[Elecraft] [K3] Serial interface noise issues

[Jim Brown]

On 11/1/2011 6:58 PM, Rick Bates wrote:
> Not always.  It may be considered poor design but if the in/output goes
> directly to a differential amp with little or no 'padding' to chassis
> (creating the comparison to 'different'), it may have a floating ground and
> actually cause problems if connected to ground.

You have described a BALANCED input.  I'm specifically talking about 
UNBALANCED inputs and outputs.

> In the case of Motorola radios, you'd have to ask them why, but never assume
> that either side of a speaker (for example) is chassis grounded,

I'm talking about UNBALANCED outputs and inputs for line level 
interconnects, not loudspeakers.

> or you'll
> blow up a very expensive audio amp (like on the popular GM300).  A careful
> resistance test should be done before connecting audio ground to chassis.

Such an output should NOT be used to interconnect equipment.

> In general, we agree that chassis to chassis bonding SHOULD be encouraged to
> reduce noise (and is electrically safer having everything at the same ground
> potential).  But it does NOT always solve problems.

You are badly mistaken. Indeed, NEC REQUIRES that every chassis 
connected to mains power be bonded to ground, and that ALL grounds be 
bonded together.

> I've seen cases where one (presume a computer to radio) chassis-chassis
> connection made the problem worse (ground looped hum), because it introduced
> more noise into the radio, even if plugged into the same power source
> (should have been equal grounding already).

I suggest that you study my tutorial on audio interfacing that is part 
of my RFI tutorial, and also the Power Point on Ham Interfacing, both of 
which show WHY power-related noise is coupled into unbalanced 
interfaces, and WHY chassis-to-chassis bonding kills it, and without the 
need for a transformer.

> Audio isolation transformers
> were the solution, pass only the audio.

They were YOUR solution, but they are not the only solution, they are 
not the simplest solution, and they are not the lowest cost solution.

> It might be against common practice, but reality trumps theory.  Chassis to
> chassis grounding may have any range of results (but certainly try it).

Not if you do it thoroughly.

> And in the case of RF, remember what may be good DC ground isn't always RF
> ground.

It is a MAJOR mistake to treat audio, RF, and power differently.  It is 
this fuzzy thinking that CAUSES  the Pin One Problem, and fails to get 
to the heart of what is really going on.  Again, I suggest that you 
study my tutorials on this.  I'm not just "some guy with an opinion," 
I'm chair of the Tech Committee on EMC of the Audio Engineering Society, 
vice-chair of the AES Standards Committee WG on EMC, and principal 
author of all existing AES Standards on EMC. I had my first job in 
broadcasting in 1959, and I've designed and used a very wide variety of 
audio systems, from the very large to the very small, in a wide variety 
of situations, including VERY high RF environments. In other words, I 
know quite a bit about the real world.

73, Jim Brown K9YC