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
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