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'scope on CO line ?'
2000\02\01@090229 by Duilio Foschi

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I'd like to investigate strange noise arriving to my CO line.

Can I attach my scope to the CO line ?

Are there risks I should be aware of ?

Thank you

Duilio Foschi

2000\02\01@094605 by Chris Eddy

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Duilio;

I would suggest going into a differential (or instrument) amp.  That
will maintain the fidelity of the noise that you may be hearing, and
give you an oportunity to amplify it neatly.  Also, the big pitfall is
the possibility of a ring occuring.  If you get a ring signal, the 90VP
( Approx, if I recall) is massive and will pop an unprotected part.
Some of the Analog Devices parts have high input ranges.  If in doubt,
use some series R's and a pair of TVS diodes to protect the part.

Chris Eddy

Duilio Foschi wrote:

> I'd like to investigate strange noise arriving to my CO line.
>
> Can I attach my scope to the CO line ?
>
> Are there risks I should be aware of ?
>
> Thank you
>
> Duilio Foschi

2000\02\01@150622 by Gordon Varney

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WARNING!!!  DO NOT CONNECT YOUR SCOPE TO CO LINE!!!!!!

You must use an isolation transformer or a battery operated scope to connect
to mains power.....

Gordon Varney

> I'd like to investigate strange noise arriving to my CO line.
>
> Can I attach my scope to the CO line ?
>
> Are there risks I should be aware of ?
>
> Thank you
>
> Duilio Foschi

2000\02\01@161822 by Chris Eddy

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Um, ah, when you said CO, did you mean Central Office, IE telephone line?
That's what I thunk you said.
Chris Eddy

Gordon Varney wrote:

{Quote hidden}

2000\02\01@223026 by John Mullan

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You can attach a scope.  Most probes are high enough impedance (or
selectable) but watch your isolation.  If the ring signal comes along while
you are measuring you my fry something (or get a buzz yourself).

But yes, you can use your scope.

John


-----Original Message-----
From: pic microcontroller discussion list
[spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU]On Behalf Of Duilio Foschi
Sent: Tuesday, February 01, 2000 8:59 AM
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject: scope on CO line ?


I'd like to investigate strange noise arriving to my CO line.

Can I attach my scope to the CO line ?

Are there risks I should be aware of ?

Thank you

Duilio Foschi

2000\02\02@024419 by Dr. Imre Bartfai

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Hi,
excuse for being under-informed: what does the CO stand for? (I associate
immediately to carbon monoxyde, and can not get rid of it. Of course, in
this context, it does not make any sense...)
Regards,
Imre

On Tue, 1 Feb 2000, Gordon Varney wrote:

{Quote hidden}

2000\02\02@141709 by Severson, Rob

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> -----Original Message-----
> From: Dr. Imre Bartfai [rootspamKILLspamPROF.PMMF.HU]
> Sent: Wednesday, February 02, 2000 1:12 AM
> To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
> Subject: Re: scope on CO line ?
>
>
> Hi,
> excuse for being under-informed: what does the CO stand for?
> (I associate
> immediately to carbon monoxyde, and


> can not get rid of it.

OPEN YOUR WINDOWS!

(Central office)



{Quote hidden}

2000\02\02@145010 by Richard Prosser

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Duilio

I don't see a major problem. Just a couple of points:-
1. Both tip & ring should be DC isolated from ground - so don't connect an
earth lead directly to the line.
2. A high impedance differential input would be best. - for example a two
channel scope in A minus B mode (& equal gains)
3. Watch out for a ring signal & spikes. Some form of overvoltage protection
(line-line & each line-earth) should be used. (e.g. a series resistor with a
mov or back-back zeners across the inputs)
4. if it's just the noise you're looking for - maybe a transformer with a DC
blocking cap. in series with the primary would be the simplest approach.

Richard P

{Quote hidden}

2000\02\02@153229 by Sean Breheny

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Hi Richard,

About using the two channels of a scope, will that really work? If the
line is really isolated from ground, it shouldn't be possible for it to
source or sink ANY current to/from the scope inputs. Yet, they are not
infinite impedance (to ground that is) so you would theoretically need a
few uA into each scope input to yield the correct voltage across the 1
Meg input impedance. The impedance from channel to channel should be MUCH
higher than 1 meg ohm, so I don't think the current could be supplied
that way.

