[SI-LIST] Can I help?
- From: Ralph Morrison <ralphmorrisonee@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 1 Oct 2013 21:14:35 -0700
Hello to the SI list.
I am new to this list but not new to problems of board design. I
have read many of the discussions submitted to the list and I want to
help. That is easy to say but not easy to do as I face a barrier
that I will call "language". With out establishing some rules, I can
be totally correct and at the same time be totally misunderstood. So
the problem I face is where to start so I can be effective. Each
engineer has a vocabulary and a set of mental images that supports his
or her understanding and I am going to challenge many of them. To
some of you I will be a thorn in the side and to others perhaps a ray
of hope. It is not an easy path to take as many assumptions will
have to be discarded. This is a difficult path to follow as it hits
everyone in their ego. Letting go of a weak support structure can be
tough.
Nature solves every electrical problem and never makes a mistake. We
seek ways to control electricity so we can achieve our goals. If
Nature's goals are the same as our goals then we can have our way.
Nature will not budge. To determine Nature's goals we need to define
electricity as Nature sees it. It is the electric field around a
group of electrons called a charge and it is the magnetic field
associated with a moving stream of electrons called a current. It
takes work to create a field as a force is needed to move charges or
increase the current. We have learned how to move these fields in our
circuits. We use the presence of a field to represent logic. We use
some of these fields to operate components. Under the right
conditions these fields can leave the confines of conductors and
radiate at the speed of light. These fields are electricity. These
fields are what Nature pushes around. To understand Nature we must
understand these fields. Below 10 MHz we could get away using
circuit concepts. In todays GHz world, circuit ideas can lead us
astray. We need to use fields as Nature does not read schematics,
follow diagrams or read color codes. The trouble is field theory can
be very complicated and Nature could care less. Avoiding fields
because of these difficulties is the big problem. There are ways to
appreciate fields without getting wound up in details. It need not
be complicated. It takes a change in approach.
In physics once a principle is accepted it must be universally
accepted. Light from the sun is the flow of electromagnetic field
energy. There are no wires. This means that fields carry energy.
There are no exceptions. There are no frequency limitations. This
means that utility power is carried in fields. The energy from a
battery flows in a fields. The purpose of conductors is to direct
where the energy flows. Conductors also act to steer fields away
from critical areas. Without this viewpoint the true nature of the
problems in board design will not be explained.
To illustrate the problem consider the word "impedance". It is used
in electrical engineering to calculate current flow for sine wave
voltages when reactances are involved. The term is basic to circuit
theory. When transmission line theory was developed it was a world of
sine waves. Today with logic signals having picosecond rise times
the word impedance has survived even when individual sine waves are
not involved. We obviously need some general way to express
opposition to current flow but a new term has not evolved.
In transmission line theory we call it characteristic impedance.
This unit measures the ratio of current to voltage for a step voltage
applied to a pair of conductors. It is correctly the square root of
the ratio of inductance to capacitance per unit length. The
capacitance can store electric field energy and the inductance can
store magnetic field energy. The characteristic impedance is the
ratio of a possible E or electric field and the H or magnetic field at
a given point in space even if no fields are present. It is a
measure of conductor geometry. But few of us accept this definition.
By matching impedances (characteristics) we are providing a smooth
transmission path for fields to follow.
My new book Digital Circuit Boards - Mach 1 GHz published by John
Wiley takes these field ideas and explains what really happens on a
circuit board.
I close by giving you food for thought. A baby carriage can go 6
mph. We need automobiles to go 60 mph. To go 600 mph the technology
must move to jet aircraft. 6000 mph is the domain of space travel.
That's a change of three orders of magnitude. Early logic was was
around 1 MHz. Little by little we have progressed to where todays
logic is well above 1 GHz. This is also more than a three orders of
magnitude change in speed and yet we are still trying to design using
a very similar technology and using the same terminology. There is a
need for change. Can I help?
Ralph Morrison
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