[RFI] DigiKeyer II

Jim Brown jim at audiosystemsgroup.com
Thu Dec 19 15:25:44 EST 2013

On 12/19/2013 11:50 AM, Ed Douglass wrote:
> I would like Jim Brown to respond:  does running fat copper from each piece of equipment to a bus bar (as Peter Laws says he is doing) comply with your direction to run fat copper between each piece of equipment and the one next to it?  (Bonding links become too long at RF?  Creating loops which have inductance and in effect, become receiving antennas? Creating multiple DC paths between components because of cable shields?)

All three issues are at play -- minimizing hum and buzz requires very 
low resistance. Longer conductors begin acting as antennas.

The most common root cause of RFI is "The Pin One Problem," so named 
because it was first discovered by audio professionals on the mic inputs 
of equipment.  Pin One of the XL-connectors used for balanced audio is 
the shield contact.  The only proper connection for a cable shield is to 
the shielding enclosure. A Pin One Problem exists when the cable shield 
goes somewhere else. The most common example of a Pin One Problem is a 
connector mounted to a circuit board, with the shield contact going to 
circuit common rather than to the chassis. When shield current is 
present, it wanders around signal common until eventually it finds a 
return path, usually the power system green wire, or to some other wire 
that takes the chassis to "ground."  As that shield current wanders 
around, it creates IR (or IZ) drops at random points around circuit 
common, and that gets added to the signal. If the shield current is hum 
and buzz, it's added directly. If the shield current is RF, it is 
detected by the input of every gain stage that sees the IR or IZ drop, 
and then amplified.

When we bond chassis to chassis, we are, in effect, shunting current 
that could be on the cable shield to the bonding conductor instead, so 
it never gets inside the box.

There's an additional coupling method when unbalanced wiring is used. 
Here, whatever voltage difference exists between one chassis and another 
is added in series with the signal. A common source of voltage 
difference is leakage current flowing on the green wire. Most of this 
leakage current consists of triplen harmonics (3rd, 6th, 9th. etc.) of 
the power frequency, and is heard as buzz. Bonding gear from chassis to 
chassis shunts this current away from signal circuits, and to be an 
effective shunt, the resistance of the shunt must be much lower than the 
resistance of the cable shield. Thus, short, fat is critical. The good 
part here is that chassis to chassis bonding is a simple and effective 
solution to both mechanisms, and is entirely consistent with good 
engineering practice for lightning protection.

For more about the Pin One Problem, see k9yc.com\publish.htm

Interconnecting cables that excite Pin One Problems at RF should always 
be as short as practical, but also benefit from an effective ferrite 
choke. The choke must have enough resistive impedance at the frequency 
of interest to significantly reduce the RF current. That means multiple 
turns (at least 4-5 if the frequency of interest is HF.

73, Jim K9YC

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