FreeLists / si-list / [SI-LIST] Re: Why is capacitor with high ESR

["Slawek Guzek" <sguzek@xxxxxxxxx>]

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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