[CQ-Contest] Optical and galvanic isolation

[Doug Smith W9WI]

On Wed, 2006-12-20 at 21:07, Dallas and Lucy wrote:
> > Bill, W6WRT wrote:
> 
> > 2. Plug the AC cords of all power supplies, including the computer,
> > into one wall outlet, using a multiple jack extension as needed. Using
> > one outlet on one wall and another outlet on another wall is asking
> > for ground loop trouble if the outlets are on different circuits
> > within the house. You need a single point ground.
> 
> Single point ground?  I don't believe this exists for even a modestly 
> equipped station.  Please identify where you think the single point is.

A single-point ground is certainly a good goal, though one that's
probably impossible to achieve in most ham stations!

In a broadcast station, audio wiring is generally balanced.  The shields
on audio wiring are often connected at only one end, ensuring no current
flows in the shield.  All grounds are (often) brought back to the
electrical entrance panel, and brought to a common earth ground at that
point.  If the transmitter is in the same building, it's tied to the
same ground.  If it's in a different building, the connection between
the two is often not conductive.  (either it uses microwave radio or a
non-conductive optical fiber)  (I've worked at stations where a
hard-wired connection existed between the offices and the transmitter. 
And regretted it every time the tower got hit by lightning, a common
occurrence when your tower is 400m tall!)

Anyway, the point is that if there are two different ground paths
between two pieces of equipment, a circuit is created and AC current can
flow.  Not enough current to damage anything. (usually)  But you've got
a LOT of audio gain in your equipment, especially in the mike circuit,
so it doesn't take much current to get something that can be amplified
into a real problem.  You want, where possible, to ensure there is one
and only one ground connection between any two pieces of gear.  With our
unbalanced audio interfaces, this is probably not possible to achieve in
many cases.  However, often a well-placed transformer can break up an
unwanted ground circuit.  (and since we aren't nearly as demanding of
frequency response as a broadcast station, we don't have to spend
obscene amounts on our audio transformers)

Given the amount of trouble I've had keeping hum & noise out of
broadcast signals even with careful attention to grounding, I'm *AMAZED*
the typical SO2R ham station (including mine) works as well as it does!
-- 
Doug Smith W9WI
Pleasant View (Nashville), TN  EM66
http://www.w9wi.com