[Elecraft] K3 RFI in transmitted audio

[Jim Brown]

On Sun, 12 Sep 2010 15:08:34 -0400, Joe Subich, W4TV wrote:

>Bonding unit to unit ENCOURAGES common mode currents to flow 
through
>all of the units in series.  Bonding to a single LOW IMPEDANCE 
ground
>moves the common mode and RF currents to the ground and *NOT* the
>other equipment.

This is a common misconception that results from muddy thinking. It 
is the big lie, repeated over and over again until somehow everyone 
believes it. DC and low frequency currents follow Ohm's Law -- that 
is, the path with the lowest DC resistance. 

First, the issue here is AUDIO AND DC, NOT RF, and it is the 
injection of AUDIO noise into unbalanced connections. 

Let's say that you bond all equipment to your MAGIC SINGLE POINT, 
and it takes 3 ft of wire to get there. If two pieces of equipment 
are only three inches apart and you bond directly between them, 
CHASSIS to CHASSIS, the wire might be only 6 inches long. Assuming 
the same size wire, the path through that MAGIC SINGLE POINT has 12 
times the resistance as compared to the direct path. That's 22dB 
MORE NOISE (20 log of the voltage, and the voltage is proportional 
to the resistance) that any circulating current would add to an 
unbalanced connection between that equipment. 

Second, the bonding together of equipment at RF is for LIGHTNING 
SAFETY protection. It is NOT part of a solution to RFI or RF noise. 
What matters is that everything be maintained as close to the same 
potential as possible, and that is achieved by LOW INDUCTANCE bonds, 
and by bonding EVERYTHING together. 

Providing a low resistance bond between interconnected equipment 
also puts a band-aid on low frequency pin 1 problems, because most 
current that flows between equipment takes the lower resistance 
chassis-to-chassis bonding path rather than the higher resistance 
path via cable shields and onto the signal return bus. 

For those who haven't heard, a "pin 1 problem" is the connection of 
a cable shield to internal wiring of equipment rather than to the 
chassis. The only proper connection of a cable shield is the 
CHASSSIS. When noise current flows on a signal return trace, it gets 
added to the signal chain at one or more points, where it appears as 
noise (hum, buzz, or that modulation of the V- rail), and if it's 
RF, it will be detected by the first semiconductor junction it 
encounters. Pin 1 problems are the primary causes of hum, buzz, and 
RFI in virtually ALL systems. When you see a connector whose shell 
is insulated from the chassis, or where a mic shield goes to "audio 
ground," it's a "pin 1 problem." It's CALLED a pin 1 problem because 
this issue was first clearly described and understood in the pro 
audio world, where the shield contact of an XLR connector is pin 1. 

73, Jim Brown K9YC