Re: Doubt about decoupling capacitors in high-freq PCBs

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 02/22/05 

Date: Mon, 21 Feb 2005 21:50:41 -0800

"Rick" <rick@nospam.com> wrote in message
 news:vozSd.29895$911.1218@fe2.texas.rr.com...
[Apparently quoting John Larkin]
>> If you do a multilayer board with a thin dielectric between the ground
>> plane and power planes (or even power islands) it doesn't matter much
>> where you put the caps; the planes themselves are a superb capacitor.
>> I sometimes add a few SMA connector footprints to a board layout so I
>> can TDR the planes. As near as my Tek 20 GHz TDR can tell, a typical
>> power/ground plane pair looks like an ideal capacitor, and adding
>> bypasses anywhere just makes it look like a bigger ideal capacitor.
>>
>
> But isn't the point of decoupling CAPs twofold - to stabilize the energy
> source and to shunt higher frequency signals to ground ? Wouldn't having
> only a large capacitance value from the physically large power and ground
> planes be counter productive to that ?

But the point is, the high frequencies need not reach
the larger discrete capacitances. They are shunted
thru the nearby dielectric between the planes.

The fact that higher frequencies are successfully
shunted can be observed in the low magnitude of
voltage disturbances arising from the impulse-like
load currents that accompany output edges. 

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

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