Yes, I think I understand a tiny bit about this.
But can you tell me about common mode verses differential.
as referenced to ground.

It seems to me AC can leak both ways into a system then through some
mysterious force infiltrate a system in a differential form
causing a dc or (low freq ac) drift.

It seems to me at Seismic frequencies you never operate
a proper balanced transmission line system because the wavelength is too long.
You need like 1/2 wavelength or multiple thereof.

Twisted lines only insure that common mode signals
are distributed evenly on both differential wires.
It does not stop common mode or even attenuate them.
You must rely upon the CMMR ratio to attack the
unwanted common mode stuff.

Use twisted pair along with 100% shielding
in my opinion is the only way.

This is all laymen talk since I'm not an engineer.

Comments,
geoff

I can not spend time going back to college,
at my age, the opportunity costs are too high.


-----Original Message----- 
From: Jim Santee
Sent: Wednesday, November 03, 2010 2:27 PM
To: psnlist@..............
Subject: Balanced line

Balanced lines are critically important to keep out unwanted signals i.e.
"10-4 Good Buddy" stuff. I have worked in one megawatt transmitting
facilities and all critical signal lines are balanced lines.

Here in a nut shell is a wikipedia outline of how they work:
http://en.wikipedia.org/wiki/Balanced_line

What is done in many applications where you have to transition from
unbalanced to balanced circuits is to have an opamp circuit or transformer
do the transition. The main problem for seismic issues is the very low
frequencies. This is a consideration you have to plug in to your system
design.

There are two basic ways of doing the balanced line concept. You can use
"Twinax" coaxial cable and connections
(http://www.altex.com/Belden-Plenum-TwinAx-Electronic-Instrumentation-Cable-100-Ohm-89207-BEL-P144273.aspx.)
This is very expensive - I truly mean EXPENSIVE! The other way is with
Belden Belfoil wire. This a twisted pair wire in a foil covering with a
drain wire for the ground. For most audio work this performs nicely. Another
way to do this is to get high quality studio microphone cable. Use only good
cable no matter which way you go. You can buy the Belden cable as surplus to
save money. None of this is direct burial. If you want burial you will need
to get out your check book. PVC conduit is very cheap and you can pull a
number of wires in the same run.

If you follow the balanced line concept with good station engineering with
solid grounding and RFI isolation a lot of problems will disappear.

Power line issues: Having worked in electronic security power line issues
are a big thing. Here are some ebay items similar to what I have:
http://cgi.ebay.com/PULIZZI-PC2773-22-PLUG-POWER-DISTRIBUTION-UNIT-/160487265348?pt=LH_DefaultDomain_0&hash=item255dc95444

http://cgi.ebay.com/Pulizzi-Z-Line-T982A2-Power-Distribution-Unit-New-/350401176294?pt=LH_DefaultDomain_0&hash=item519589a2e6

If you are building your own distribution system I might suggest:
http://cgi.ebay.com/CORCOM-20VR6-POWER-LINE-RFI-FILTER-20A-120-250VAC-NEW-/370316412063?pt=LH_DefaultDomain_0&hash=item563894049f
or
http://www.alliedelec.com/search/searchresults.aspx?N=0&Ntk=Primary&Ntt=feed+through+capacitors

For incoming signal lines I suggest
http://cgi.ebay.com/100PF-500V-FEED-THROUGH-CAPACITOR-blb151-/180250623900?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item29f7c63f9c.
These are feed through capacitors. I have used these in military
applications many times. They work and keep the junk off the signal line.
They are also used on balanced lines in some applications.

Right now I am restoring older transmitters and repairing test equipment. So
it is essential I keep everything "electrically quiet".

Jim











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