[Elecraft] [K3] Serial interface noise issues

[Rick Bates]

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.  

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, 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.

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.  

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

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

And in the case of RF, remember what may be good DC ground isn't always RF
ground.  Ask any mobile HF operator (striving to get very low RF resistance
to ground to improve antenna efficiency).  ;-)  With RF, more grounding
(overkill) is often MUCH better and always use plenty of low RF resistant
metal because of skin effect (braid or at least multi-strand, not solid
wire).  A lot of the crud generated by electronics is actually low level RF
so it should be treated as such.  Fortunately, it also works well at DC to
audio frequencies as well.

Rick WA6NHC

-----Original Message-----
From: Jim Brown

On 11/1/2011 4:57 PM, Rick Bates wrote:
> Note that on some computers, chassis ground is NOT audio ground and I'd
> suspect the same is true of some radios (true for at least one of my
> Batwings).

It SHOULD be.  If it is not, the mfr goofed. THAT'S what a Pin One 
Problem is!  Further, unless the audio interface is BALANCED, audio 
return will always be referenced to the chassis, but it may be a lousy 
path.

>   Chassis to chassis bonding may increase noise in that case.

WRONG.  This is another of those widely held misconceptions that is 
WRONG. Chassis to chassis bonding is ALWAYS good practice, and ALWAYS 
minimizes buzz and RFI.