What does a good DSL waveform (signal) look like?

By mpersic ·
Does anyone know what a clean SHDSL signal should look like on an oscilloscope? We are using two Westermo DDW-120 Ethernet Extenders and they seem to be resetting. I think induced noise on the copper pair between the two extenders is causing a problem. The extenders are dropping out. Does anyone have a reference picture of the DAL signal and/or any information on characteristics of a dsl signal.

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Firstly I've never looked but as any signal

by OH Smeg Moderator In reply to What does a good DSL wave ...

But a signal like this would vary depending on what is being transmitted I think you'll find that you need 2 CRO traces and compare the Waveforms. One at the Starting point and one at the receiving point and check for differences.

As this can have a fairly large distance like 6 miles between sender and receiver depending on your installation you may need a couple of DSO and compare the waveforms after the event.


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Too long

by Twors88 In reply to Firstly I've never looked ...

The cable could be too long, producing too much crosstalk. But, both of you are way more in depth about this issue than i can go.

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test equipment

by bboyd@ In reply to What does a good DSL wave ...

Not sure you will see anything useful with just a scope. put a generator at 276k and watch for noise using a scope might work.

Better to use a simple cat5/phone line scanner the installers check lines with.

One thing to note is SHDSL cannot be used on a line with a phone splitter/microfilter.

Noise is usually not a problem in twisted pair systems. Induced current on the line is canceled by the opposite current on the pair line.

Because its a two pair system you can improve error tolerance by dropping data rate.

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That device has it's own diagnostics tool

by robo_dev In reply to What does a good DSL wave ...

Do not reinvent the wheel:)

Information from diagnostic tool
? Software release
? Serial number
? DIP switch settings
? If the unit is configured as CO or CPE
? Ethernet link status
? Ethernet data rate
? Ethernet duplex
? System uptime
? DSL uptime
? DSL negotiations
? LFF status
? DSL link state
? DSL data rate
? DSL noise margin (information is sampled and continually displayed)

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Used the Diagnostic Tool

by mpersic In reply to That device has it's own ...

Yes, I have the diagnostic tool (using one at each end). The tool does show the signal to noise ratio and link state. However, it doesn't show "why" the link is down. With the oscope I was hoping to identify a frequency or noise burst that was causing the link loss. If I could see the noise then I have a better chance at determining the source.

Interesting that the POTS telephone (which shares the same 50 pair commline) is complete static a lot of the time when the link is sometimes up and always when the link is down.

Replacing the commline is not an option (due to resources) and the commline was installed new. The conductors all check out at about 150 Ohms from end to end.

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by jangirke In reply to What does a good DSL wave ...

I don't know anything about it but my stereo was giving me static and I used ferrite cores to suppress it.

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Bad Device or Premise Wiring

by TheChas In reply to What does a good DSL wave ...

If you have a lot of static on your phones, then you either have a bad device on the phone line, or your premise wiring is bad.

I would start by taking a close look at your POTS / DSL splitter.
Next, check with the DSL off, plug a phone straight into the main line with no other device connected and see if you still have static.
If no static on a direct connection, the problem is a device or your wiring.
If you have static on the incoming line, the problem is between your building and the central switch.

My guess is that you will find either a phone cord that has been rolled over by a chair or pinched in a door. Or, a line in a wall that has been rubbing on a staple for years.

Taking a quick review of DSL technical data, a low frequency spectrum analyzer is going to be a better tool than a scope to look at the DSL signal. The signal can vary between 26 KHz and over 1 MHz.

While this article is for designing DSL receiver circuits, it does provide decent technical data.

One other point, the phone signal is balanced and most scope inputs are unbalanced. To see a signal of any value you either need an isolated scope with a differential input, or a 2 channel scope that allows you to view the sum or difference of the 2 channels.


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