Hi. I want to understand adsl better so I can diagnose a faulty or
weak signal when I see it. My own broadband at home is standard adsl
through a BT line (with filter). It's always worked well so I tested
it on an oscilloscope. When I put the scope probe onto the terminals
of the RJ11 modem plug here is a picture of what was on my scope.
http://www.zen73857.zen.co.uk/adsl
According to this picture the adsl signal is on a 20millisecond
cycle - which translates to 50Hz - and this doesn't tally up with what
I've read about adsl on the internet, which is 25Kz and up. Was I
getting some interference or leakage from the mains? This pic also
shows AC at about 2.5volts - is that correct for adsl?
Thanks for any help.

"tg" <tg@nospamevereverever.net> wrote in message
news:476eaed5$0$13925$fa0fcedb@news.zen.co.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).

In an earlier contribution to this discussion,
tg <tg@nospamevereverever.net> wrote:

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!
--
Cheers,
Roger
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"tg" <tg@nospamevereverever.net> wrote in message
news:476eaed5$0$13925$fa0fcedb@news.zen.co.uk...


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
http://www.mauritiustelecom.com/down...rners/ADSL.pdf
and whilst I agree such pretty pictures are instructive, in practice
testing in the analogue domain like this is not used to
test ADSL lines very much outside the R&D labs.

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


--

Graham

%Profound_observation%

"Roger Mills" <watt.tyler@googlemail.com> wrote in message
news:5t83koF1cis4hU1@mid.individual.net...


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

--
Graham

%Profound_observation%

"Roger Mills" <watt.tyler@googlemail.com> wrote in message
news:5t83koF1cis4hU1@mid.individual.net...
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.

"Graham." <me@privacy.com> wrote in message
news:fkmm7s$4st$1@registered.motzarella.org...

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

tg wrote:

www.kitz.co.uk

tg wrote:
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.

The Natural Philosopher wrote:
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.

"Marra" <cresswellavenue@talktalk.net> wrote in message
news:081cf634-a983-40fc-860d-dad8a90d662b@s8g2000prg.googlegroups.com...
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.



--
Graham

%Profound_observation%

In message <fkocvt$bns$1@registered.motzarella.org>, Graham.
<me@privacy.com> writes
--
Peter R Cook

"Graham." <me@privacy.com> wrote in message
news:fkocvt$bns$1@registered.motzarella.org...
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.