From: mwandel@bnr.ca (Markus Wandel)
Subject: Re: Phone line monitor
Organization: bnr
Date: Tue, 10 Nov 1992 13:51:45 GMT

In article <27344@oasys.dt.navy.mil> curt@oasys.dt.navy.mil (Curt Welch) writes:
>In sci.electronics, kharstin@MtHolyoke.edu (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?

Use a good quality audio transformer (the output transformer from an old
tube audio amp can handle the full ring voltage (on the primary) without
saturating).  Couple to the line through a capacitor, say a 250V 1uF type.
If the other phones don't ring OK any more, the transformer can't handle
the ring voltage so put a 10K ohm resistor in series with the primary side
too.

Put the 'scope on the secondary of the transformer.  Now you can listen to
the line whether it's on hook or off hook.  Play with the phone, let the dial
tone time out repeatedly, hang up without dialing, send corrupted dial pulse
sequences etc.  Then leave it on hook for a while and listen to the neat noise
of the automated tester hitting your line in response to the 'trouble' --
at least that's what happens on the DMS100 type lines I've played with.

A transformerless kludge that should work -- if you have a 2-channel scope:
Use AC coupling, one probe on each side of the line, and differential mode.
Disconnect from the line when not in use.  The high impedance of the scope
(infinity at DC, 10M at signal) should prevent interference and the
differential mode should cancel out the common-mode hum.

About unbalanced phone lines:  Your phone line is usually a twisted pair of
wires in a thick cable with hundreds of other phone lines.  None of them
interfere with each other because they are all twisted independently, and
all are balanced -- the exact same current flows in both wires but in 
opposite directions.  Suppose you unbalance the line at your end by
introducing an impedance to ground in one branch.  Now not all the current
that flows to you through one wire goes back through the other.  A net
magnetic field is generated by the twisted pair and the other other pairs
will pick up some of it.  Everybody can hear you.  DTMF tones, dial pulses,
receiver-off-hook noises, and distorted ringing voltages are LOUD.
Complaints pour in.  Phone company does lots of testing, finally tracks down
the source of the interference, and comes to your door VERY UNHAPPY only to
find your homemade device.  This is NOT from experience with the real phone
company.  But I have a homemade system with unbalanced lines (one side is
ground, the other side is signal).  Combining two lines in JUST ONE lousy
25ft phone extension cord (two pairs, not twisted) results in enough
crosstalk that I can hear a busy signal from one line on the other line.
Telephone receivers are very sensitive.

Markus Wandel
markus@pinetree.org <-- NOT the source of this posting
Newsgroups: sci.electronics