Topband: plasma TV RFI?

[Telegrapher9 at aol.com]

Jon,

Tom's experience with plasma TV's supports my theory of the coupling 
mechanism. He found that common mode impedance was not very effective. 

Theory: A possible equivalent circuit for the plasma TV EMI is an RF source, 
a capacitor to ground at one end, and a wire to ground at the other end. We 
have a loop with 30 pF in series. The CM (Common Mode) current along the wire 
radiates 1.8 MHz energy. The 'wire' is the AC power ground wire and/or the TV 
signal cable. The capacitor is the plasma TV screen capacitance to ground.

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.

But it is worth a try. I would start by disconnecting all cables, except of 
course the power cable, from the TV. Connect a battery operated source of video 
if that makes the TV happy. If the problem still exists then attack the power 
cord with a big, high-mu ferrite core. Using multiple cores taped together 
will help. Two cores give 2X the inductance of one core. When the EMI is lowered 
to an acceptable level connect the TV signal/video cable. If the EMI comes 
up, attack it the same way. Looping the power cable and the video cable through 
the same ferrite is the best from an EMI standpoint but might be difficult to 
do. 

If this helps but does not lower the EMI to an acceptable level then we need 
to resort to unusual methods. At that point I can build you an active CM 
"choke." I think we can shoot for a reduction in CM current of 20 dB using this. 
Maybe more. I can build it up and get it working if you'd like to try it. 

    Dave WX7G