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

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From: S. Weir (weirsp@atdial.net)
Date: Sun Sep 24 2000 - 23:41:44 PDT


Keng-Kok,

The resonant frequency of the box gives some measure of how difficult
common mode suppression will be at certain frequencies. The closer you get
to the box resonance, the less attenuation you will get. However, your
problem does not sound like box resonance.

100-150MHz emission is pretty broad-band. Usually, emissions will line up
either on clocks and their harmonics, or on carriers.

There are a couple of reasons that ferrite inside the box didn't
work. Clamp-on ferrites work to defeat common mode noise between the sum
of the conductors, and shield in the cable and earth. If your cable is
shielded to the box beyond the ferrite, then you get attenuation between
the box, and the driving end of the cable. But, since the ferrite is
inside the box, you do not get attenuation between the box common mode and
earth.

If the cable is not well shielded to the box, then the answer may be in a
brain-teaser presented by Doug Smith about 8 months back. Your 430mm cable
extension represents close to 1/4 wavelength in the 100-150MHz range for
Cu. By placing a ferrite on the cable at the box exit, the cable has a
high impedance at that end, and the 1/4 wavelength causes an impedance
inversion at the other. So, the free end of the cable ends-up with a low
impedance which does not couple well to free-space, and your EMI gets
attenuated. If you double your cable length on the outside, you may find
that you EMI goes up by more than 10db, with the same ferrite configuration.

Regards,

Steve.
At 10:12 AM 9/25/00 +0800, you wrote:
>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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