[Elecraft] K3 RFI in transmitted audio

[Joe Subich, W4TV]

 > 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.

And the proper low impedance ground fits that requirement but the
common point must be proper - not just a couple of ground rods.

 > 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.

That's fine with two pieces of equipment that are physically close
to each other.  However, if one has several pieces stretched along
a long bench with a power supply at one end, bonding box to box
could conceivably cause common mode currents to flow through all
of the boxes causing who knows what kind of abnormal operation -
and pin 1 problems!

 > 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.

But that's only a band aid.  Address the issues by properly connecting
the cable shields to the chassis and bonding the the signal returns
to the circuit common properly (e.g. removing things like RF chokes
between signal returns and the chassis).

73,

    ... Joe, W4TV

On 9/12/2010 4:52 PM, Jim Brown wrote:
> 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
>
>
>
>
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