Topband: plasma TV RFI?

[Jim Brown]

On Wed, 6 Dec 2006 05:34:28 EST, Telegrapher9 at aol.com wrote:

>Let's say it is 30 pF to ground. That is -j3000k ohms at 1.8 MHz. And let's 
>say the wire to ground is 0 ohms. The loop impedance is -j3000 ohms. Now let's 
>say we add a 100 uH CM inductor in the wire-to-ground end. The CM inductor is 
>+j1000 ohms. The loop impedance is now -j2000 ohms. We have just increased the 
>CM current and the EMI. Before we see any improvement the CM inductor 
>impedance must be over 6 k ohms. We must go to 9 k ohms before we see a 6 dB 
>reduction in the CM current. To obtain +j9 k ohms we need 800 uH. Using a ferrite 
>toroid having a mu of 500 and a length of 1" we need to loop the 'wire' through it 
>about 9 times. That seems doable. With a +9 k ohm inductor we are probably 
>around parallel resonance, so that may help. But 6 dB has reduced the EMI by 
>just one S-unit.

Well, sort of. You will find a more realistic version of this analysis in the 
tutorials and AES paper that are on my website. I call this "threshold effect" for 
want of something better. That is, the impedance you add must be sufficient to make 
a significant dent in the current. 

BUT: this analysis misses entirely the most fundamental principle of using ferrite 
chokes -- for suppression, we want RESISTANCE, NOT reactance. Resistance ALWAYS 
reduces the current, but reactance can increase it, as you've shown. 

A ferrite choke is inherently a parallel resonant circuit (and for some materials 
and/or chokes, two parallel resonances in series). When we wind multiple turns 
around a ferrite core, we multiply the values of both R and X below resonance by 
the square of the turns ratio, AND move the resonant frequency down. The graphs of 
the measured data clearly show this. Chokes wound on #43 material have only one 
parallel resonance. Chokes wound on #31 have TWO parallel resonances in series, so 
they have greater suppression bandwidth (roughly one octave more). The upper 
resonance of #31 is the same as for #43. The second resonance is lower in 
frequency, which is what gives us the greater bandwidth (and what makes #31 the far 
superior material below about 7 MHz). 

One of the major misconceptions about the use of ferrite chokes is that we are 
using their inductance to solve problems. WRONG! It is their RESISTANCE that does 
the heavy lifting. 

Again, I suggest that you study my AES paper and tutorial. My colleague spent a lot 
of time measuring, and I spent a lot of time studying both the data and the 
literature. 

http://audiosystemsgroup.com/publish

73,

Jim Brown K9YC