I agree that you can use a scope this way when you want to see the
voltage across a component in a circuit and you cannot connect GND to
either side of the component BECAUSE it is NOT gnd isolated and that
would cause a short. However, if you have an isolated device, I think
that scope GND needs to be connected to it somewhere.

Sean


On Thu, 3 Feb 2000, Richard Prosser wrote:

{Quote hidden}

2000\02\02@163034 by rottosen

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Sean Breheny wrote:

> Hi Richard,
>
> About using the two channels of a scope, will that really work? If the
> line is really isolated from ground, it shouldn't be possible for it to
> source or sink ANY current to/from the scope inputs. Yet, they are not
> infinite impedance (to ground that is) so you would theoretically need a
> few uA into each scope input to yield the correct voltage across the 1
> Meg input impedance. The impedance from channel to channel should be MUCH
> higher than 1 meg ohm, so I don't think the current could be supplied
> that way.
>
> I agree that you can use a scope this way when you want to see the
> voltage across a component in a circuit and you cannot connect GND to
> either side of the component BECAUSE it is NOT gnd isolated and that
> would cause a short. However, if you have an isolated device, I think
> that scope GND needs to be connected to it somewhere.
>
> Sean
>

No the scope really is a diferential amplifer when used in A-B mode. By your
reasoning, you would not be able to measure the voltage of a dry cell using the
scope inputs in A-B mode.  :-)

2000\02\02@164114 by Sean Breheny

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OOPS! You are right, I realize my mistake: the two scope inputs (A and B)
share GND, so they really only have 2 meg ohms of impedance between them,
so there is a return path.

Sean

On Wed, 2 Feb 2000, Richard Ottosen wrote:

>
> No the scope really is a diferential amplifer when used in A-B mode. By your
> reasoning, you would not be able to measure the voltage of a dry cell using the
> scope inputs in A-B mode.  :-)
>

2000\02\02@174802 by bill

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Probemaster (http://www.probemaster.com) sells an active
differential probe (about $300 US) that's great for this kind of
thing. I had one where I worked a few years ago and found it
quite useful.


---
                                       Peace,
                                       William Kitchen

The future is ours to create.

2000\02\02@183935 by Wagner Lipnharski

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Tektronix has a special device to scope phone lines. It is a simple high
impedance transformer (10k ?) at the phone line side, and something
around 300 Ohms to the scope side, so even a 60Vac ring signal will be
reduced to 2Vac at the scope side.

I think you can use *any* 110 x 6 Vac small power transformer to this
job, problem is you will not see so much high frequencies.  If you say
"noise" at the phone line, I suppose you mean "audio noise", and this
should be present at the 6Vac side of the transformer when you connect
the 110Vac side to the phone line.

Never connect scope ground to phone lines...

2000\02\03@032504 by Nick Ray

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Sean Breheny wrote:

>No the scope really is a diferential amplifer when used in A-B mode. By your
>reasoning, you would not be able to measure the voltage of a dry cell using the
>scope inputs in A-B mode.  :-)

AIUI only if it's a differential scope. Otherwise B is inverted and added to A.
         Fine for small signals but looking for noise on larger signals would
         lead to clipping or distortion.

2000\02\03@142623 by Richard Prosser

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Sean
But the line isn't isolated. The exchange ground (battery positive) is very
well connected to earth. What I am trying to avoid is connecting either tip
or ring to earth via the scope probe. Doing this could trigger the switch
into a protection mode and would also upset the line termination impedances
- thereby most likely changing the nature of the noise you are trying to
look at.

Of course, if the "48V" (or whatever) line voltage is outside the common
mode range of the scopes' inputs then you may still get problems. Hence my
suggestion to use a transformer & blocking cap to allow the scope to be used
"normally"

Richard P

> {Original Message removed}

2000\02\03@213450 by rottosen

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Nick Ray wrote:

> Sean Breheny wrote:
>
> >No the scope really is a diferential amplifer when used in A-B mode. By your
> >reasoning, you would not be able to measure the voltage of a dry cell using the
> >scope inputs in A-B mode.  :-)
>
> AIUI only if it's a differential scope. Otherwise B is inverted and added to A.
>           Fine for small signals but looking for noise on larger signals would
>           lead to clipping or distortion.

Right you are. The inputs have no common mode rejection as a true differential
amplifer does. If the signals are kept within the display area of the screen then
this problem will not be seen. Of course neither will the noise.

-- Rich

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