> Carolyn Dan Harris wrote:
> 
> Can someone recommend a good reference book for intercom wiring?  What
> is the spec/nomenclature of the shielded cable?  Typical circuit
> schematics?  Dos and don'ts?
> 
> I work in electronics so I just need some direction.  My Citabria has
> an alternator squeal that I suspect was "installed" by the last person
> to wire it (before my ownership).  Inspection of shielded cables to
> the headset plugs revealed a ground *inside* the shield.  Yuk!

Dan,

The correct way to install an intercom system in an aircraft is to build
(and test) the entire
wiring harness, jacks, etc on a wooden-top kitchen table, without
relying on "grounding" any of the jacks to anything. If it works on the
table-top, it should also work when you mount it in the 
airplane. A properly installed intercom uses plastic "shoulder" washers
to electrically isolate all of the jacks from the airframe (more on this
later).

All of the barrels of the Mic and Phone jacks are returned to a
"floating" common tie point near the rear of the intercom (see the
intercom installation schematic). Assuming a four-station intercom,
there will be four pairs of jacks; pilot, co-pilot, pass 1 and pass 2.
It is ok to run a single common return wire from each jack pair, or you
can separate the mic-lo from the phone-lo and run two wires. Even if you
separate them, they tie to the "floating" common tie point near (or
inside) the intercom, anyway. The common-mode coupling along a common
return line from mic to phone doesn't cause me any problems.

Shielding is optional. If you want a "shield", it has to be a separate
conductor which encloses
all other wires. If you are running a shield, then dont connect it to
anything at the jack end (leave it float); connect it to the common tie
point only at the intercom end. Alternatively, you can use the shield on
shielded wire as the return for the mic-lo and phone-lo as described in
the previous paragraph, in which case it does have to tie to barrel of
the respective jacks. 

In some installations I have used three conductors inside a shield
running to each station: Mic hi, PTT, Phone Hi, with Mic Barrel and
Phone Barrel returned through the shield, and it works fine. A more
delux version would be four conductors and a shield: where Mic barrel
and phone barrel are on the fourth wire, and the shield carries no
signals. An even more delux version is separate shielded wires for mic
and phone, with or without the shield being used to carry any signals.

Now that the entire harness is complete on your wooden table top,
including the common returns from all of the jacks, it is necessary to
mount it in the airplane. As I mentioned earlier, the four sets of jacks
(eight jacks) all need to be kept electrically isolated from the
airframe. PS engineering and other intercom companies provide the
plastic isolation washers with their intallation kits. Keeping the jacks
from "grounding locally" at the jacks is what this is all about. The
only ground path from each jack must be back to the floating ground at
the back of (or inside) the intercom.

For example, see http://www.ps-engineering.com/diag-pm1000-11902.html

This diagram shows that all of the jacks are connected to the rear
connector of the intercom, and there is only one ground connection from
the intercom to the air frame (see the shaded-in diamond ground symbol
on the diagram.) 

This is the key to prevent the dreaded alternator whine and strobe
squeal from coupling into your intercom audio. Any installation that
allows the jacks to touch the airframe metal, or actually relies on the
headphone/mic signals to flow along the airframe back to the intercom
will have common-mode noise coupled into the mic and/or the phone
circuits. 

Another name for coupled-common-mode noise of this type is
"ground-loop", and is caused when the rather large alternator and strobe
currents which already flow along the airframe take a short cut through
your intercom audio wiring. The act of floating all of the jacks, and
using a "single-point-ground" breaks the ground-loop, and keeps you from
hearing the whine and squeal. 

The reason that I said the shielding is optional is because in my own
Skylane, none of the intercom
wiring is sheilded, but all of the jacks are isolated, and I have not a
trace of whine or squeal.
I have a kit-built RST intercom installed. 

I just bought a Piper Pacer with a Signatronics four-place intercom
installed. It has a lot of alternator whine in the intercom. Sure
enough, all eight of the jacks are tied directly to the airframe (no
isolation washers on the jacks). As soon as it gets warmer in my
unheated hangar, I am going to isolate the jacks, and make sure there is
a separate return wire from the barrels of the respective jacks back to
the rear of the intercom. Interestingaly, Signatronics provides the
shoulder washers with their jack kits, but their installation
instructions show the barrels of the various jacks connected to
airframe. This is WRONG, WRONG, WRONG!

Isolating the jacks from ground will cure 98% of the problem. Shielding
may (or may not) do anything
at all.


MikeM
Skylane '1MM
Pacer '00Z