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Re: [RFI] Ferrite Beads from DX Engineering

To: "rfi@contesting.com" <rfi@contesting.com>
Subject: Re: [RFI] Ferrite Beads from DX Engineering
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Mon, 11 Jan 2010 15:10:46 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On Mon, 11 Jan 2010 16:45:22 -0600, Peter Laws wrote:

>Well, I mean not "really" as in "you don't know what you're talking
>about" but as in "wow, the SAE types spend a lot of time getting those
>rise-fall times just right so that the car meets the applicable
>standards for fuel consumption and emissions" ...  Although not
>mandated, there are also drivability concerns as well ...

I can't comment on the automotive aspect of the design problem, but the 
electronics and EMC part is solid and well known. 

>Just how much would chokes change that?  

That's hard to predict. It will certainly vary with the plug circuitry 
and the ferrite part(s) being used. HOWEVER -- it is also well known 
that rather small changes in the switching waveform can make a large 
difference in harmonic content. If I were working on the problem in a 
vehicle, I'd have a scope with a piece of wire attached to its input 
set up next to the engine and watch the waveshape as I worked on the 
plug wires. Even better, a spectrum analyzer. 

>And how does this compare to the old "resistor plugs" that were sold
>to make the ticking on the car radio go away?

Same fundamental idea, except that a ferrite part has much less effect 
at DC and low frequencies than does a resistor, and has more effect at 
high frequencies. In other words, ferrites can probably provide more 
suppression with less effect on motor performance. 


Jim Brown K9YC

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