Open Reach as they seem to call themselves these days, although part of a conglomerate they act as separate entities and BT internet has to book and pay for an engineer to go to the exchange.On one occasion my own service went down due to BT internet tweaking the equipment at the exchange remotely from London in order to improve the service speed instead it killed it dead and it took over a week to get it sorted and the next month they did it again, they obviously have not heard if it ain't broke don't fix it.

author: grumpydoc
author_rank: Super Contributor
timestamp: July 10, 2012, 09:44:06 pm
content: Don't forget BT were made to split out BT wholesale/Open Retch to provide a "level playing field" for competition amongst ISPs.The result, of course, is that sometimes the left hand doesn't know that the right hand exists - much less what it's doing.

author: baljemmett
author_rank: Unknown
timestamp: July 11, 2012, 12:48:55 am
content: Quote from: G7PSK on July 10, 2012, 09:13:32 pmEven worse than that, Wholesale and OpenReach are two separate divisions -- OpenReach look after the lines and exchanges, Wholesale provide access to the services BT run on OpenReach's copper, and Internet are the ISP (part of Retail, who also handle other consumer and small business sales) I believe there are a few other tentacles for business services, IT outsourcing, etc., and not to forget Dabs and Plusnet who are both BT Group companies!

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 11, 2012, 01:51:43 am
content: ADSL uses lower frequencies for upstream and higher frequencies for downstream, so if your scope has FFT, you should be able to tell if the modem is sending and if the ISP is sending. Do note that it's a differential pair so for what you're doing, use two probes, one for each side, AC coupled, and take the differential.

author: Psi
author_rank: Super Contributor
timestamp: July 11, 2012, 01:51:49 am
content: Yeah, connect the modem to the phone line where it first connects to the building/house. (So everything else is disconnected)If it wont connect there then its definitely not an issue at your end.Also, i'm going to say this, i know it's obvious, but i see people fail to understanding it quite often, so...The filter is not for the adsl modem, it's for the phoneline.If you only connect the modem and have no phones connected then you shouldn't be using a filter anywhere.

author: _Sin
author_rank: Regular Contributor
timestamp: July 11, 2012, 08:42:57 am
content: Last time I had a BT line issue, the ultimate solution was to move house. Intermittent fault, so a gamble every time I wanted an engineer to come out as they always wanted to bill me if it didn't show itself while they were there. Managed to convince each engineer that the fault existed, but each time they fucked off back to the exchange to "change some wires".The fault wasn't in the bloody echange. I could've told them where the fault was. In fact I did, every time. Did they listen? No. Customer is always wrong in BT world.If I hadn't moved, I'd probably have just gone and fixed it myself, though you're not supposed to bugger about with BT's own wiring...TBH at least I had some kind of service. It's shockingly poor in my current place.

author: grumpydoc
author_rank: Super Contributor
timestamp: July 11, 2012, 02:13:26 pm
content: QuoteWell, the ADSL line card in the exchange is sending.Note however, unless you live very close to the exchange you will find the downstream signal considerably attenuated - certainly below the noise floor of a typical 'scope input.A dedicated spectrum analyser might be able to show it.

author: G7PSK
author_rank: Super Contributor
timestamp: July 11, 2012, 02:33:14 pm
content: Quote from: grumpydoc on July 11, 2012, 02:13:26 pmIf a scope has trouble seeing the signal due to so low a voltage, My scope goes down to 1 mV, how can the modem see the signal above the noise, It is hardly surprising that the things can be so bloody temperamental and perhaps remarkable that in my case the things work at all, as I am three and a half miles from the exchange.

author: bilko
author_rank: Frequent Contributor
timestamp: July 11, 2012, 03:07:13 pm
content: Some ADSL modems have line monitoring and testing diagnostics. When I was in the UK I lived in a rural location, I often had problems with the ADSL connection. I used a Zyxel p660HW61 (I think) it could display the the number of signal pairs in use and the near end and far end dB's. Excellent tool.Take a look at this, it shows some of the capabilitieshttp://www.dslreports.com/faq/14209

author: grumpydoc
author_rank: Super Contributor
timestamp: July 11, 2012, 04:11:06 pm
content: QuoteBecause the ADSL signal has a lower bandwidth (1 or 2MHz) so it's easier to design a low noise input - this is less so for VDSL which can have up to 30MHz bandwidth. In fact the 30MHz variety of VDSL is allowed not to use the lower frequencies for this reason.For ADSL the transmitter can go up to (IIRC) +17dBmW - assuming a 600ohm reference that's approx 5.5V of signal. My ADSL line has (or had, I'm now on FTTC) 63dB of attenuation - or a factor of ~103- i.e 5.5mV at the far end. Of course that's an average - low frequency bins will have lower attenuations and have higher voltage by the time it gets to the end users modem and higher frequency bins will be even less - but probably not used because of the low SNR for those binsAt the higher frequency end you're fishing around close to the 'scope's noise floor and aren't going to get all that useful a result from a 'scope based FFT.Far better to use the modem's ability to do diagnostics together with something like DMTTool which can show you graphs of signal to noise ratio, bits per bin etc.

author: i4004
author_rank: Regular Contributor
timestamp: October 10, 2016, 03:14:47 am
content: Quote from: G7PSK on July 10, 2012, 05:41:02 pmalbeit pretty old thread, sometimes you just can't find stuff you want on the web, so there, fresh:https://youtu.be/K3tgvBbx-WU2 channels, 2 probes, a-b mathunsurprisingly it looks like digital bitstream.

author: Halcyon
author_rank: Unknown
timestamp: October 10, 2016, 03:28:30 am
content: Quote from: bilko on July 11, 2012, 03:07:13 pmYep. The Alcatel SpeedTouch Home and SpeedTouch Pro modems were great for this feature if you used it along with theAlcatoolapplication (Alcatool basically gives you a GUI interface to the modems telnet interface). Among the features it allowed you to graph line conditions at the various frequencies across the ADSL band. Very handy!

author: bktemp
author_rank: Super Contributor
timestamp: October 10, 2016, 12:43:00 pm
content: Quote from: i4004 on October 10, 2016, 03:14:47 amI can't see any digital datastream. It looks like the scope can't handle the large common mode signal (sinewave) and all you get is garbage from subtracting both channels in the digital domain.

author: CJay
author_rank: Super Contributor
timestamp: October 10, 2016, 03:26:35 pm
content: If you've taken everything out all the way back to the master socket (you do *NOT* need a filter to make ADSL work) so there is no extension wiring inside the house still connected then they cannot charge you for a fault in the house, the line up to the master socket belongs to them and they have an obligation to make it work for you as part of your line rental contract.Stop dicking around with it and insist they come and fix it.if you start explaining to them what you've done and connected to their phone line then they would be well within their rights to refuse to offer you a simple POTS if you're unlucky enough to find someone who gives a toss.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 10, 2016, 03:31:29 pm
content: Quote from: CJay on October 10, 2016, 03:26:35 pmSomehow I hope it has been fixed after 4+ years

author: CJay
author_rank: Super Contributor
timestamp: October 10, 2016, 03:33:52 pm
content: hahaha, yes, my bad, I saw the recent replies and didn't realise the orginal post was that oldStill valid though, ADSL fault finding as a subscriber in the UK is really that simple, unplug everything to the master, if it still doesn't work make them come and fix it.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 10, 2016, 10:28:29 pm
content: Quote from: PA0PBZ on October 10, 2016, 03:31:29 pmI dunno--some ISPs in Oz come close!