[SI-LIST] : Cavity Resonant frequecy and radiated EMI

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From: KHOO,KENG-KOK (HP-Singapore,ex4) (keng-kok_khoo@hp.com)
Date: Sun Sep 24 2000 - 19:12:06 PDT


Hi all,

I have somne questions on the cavity resonant frequencies. I found a formula
in Noise Reductions Techniques in Electronics System by Henry Ott Pg.199
that says

f = 212/L,

where:
L is the largest dimension of the cavity in meters
f the lowest resonant frequency in MHz

I have a rectangular box with L=140mm so f = 1.514GHz.

There is a cable running out of the enclosure and the cable lenght inside
the enclosure is 210mm and 430mm on the outdside of the enclosure. Radiated
EMI measurement shows cable emission around 100-150MHz.

             Enclosure
      210mm | 430mm
   --------------|----------------------------------
      Inside | Outside

Ferrite core works well in reducing the 100-150MHz emission when it is put
on the outside of the enclosure but did NOT have any effect when put on the
inside portion of the cable.

Questions:
How do I relate the 1.5GHz resonant frequency to the 100-150MHz emission
from the cable?
Have I interpreted the formula wrongly?
Where did the 212/L formula came from?

Thanks a lot.

KHOO KK

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