On Wed, 6 Dec 2006 05:34:28 EST, Telegrapher9@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
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