[Elecraft] DX Doubler/K3 issue

[Tom W8JI]

> There are two common causes of this problem. One is the
> potential difference between the two radios. Bonding and 
> using
> the same outlet will fix that.
>
> The second common cause is a magnetic field that couples 
> into
> the loop between the two radios and the DXDoubler. That 
> field is
> most often is established by a big power transformer very 
> close
> to the rig (either the linear supply for the rig(s) or the 
> power
> transformer for the power amp. The field is coupled to the 
> audio
> two ways. First as the induced voltage, and second as 
> current
> flowing into a pin 1 problem at either (or both) end(s).

There actually is a much more common and much more severe 
issue that gets almost totally ignored, dc current paths 
from external power supplies.

The high supply current radios draw causes  a ground loop 
when we use low voltage supplies that have the negative 
supply voltage grounded at the power supply or any other 
point in the system other than at the radio. When the radio 
has any sort of amplitude varying RF output, the rapidly 
varying dc input current creates a pretty good voltage drop 
across the negative supply lead. Since that lead is grounded 
at more than one point, it drives the supply negative to a 
different potential at different points in the system, and 
the rig's chassis and other chassis "wiggle around" at 
different potentials at the RF envelope's modulation rate as 
PA current varies from quiescent to 20 amps or more peak 
current on envelope peaks.

The effect is almost indistinguishable from rectification of 
the RF envelope. As a matter of fact many blame this problem 
on RFI because it sounds like RFI even though it is "AFI".

While bonding all the chassis helps a great deal, the real 
issue is a design shortfall in the negative rail wiring of 
the power supply and our equipment. The best fix for this 
problem is to not have common chassis grounds on low voltage 
sensitive signal leads, a good design isolates the sensitive 
signal leads with isolation transformers. This problem is a 
whole lot like the problem we create in vehicles when we run 
the negative radio lead to the battery negative post. People 
designing gear have to learn to not create a harmful path. I 
can't imagine having an audio interface device that connects 
audio and signal lead grounds all together at low 
frequencies or dc!

The voltage induced this way is typically hundreds of times 
higher in magnitude than voltage induced by flux leakage 
from power transformers. Worse yet, it is an extremely low 
impedance source making it very difficult to "bypass".

73 Tom