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Author Topic: Viewing signal  (Read 1968 times)
WB6MIO
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Posts: 20




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« on: July 24, 2007, 10:13:54 AM »

I would like to view my DSL signal coming into my gateway with an O scope. I think that phone stuff is balanced with no ground reference.
I was thinking of coupling the scope probe ground and signal with a pair of decoupling caps.
Would that work, or would I just see a bunch of hum?
Do you need some kind of transformer, or ?

How about two channels of my scope (still no gnd, though)
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AA4PB
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Posts: 15042




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« Reply #1 on: July 25, 2007, 07:28:46 AM »

Phone signals are balanced so anything you do to unbalance it, like grounding one side, will result in significant 60Hz pickup.

You could use an isolation transformer if you had one that would pass the high frequencies required for DSL. Another option might be to use a battery operated scope or perhaps to use an AC isolation transformer on the scope power. The idea here being to "float" the scope chassis above ground.
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Bob  AA4PB
Garrisonville, VA
WB6MIO
Member

Posts: 20




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« Reply #2 on: July 25, 2007, 07:56:34 AM »

The simplest would be to float my scope.
If that doesn't work out, I could wind a 1 to 1 transformer on a ferrite toriod.
Anyway, hadn't thought of that...Thanks!
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NI6B
Member

Posts: 1




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« Reply #3 on: July 26, 2007, 07:50:19 PM »

If you have a two channel scope the best way to look at a DSL signal is to use the two channels in a differential input mode with the following settings

1.   A+B with B inverted
2.   Use the same probe sensitivity on both probes (1x is sufficient). The DSL circuit Z is 100 ohms and you want to have >> 10K Ohms across the pair to prevent significant attenuation of the signals.
3.   Set the input Z of both channels to the 1M Ohm setting. 50 ohms will not work see #2.
4.   Be sure to set the channel gain of both channels to the same setting as well. Typically 100mV/div should work.
5.   Use AC coupling on both. Presumably you are downstream of the DSL filter/splitter and no DC battery for the phone should be present but just in case use AC coupling.
6.   Connect each probe to one of the pair
7.   If you have ground clips at the ends of the probe connect them to each other but not to anything else

Because the probes have some stray capacitance when you connect them they may disturb the DSL MODEM and cause a retrain so give it a few seconds to “see” the signal.

DSL signals will look like random noise on your scope. What is more interesting, if your scope has the ability to do an FFT, is to see spectrum.

Good luck!

73s
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WB6MIO
Member

Posts: 20




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« Reply #4 on: August 02, 2007, 09:54:09 AM »

Thanks for the good advice guys!!
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