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Re: [RFI] Fair Rite #31 Mix Material

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Fair Rite #31 Mix Material
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Sun, 11 Jan 2009 20:07:47 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On Sun, 11 Jan 2009 20:40:59 -0700, Larry Benko wrote:

>A good first order approximation for all cores is:

>Effectiveness = (n^2)*u*length*log(OD/ID)
>where n^2 = #turns squared , u = permeability (43 matl.=850, 31 matl.= 
>1500), length = dimension that is perpendicular to the hole, OD and ID 
>are the outside diameter and the hole diameter (all in same units).

This simple formula applies ONLY in the frequency range where the 
equivalent circuit is R + L in series. That circuit applies at low 
frequencies, in the spectrum where the part is used as an inductor. A 
single turn (that is, a wire running through a core once) of #31 or #43 
resonates in the 150-200 MHz range, depending on its dimensions. 

Ferrites are used for suppression in the spectrum where their 
equivalent circuit is a parallel resonant circuit of very low Q. My 
website includes an AES paper showing my development of that high 
frequency equivalent circuit, complete with a lot of measured data for 
toroids of a common form factor with different materials. It has long 
been widely known (but often lost sight of), that when used for 
suppression, we're looking for RESISTIVE impedance, not inductance. My 
paper and several associated tutorials (one written for hams, another 
for pro audio people) develops this notion and shows why it is true, 
but I have seen it stated in literature that dates back several decades 
and I suspect the concept is far older.  

The website is http://audiosystemsgroup.com/publish 

>I am probably preaching to the choir but the impedance of the circuit
>is just as important as the impedance of the choke when it comes to 
>reducing common mode currents.  

Absolutely, and that impedance is virtually always complex. My tutorial 
develops that concept, showing, among other things, that an inductive 
choke on a capacitive length of cable INCREASES the current.  

73,

Jim K9YC



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