[Elecraft] [K3] Serial interface noise issues

[Jim Brown]

On 11/1/2011 4:51 AM, Thomas Horsten wrote:
> I guess both of these problems could be down to bad/no screening of the
> little chip in the dongle, I tried enclosing it and the cable in kitchen
> foil but with no discernible difference. Any suggestions,

The more likely cause is a Pin One Problem on one or both ends of the 
cable. The only proper termination of a cable shield is to the shielding 
enclosure at both ends. A Pin One problem, first acknowledged and 
described in the world of pro audio by Neil Muncy, ex-W3WJE, occurs when 
the cable shield goes PAST the shielding enclosure to wiring inside the 
box. This causes any common mode current flowing on the shield to wander 
around inside the box until it eventually finds a path to the chassis, 
and from there to "ground."  The Problem with this is that, as the 
current wanders around circuit common inside the box, voltage is dropped 
across the inductance of the conductors (usually circuit traces) that 
make up circuit common, and that voltage is injected into the signal 
path, detected, and amplified.  THAT is the Pin One Problem, and it is 
the most common cause of RFI in today's world.

One solution is to FIX the improper wiring, and sometimes that is 
possible, but often it is not.  The other solution is to "put a 
band-aid" on the Pin One Problem by killing the current that excites 
it.  In the case you describe, the solution is to form a common mode 
choke by winding 4-6 turns of the serial cable around a #31 or #43 
ferrite core made by Fair-Rite.  This forms a low-Q parallel resonance 
in series with the common mode circuit, which looks like a high 
resistance near resonance.  Like any other coil, we put the resonance 
where we want it by using the "right" number of turns (which sets the 
inductance) and the "right" spacing between them (which sets the 
capacitance).  The nature of the two ferrite materials I've recommended 
is that a lot of RESISTANCE is coupled from the lossy ferrite core, so 
the resonance is VERY low Q (typically about 0.5), and thus very broad.

You should also bond between the computer chassis and the K3 chassis 
using a short conductor. In the US, I would recommend  #10 AWG, which is 
0.1 inch diameter. This minimizes the hum and buzz produced by leakage 
currents in the mains power system that also excite the Pin One 
Problem.  The chassis of most laptop computers can be accessed at the 
retaining screws of a D-connector.

For (much) more information on these issues see my various tutorials at 
http://audiosystemsgroup.com/publish.htm

73, Jim Brown K9YC