NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/sci/electron/18728

Text File | 1992-11-11 | 1.7 KB | 51 lines

Newsgroups: sci.electronics
Path: sparky!uunet!email!eiaut2.tuwien.ac.at!ch
From: [e-mail] (Christian Kral)
Subject: Re: Phone line monitor
Message-ID: <[e-mail]>
Sender: [e-mail]
Nntp-Posting-Host: eiaut2.tuwien.ac.at
Organization: Technical University Vienna, AUSTRIA
References: <[e-mail]> <[e-mail]>
Date: Wed, 11 Nov 1992 13:01:57 GMT
Lines: 38

In article <[e-mail]>, [e-mail] (Curt
Welch) writes:
|> In sci.electronics, [e-mail] (Ken Harstine) writes:
|> >I would not recommend that you tie any portion of
|> >this circuit to ground, as this would unbalance the phone line, and
|> might
|> >cause distortion of the sound.
|>
|> This reminds me of a problem I had.  I tried to use an oscilloscope
|> once
|> to look at a phone line and some phone interface circuits I was
|> playing
|> with.  Hooking up the scope caused a large hum to be introduced in
|> the
|> circuit, which prevented me from making any useful measurements.
|>
|> So, my question.  How do I use a scope to look at a phone line?  Do I
|> need
|> an isolation transformer between the power and wall outlet?  Or is
|> there
|> some type of probe I could buy that would provide the needed
|> isolation?
|>
|> Curt Welch

Hi Curt,

There is a simple possibillity of getting rid of this kind of problems,
using a two channel scope which can add both inputs and invert one.
If you hook up both probes to the two telefone lines, leaving the
GND-connectors of the probes away, you can measure simply the
differencial signal which is just what you want, regardless of the
common voltage.

Try it,

Christian.