[Top] [All Lists]

Re: [RFI] Do we neglect problems and fix symptoms?

To: rfi@contesting.com
Subject: Re: [RFI] Do we neglect problems and fix symptoms?
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 30 Dec 2010 12:18:09 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On 12/30/2010 11:36 AM, Roger (K8RI) wrote:
> Chokes work very well in many instances, but I think
> we some times over look the problem and fix the symptoms.

My published advice on the topic of RFI is two important premises.  
First, is the assumption that with any commercial product, the best 
solution is generally one that does not require modification of the 
equipment. In other words, the most practical solution is one that can 
be applied "outside the box."  There are three good reasons for this.  
Staying outside the box is usually less expensive, it avoids warranty 
problems, and it avoids changes in internal wiring that could result in 
instability or other malfunction.  That is, if the designer was dumb 
enough to build a product with poor EMC properties, changing the way he 
has implemented circuit common might make the product unstable.

The second premise is that we need to understand the fundamental 
mechanisms that are coupling RFI so that we can not only diagnose which 
one(s) are coupling RFI in a given system and apply suitable fixes, but 
also so that designers of equipment can avoid those design errors in 
future products.

It is generally well known that the most common causes of RFI are Pin 
One Problems, poor internal shielding (and/or poor layout of circuit 
boards and internal wiring), poor interconnect wiring (poorly shielded 
cable, or paired cable that is not twisted), and inadequate filtering of 
inputs and outputs. In some cases, it takes two errors to excite the RFI 
-- poor interconnect wiring PLUS poor filtering.

While few would argue that eliminating a Pin One Problem is a more 
direct way of eliminating it, most products are built in such a way that 
it is not practical to do so without major surgery. The same is true of 
shielding and circuit layout errors, and the addition of filtering 
components. With most real world products, the only practical fix for 
ALL of these design errors is at the design stage.

Thus, the most practical "in the field" fixes for most RFI are 1) good 
common mode chokes on all wiring that could act as an antenna, 2) robust 
braid shields on all unbalanced wiring, and 3) twisted pair for anything 
else. And, of course, good engineering practice includes 
chassis-to-chassis bonding of interconnected equipment in close 
proximity, especially when the connections are unbalanced.

73, Jim Brown K9YC
RFI mailing list

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