trying to understand adsl (in the UK)

You need to look for signals in the 1M+ range. What you have there is just interference which the ADSL modem has to filter out. The actual ADSL signal will be in the millivolts range I would think (depending on losses).

Was the modem cable actually connected to a modem or router, or was it open circuit. If the latter, there'll be no synch signal, since it takes two to tango!

You have connected the scope directly to the BT line the unfiltered port on the filter *is* a direct connection to the line. The line is balanced transmission line for voice (PSTN) and ADSL, unless you have a scope that is isolated from mains earth when you connected the probes you would have earthed one side of the line. Pick up a phone and listen when the scope is connected and you will probably hear the result, a deafening mains hum. Even if you got the RF waveform to display clearly on the scope it wouldn't be very instructive. A spectrum analyser would show you the DMT up and down spectrum

Oh, and I suppose you know that connecting un-approved equipment is frowned upon in some quarters?

Good point, if your local modem is disconnected the DSLAM (the modem ar the exchange) goes to sleep.

thanks for your response roger, I didn't know it had to be connected to a router. I was testing on an open circit, just touching the terminals on the end of a moden cable with the probe. In response to what dennis@home said earlier I did a test again this time using a volt setting of 20mV and a timer setting of 20u secs and I got several strands of sine wave mixed together but I don't know what this was.

I'm not getting very far with this. Maybe I'll have to buy one of those proper ADSL testers that are outrageously expensive....

go hear and read.

I suggest you learn ti use a scope befre you try and use it to debug ADSL..

Thats mains hum.

No adls signal at all - leastways that's what the 'fur' on the hum is, probably. If the ADSL was active.

the scope is about he least useful thing to diagnose an ADSL lime there is. A spectrum analyser at a few thousand would be better, but best of all is a ADSL router.

In which you will no doubt learn that the ADSL signal is composed of a varying number of 'BINS' each 4KHz wide. The total number of these depends on the available reliable bandwidth of your line. You will never be able to discriminate between these with an ordinary oscilloscope. It will just look like 'noise' i.e. lots of 'shash' About the only way to see these without resort to an expensive spectrum analyser is to use an ADSL modem like a Dreytek Vigor which will display the amplitude, signal to noise and number of binary 'bits' in each 'BIN'

As a previous poster says, all you were seeing is mains hum.

Mike

Your not allowed to attach scopes to a BT line !!!!!! You are breaking the law.

Try a new modem. Disconnect your phone to see if that is interfering.

It that all fails get your ISP in to fix it.

To be fair he isn't trying to fix anything, he is trying to gain some basic understanding of the technology involved.

I doubt if I would have the knowledge of electronics that I have today if I was discouraged from experimenting with stuff I didn't understand when I was younger.

The "health and safety" culture has meant amongst other things that youngsters don't even learn how to fit a 13A plug at school, and we accept without question that a fault on "the PCB" in a boiler etc necessitates the replacement of the PCB.

In message , Graham. writes

Or the boiler!!

thankyou for responding to this post in a way far better than I would have. You're right, I'm a guy trying to gain some understanding of things I have to deal with. You're also right about the health and safety culture we live under now, so many red lights stop you at things you try to do. Such a shame. You can read and study the academics of things for years but life's taught me the only way you're really going to learn about things is to roll your sleeves up and get your hands dirty.