Bob,

I'll try my best. I usually place the listeners in Boston or New York. .They
are in Woburn and they get a call from their best friend to inform them that
he won a million dollars, and will share it with them. But, they must get to
Logan airport at exactly 6:00pm. So we set out for the airport and the first
thing that happens is a traffic jam on RT93. We finally get through that jam
only to get stuck in the Callahan tunnel. Well, to make a long story short,
we finally get to Logan airport at 7:00pm, only to see our friend waving
good-bye from the airplane as it taxi's down the runway.

A week later, we get another call with the same type situation. But this time
we're smarter, and contact the state police, to set up road blocks. These
roadblocks get us throught the RT93 jams. We also have them clear a lane
through the tunnel and control all the traffic lights to the airport. We get
to the airport on time, and receive our windfall.

What we really did was remove the impedances from our path, that prevented us
from getting to point "b" at a particular time. Controlled impedance is very
similar. There are a number of things, such as the properties of the
materials selected (dielectric constant), the thickness of the copper, the
distance between the traces and the reference plane, etc, which causes a
signal to arrive at point "b", at a specific time. In todays high speed logic
circuits, certain events happen a the presence of a signal, and other events
happen at the absence of a signal.

Designing for these types signals, requires a knowledge of  materials and
effects of the board manufacturing process.

I hope this little tail helps. If you have any further questions, please feel
free to contact me.

Gary Ferrari
Tech Circuits, Inc.
Wallingford, Ct
203 269-3311