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Steve Roach
7. Signal Conditioning in Oscilloscopes
and the Spirit of Invention
The Spirit of Invention
When I was a child my grandfather routinely asked me if I was going to
be an engineer when I grew up. Since some of my great-uncles worked
on the railroads, I sincerely thought he wanted me to follow in their foot-
steps. My grandfather died before I clarified exactly what kind of engi-
neer he hoped I would become, but I think he would approve of my
interpretation.
I still wasn't sure what an engineer was when I discovered I wanted to
be an inventor. I truly pictured myself alone in my basement toiling on
the important but neglected problems of humanity. Seeking help, I joined
the Rocky Mountain Inventors' Congress. They held a conference on
invention where I met men carrying whole suitcases filled with clever
little mechanical devices. Many of these guys were disgruntled and
cranky because the world didn't appreciate their contributions. One of
the speakers, a very successful independent inventor, told of a bankrupt
widow whose husband had worked twenty years in isolation and secrecy
inventing a mechanical tomato peeler. The tomato peeler had consumed
the family savings, and the widow had asked the speaker to salvage the
device. With sadness the speaker related the necessity of informing her
that tomatoes were peeled in industrial quantities with sulfuric acid.
Apparently the inventor had been too narrowly focused to realize that
in some cases molecules are more powerful than machines.
I didn't want to become disgruntled, cranky, or isolated and I didn't
even own a basement. So I went to engineering school and adopted a
much easier approach to inventing. I now design products for companies
with such basic comforts as R&D budgets, support staff, and manufactur-
ing operations. Along the way I have discovered many ways of nurturing
inventiveness. Here are some techniques that seem to work:
Give yourself time to invent. If necessary, steal this time from the un-
ending rote tasks that your employer so readily recognizes and rewards. I
try to work on things that have nothing to do with a particular product,
have no schedule, and have no one expecting results. I spend time on
highly tangential ideas that have little hope for success. I can fail again
and again in this daydream domain with no sense of loss.
65
Signal Conditioning in Oscilloscopes and the Spirit of Invention
Get excited. Enjoy the thrilling early hours of a new idea. Stay up all
night, lose sleep, and neglect your responsibilities. Freely explore tan-
gents to your new idea. Digress fearlessly and entertain the absurd.
Invent in the morning or whenever you are most energetic. Save your
"real" work for when you are tired.
Master the fundamentals of your field. The most original and creative
engineers I have known have an astonishing command of undergraduate-
level engineering. Invention in technology almost always stems from the
novel application of elementary principles. Mastery of fundamentals al-
lows you to consider, discard, and develop numerous ideas quickly, accu-
rately, and fairly. I believe so much in this concept that I have begun
taking undergraduate classes over again and paying very careful attention.
Honestly evaluate the utility of your new idea at the right time: late
enough not to cut off explorations of alternatives and wild notions, but
early enough that your creativity doesn't go stale. In this stage you must
ask the hardest questions: "Is this new thing useful to anyone else? Ex-
actly where and how is it useful? Is it really a better solution or just a
clever configuration of parts?" Even if you discover that your creation
has no apparent utility, savor the fun you had exploring it and be thankful
that you don't have the very hard work of developing it.
Creativity is not a competitive process. It is sad that we engineers are
so inculcated with the competitive approach that we use it even privately.
You must suspend this internal competition because almost all of your
new ideas will fail. This is a fact, but it doesn't detract a bit from the fun
of inventing.
Now it's time to get on to a very old and interesting analog design
problem where there is still a great deal of room for invention.
Requirements for Signal Conditioning
in Oscilioscopes
Most of my tenure as an electrical engineer has been spent designing
analog subsystems of digital oscilloscopes. A digital oscilloscope is a
rather pure and wholesome microcosm of signal processing and measure-
ment, but at the signal inputs the instrument meets the inhospitable real
world. The input signal-conditioning electronics, sometimes referred
to as the "front-end" of the instrument, includes the attenuators, high-
impedance buffer, and pre-amplifier. Figure 7-1 depicts a typical front-
end and is annotated with some of the performance requirements.
The combination of requirements makes the design of an oscilloscope
front-end very difficult. The front-end of a 500MHz oscilloscope devel-
ops nearly IGHz of bandwidth and must have a very clean step response.
It operates at this bandwidth with a IMQ, input resistance! No significant
resonances are allowed out to 5GHz or so (where everything wants to
resonate). Because we must maintain high input resistance and low ca-
pacitance, transmission lines (the usual method of handling microwave
Steve Roach
Instrument Attenuator