[Top] [All Lists]

Re: [RFI] DigiKeyer II

To: rfi@contesting.com
Subject: Re: [RFI] DigiKeyer II
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 19 Dec 2013 12:25:44 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
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
RFI mailing list

<Prev in Thread] Current Thread [Next in Thread>