[SI-LIST] Re: Bypass vs Decoupling capacitors

Chris,

What I am saying is that we don't want signal current exciting the cavities 
formed by the Vcc / Gnd planes.  So, we want to insert a series ( 
decoupling ) impedance between the power distribution, ( even if it is 
planar ) and local Vcc islands for ICs.  The price that we pay for this is 
that the decoupled Vcc planes are no longer suitable as signal image 
current returns which is precisely the point.  So just like "the old days", 
early logic, or as Mike brought up vacuum tubes, we keep the signal 
currents out of the global power supply interconnect, and instead restrict 
return images to ground.

So, now the decoupled power islands need bypassed power across a frequency 
range drops to that of what is needed to support PDS requirements, rarely 
anything over 100MHz, because of the device package inductance.  That is 
equivalent to a 3ns Tr, stuff we haven't seen since nearly 12 years ago, 
when ironically, Rogers still made good money selling their bus-bar 
products for use on PCBs.  Given that more and more power is locally 
regulated, the existance of global Vcc planes is diminishing.  So more and 
more we see the questions about crossing plane splits.  Former first lady, 
and noted signal integrity expert Nancy Reagan declares "Just say 
no!"  Transport signal return images on ground planes as God 
intended.  Decoupling costs go through the floor, as do headaches 
associated with trying to calm resonant cavities being pumped at 
frequencies well above their SRFs and without paying Hitachi or Sun money 
for their patents to try and make planes look like Tx lines.

Regards,


Steve.
At 12:20 PM 8/25/2004 -0700, Chris Padilla wrote:
>Steve,
>
>You've piqued my interest here since I'm an EMC guy.  Are you saying that
>because of low-impedance planes, we get too much energy sloshing around and
>not getting absorbed so you like to put in some kind of R value in some
>fashion (expound here, please) to absorb that energy.
>
>Okay, I just read a lot into the below two paragraphs so set me
>straight.  I'd like to understand better how I can use Rs in a decoupling
>network.
>
>Thanks----->Chris
>
> >The advent of low-impedance planes did away with the need for series
> >isolation in most digital circuits, so with the series element gone, all
> >that was left of the decoupling network was the shunt capacitor on the load
> >side which looks and acts like a bypass capacitor, because it is one.  But
> >the decoupling term got carried forward.
> >
> >Since, I am a fan of putting the series impedance back in decoupling
> >networks as a way to dramatically reduce cost and improve EMC performance,
> >I like to use  the terms the way they were 30 - 35 years ago.
> >
> >Regards,
> >
> >
> >Steve.
>
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