Slavek the issues with ground fences:
1. The reflected energy hits vias which provide a path to the top of the
board and features that radiate.
2. The decoupling response exhibits a series of peaks and valleys on
quarter wave multiples.
An ideal plane termination would absorb all the energy and eliminate both
effects.
Steve.
At 09:07 AM 12/4/2004 +0100, Slawek Guzek wrote:
>Dear Zhangkun,
>
>[....]
>
> > According to the transmission line theory, when the transmission line
> is matched, there is no resonance. At the edge of
> > PCB, the EM energy is reflected because the transmission line is OPEN.
>
>I think you've made some small error in your concept. Indeed, EM energy is
>reflected IF line IS open,
>but at the end of the PCB line in not perfectly open - ground plane and
>power plane form a radiating
>element - a kind of a dipole. Some of the EM energy is radiated outside,
>because of variable E field
>lines at the PCB edge closed through air from power to ground plane
>
>
> > Traditional decoupling capacitors are of low ESR,
> > which could not damping the resonance. Therefore I need capacitor with
> high ESR, whose ESR should be matched with the
> > transmission line. On the other hand, the ESL is critical at high
> frequency. The ESL should be as small as possible. The
> > package of capacitor mainly determines ESL so that the package should
> be as small as possible, such as 0402 or 0603. The
> > capacitance should be as large as possible so that some benifit could
> be gotten at lower freqeuncy domain.
>
>
>It is much easier get close to perfect short circuit than close to perfect
>open, so in my opinion,
>from EMI point of view, it could be much more efficient to finish power
>plane with some margin to
>PCB edge, and fill up this space with vias connected to ground plane,
>spaced not more than 1/16
>(one says even 1/100) of shortest wavelength you expect to be excited
>between power and ground planes.
>That case RF energy really reflects back into PCB, noises on traces
>perhaps would be greater,
>but significantly smaller portion would be radiated.
>
>Regards,
>Slawek
>
>
>
>
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