[RTTY] RadioShack isolation transformer replacement

[Joe Subich, W4TV]

 >> 1. Most important, place the computer and radio physically
 >> close to each other and run a short, heavy wire between the
 >> two chassis.

   - snip -

 >> 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.

The  problem with this is they *cause* a different kind of problem -
one that acts and sounds like RFI.  By providing a DC return between
the radio chassis and ground *through* the computer (or any other
device that creates audio designed to modulate the transmitter) some
portion of the transceiver power supply return current can (and will
- depending on the relative resistance/impedance) flow through the
computer chassis.  This will appear *in series* with the audio from
the computer and will modulate the input audio based on the peak
power supply current.

In addition, if the AC "third wire" is not properly bonded to the
RF ground (shield of any/all coaxial cables entering the shack),
common mode RF can be driven through the chassis of the computer to
the AC "third wire" *also* causing "RFI".  Full "galvanic isolation"
(transformer coupling for audio, optoisolators for data/control, and
separate power supplies) between computer and transceiver not only
eliminate "ground loop" hum but both types of "RFI."  Although the RFI
can be addressed by proper grounding, the use of appropriate common
mode chokes on all antenna cables and good power supply management
(use of *properly sized* power cables and low resistance connectors,
etc,) doing so is sometimes difficult in second story shacks and shacks
located a significant distance from the building power/utility entry
point.

73,

    ... Joe, W4TV


On 4/4/2012 12:42 PM, Ed Muns wrote:
> Great advice and well-summarized.  Isolation transformers are a work-around
> for system grounding problems.  However, isolation transformers are useful
> when it is not easy or quick to do these two things.  I keep a supply of
> them handy for quick "fixes" when a ground loop problem arises.  Ultimately
> fixing the ground loop problem is the better approach.
>
> The same thing is true of ferrites to work-around RFI problems in the shack.
>
> Ed W0YK
>
>
>
> Bill, W6WRT, wrote:
>> 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.
>
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