Re: [SI-LIST] : Comment on Johnson's article (Now Buried Capacitance)

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Date: Mon Nov 01 1999 - 14:18:49 PST

John: (RE: Self-resonance)

The buried capacitance (BC) configuration behaves as a (very) low impedance
transmission line; hence, the phenomenon to consider/analyze is (as you
suspected) the propagation delay of perturbations between the planes. Note
that the magnitude of a disturbance caused by a sudden chip current demand
reduces rapidly with distance from the demand point as the required charge is
drawn from a capacitor whose area is increasing at the square of the radius
which is increasing at the propagation speed through the dielectric. Even
though the board will still have an electrical resonance related to the board
dimensions, the voltage perturbation reaching a given board edge is small and
the radiation resistance of the closely spaced planes is beneficially
mismatched to that of free space (120xPI, or 377 ohms); hence reduced energy
will be radiated relative to other board constructions. The energy is
reflected back from the plane edges and continues to dissipate in the

In early 1993, boards using BC were tested over frequency by Zycon/Hadco (and
I repeated and confirmed the results in my lab). Other than the resonance of
external power supply leads with the large parallel-plate capacitor that the
BC laminate creates, there were no high frequency resonances observable.
There may be some more up-to-date tests that Hadco and/or other using vendors
have performed that may shed more light on this subject, but I have not seen
this particular item addressed in any technical notes. (Responses anyone?)

BC does do a remarkable job of filtering power supply ripple and containing
frequencies from 40 MHz on up to the multiple GHz range. Since late 1992, as
a consultant and presenter, I have advocated the use of BC for high frequency
design in my own tutorials and recommended BC as a beneficial design tool in
the 1993 HP High-Speed Digital Design Symposium (presented across the US and
throughout Europe and parts of Asia). Today's high-speed systems can benefit
strongly from use of this design technique.


Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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