Date: Fri Nov 17 2000 - 12:09:15 PST
I've simulated and verified in the lab the effects that you are worried
about. I have seen that X7R caps provide a smoother decoupling, due the fact
that the higher ESR redcuces the Q of the circuit. The capacitors and their
internal inductances, as well as the inductance of the power planes, can make
a field of resonators which can interact severely to produce a poor
decoupling network. For instance, one power-ground sandwich had an impedance
that rose above 25 ohms at 60 MHz, as a result of using NP0 dielectric.
Obviously, in the case of a very large di/dt requirement, the NPO capacitor
is going to do a better job of supplying the current due to it's lower ESR.
An alternate approach would be to supply the current through many more X7R
I modelled the power-ground sandwich as a group of horizontal and vertical
interconnecting transmission lines using Pspice. Lab results using a
network analyzer proved the accuracy of the model.
Richard Ellison, consultant
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