Don,
I'm on the road so this will be short.
The big problem is with amateurs who do not take good care of
station grounding and/or use common 12V supplies for both the
transceiver and interface. Both situations allow significant
common mode currents to flow between the transceiver and the
computer (or interface) which modulates the audio (either TX
or RX).
Transformers are a convenient way to break that common mode
(ground loop) path ... something an interface designer can
not guarantee any other way.
As Chen points out, microHAM use the Bourns LPB1011 (from memory)
transformer - in current products - which is one of the better
performing transformers as long as you don't overdrive it. It
is a good 10 to 20 dB better than transformers used in some of
the low cost audio only interfaces on the market.
73,
... Joe, W4TV
On 5/14/2012 4:15 PM, Don Hill AA5AU wrote:
> 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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