[SI-LIST] the effect of box on EMI

Dear all

In the recent test, I found out that the effect of metal plane on EMI is 
obviously.

Details:
  There is a product with only one PCB of 4 layer. The EMI of the PCB is 
measured at first. Then the PCB is put on a metal plane. The EMI is lessen very 
much(>6db).

What is the theory of this case? The second and more important question is how 
to apply the effct of the metal plane by means of modification of the PCB, 
while without additional cost.

Best Regards

Zhangkun
2008.7.29 

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