Bogatin:25年的10个经验教训

stupid · 发表于 2010-10-7 09:49

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Ten lessons from 25 years of teaching electrical design.

I’ve been writing about, lecturing and teaching signal integrity topics for more than 25 years. I’ve taught more than 7,000 engineers
and personally mentored more than 30. Along the way, I’ve developed some sayings that crystallize important rules to consider when
working on signal integrity projects. Of course, these rules apply to more than just signal integrity.

I received a note from a recent student who jotted down my “rules.” He sent me a copy. This month, I thought I would share some of
the list:

1.The most common answer to all signal integrity questions is “it depends.” And, the way to answer all “it depends” questions is
by “putting in the numbers” using rules of thumb, approximations, numerical simulations and measurements.

2.The way to separate myth from reality is by “putting in the numbers,” using rules of thumb, approximations, numerical
simulation tools and measurements. All these processes are equally important and should be in the tool box of every engineer.
They each have a different balance between accuracy and cost to get an answer (cost as measured by time, money and
expertise). Use the process for each problem appropriate to your budget.

3.Watch out for the “Whac-a-Mole” effect. Often, changing one design feature to improve a performance metric has a negative
impact on another performance metric. It is like the Whac-a-Mole arcade game. For example, bringing the signal path closer to
the return path decreases ground bounce, but at some point, this will reduce the impedance of the interconnect and cause
excessive reflection noise.

4.The most efficient way to solve a signal integrity problem is to find its root cause. If you don’t know the root cause of a
problem, and it goes away when you try something, you have no idea if this was a coincidence or if this problem will creep
back in.

5.Use the Youngman Principle to turn a root cause into a design guideline. This is named after Henny Youngman, a famous
comedian of the 20th Century. One of his jokes was, “A man goes into a doctor’s office and says, ‘Doctor, my arm hurts when I
raise it. What should I do?’ The doctor replies, ‘Don’t raise your arm.’ ” If design feature A causes problem B, to eliminate
problem B, eliminate design feature A. For example, if reflection noise is caused when the instantaneous impedance the signal
sees changes, engineer the instantaneous impedance to be constant down the entire interconnect.

6.Sometimes an OK answer NOW is better than a good answer later. You often have to make decisions without all the
information you would like. This is where rough estimates are important. What is the bandwidth of an 800 Mbps DDR3 signal?
It depends on the rise time, of course, but if you don’t know the rise time, do you sit and wait until someone can measure it? If
you need an answer NOW!, you can use the rule of thumb that the bandwidth is about the 5th harmonic of the clock. The
clock is 400 MHz and the 5th harmonic is 2 GHz.

7.Always evaluate the bang for the buck from a design change using a “virtual prototype.” This is a parameterized model for
your system, and a way of simulating its performance using this model. It will help you answer “what if” questions, and lets you
measure the expected performance gain for the extra cost of a new material, design or component, before you commit to
hardware.

8.Watch out for “mink holes.” A rat hole is a convoluted path you detour down that takes away from the real goal. A mink hole is
a rat hole lined in mink: It feels really good while you are in it. Engineers love technical puzzles. Resist the temptation to track
down every little detail, or get that model bandwidth to just another few GHz. More important problems are always awaiting
attention.

9.Never perform a simulation or a measurement without anticipating what you expect to see. If you are wrong, something is off in
the problem setup, the tool accuracy, or your intuition. Either way, you will learn something by tracking down the discrepancy.
If you are right, and you see what you expect, you get a nice, warm feeling that maybe you really do understand what is going
on.

10.There are two kinds of engineers: those who have signal integrity problems and those who will. The corollary is, there are two
kinds of designers: those who are designing antennae on purpose and those who aren’t doing it on purpose.