Re: [SI-LIST] : Hopefully not a controversial question ...

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From: MikonCons@aol.com
Date: Thu Feb 24 2000 - 14:11:04 PST


Mike Benson:

To describe all facets/considerations of using moating would take an hour of
verbal discussion, but here are a few basics.

A "moat" is simply placing an isolating dielectric ring around either an area
you want to isolate from other noise sources on the same printed circuit
board (PCB) or, conversely, to contain a noisy circuit from propagating and
interfering with other susceptible circuits. The moat is commonly referred
to as an "island." One uses a "drawbridge" to provide a DC connection to the
island for power. Either the power plane, the ground plane, or both planes
could have the isolation slit; i.e., where copper has been removed. The
drawbridge implies access to some frequencies and (if desired) not to others
by use of ferrite beads or other reactive means to make the connection across
the moat. Multiple drawbridges are commonly used to provide access to
different sides of the island.

The moat does not have to completely enclose a circuit. A simple slit (a
subset of a moat) in a plane can also be effective in isolating noise between
PCB sections.

All the general precautions of determining where high-speed return currents
will flow need to be addressed when slits and moats are used.

Moating also dictates different placement (and generally different densities)
of decoupling capacitors. Each moated area needs to be self sufficient in
charge storage relative to the needs of the contained circuit(s).

Slits and/or moats can be effectively used to break up PCB resonances and
mitigate common-mode radiation from many circuits.

Moats are also used to isolate signal and power planes from PCB edges for ESD
and radiated emissions mitigation, but that's another long story. I
introduced those techniques in the 1989 to 1993 series of HP seminars on
high-speed digital design throughout the US, Europe and Asia.

All of the above requires an intuitive understanding of where and how
electromagnetic fields will be generated and propagated in a given PCB.
Unfortunately, there are no simple shortcuts or rules of thumb for effective
use of these techniques as the frequency content, number of potential
generators and offenders, I/O, and other factors are peculiar to individual
boards.

A reminder... I've only scratched the surface here, but I hope it helps.

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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