I'm bringing up this discussion again because I'm a little confused and am
trying to clear something up in my mind. I always thought
it was good practice to use audio isolation transformers between the
transceiver and soundcard and not just to eliminate ground
loops. I've connected radios directly to soundcards with no problems, but
always added the transformer later because I thought it
was the right thing to do.
I understand Bill's message below and it's a good idea. And Chen pointed out
that isolation transformers have IMD and that it's
better not to use them. So is it good practice to connect the radio directly to
the soundcard without an isolation transformer if
you don't have a ground loop? Commercial interfaces, such as Joe W4TV pointed
out with the Microham interfaces, employ isolation
transformers, so why would anyone attempt to run without them?
Thanks,
Don AA5AU
-----Original Message-----
From: rtty-bounces@contesting.com [mailto:rtty-bounces@contesting.com] On
Behalf Of Bill Turner
Sent: Wednesday, April 04, 2012 10:59 AM
To: rtty@contesting.com
Subject: Re: [RTTY] RadioShack isolation transformer replacement
Another approach, if you feel a bit experimental, is to eliminate the ground
loop in the first place. Do that and no transformer is
needed at all.
The kind of ground loop that causes hum with soundcard digital applications is
caused because the chassis of the computer and the
chassis of the radio are not at the same AC potential. It takes only a few
millivolts of difference between the two chassis to cause
60 Hz AC current to flow between the two chassis via the audio cables and the I
squared R voltage drop in the cable shield causes
hum to be generated and coupled into the audio.
There are two cures:
1. Most important, place the computer and radio physically close to each other
and run a short, heavy wire between the two chassis.
This shorts out almost all the voltage difference. No voltage difference = no
ground loop current. As a bonus, this wire will often
help a lot with EMI problems as well. Without this wire, the cables between the
computer and radio form a small loop antenna. It's
usually non-resonant and not very efficient of course, but in the presence of a
strong RF field, the loop can pick up enough RF to
cause trouble. The ground wire shorts out the "feedpoint" of the loop.
2. The AC power plug for the computer and the one for the radio should be
plugged into the same AC outlet. This connects the "third
wire" of the two power supplies together at the AC source. Different AC sockets
throughout your house can have minutely different AC
potentials on the third wire ground and those differences go directly to the
two chassis.
I have used these two methods for nearly 20 years now and I have never needed
an isolation transformer anywhere.
73, Bill W6WRT
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