Electronic Test Instruments: Intro

This guide is for the hobbyist, student, technician, or engineer who understands basic electronics and wants to learn more about how electronic measurements are made. To use instruments effectively, one needs to understand basic measurement theory and how it relates to practical measurements. Basic measurement theory includes such things as how a voltage’s waveform relates to its frequency and how an instrument can affect the voltage that it's measuring. Ideally, it's desirable not to have to understand the operation of an instrument in order to use it. Although this ideal situation can be approached, it can't be reached completely. (One does not have to know how a gasoline engine works in order to drive an automobile. However, a driver should at least understand the function of the accelerator and brake pedals.)

To minimize dealing with the internal workings of an instrument, circuit models and conceptual block diagrams are used extensively Circuit models take a “black box” approach to describing a circuit. In other words the behavior of a complex circuit or instrument can be described adequately by conceptually replacing it with a much simpler circuit This circuit model approach reduces the amount of detail that must be remembered and understood Conceptual block diagrams show just enough of the inner workings of an instrument so that the reader can understand what the instrument is doing, without worrying about the details of how this is accomplished In some instruments most notably oscilloscopes a greater understanding of the internal workings of the instrument is required since they determine how the instrument is used.

The traditional analog technologies are gradually yielding to digital techniques. However, a voltage measurement is still a voltage measurement whether an analog meter or a digital meter is used. Since the measurement is fundamentally the same this guide treats both technologies in a unified manner, while still highlighting the differences. Externally, both types of instruments do basically the same thing with some important differences in how the reading is displayed. As appropriate, these differences are explained.

This guide does not attempt to be (nor can it be) a substitute for a well written instrument operating manual. The reader is not well served by a guide that says “push this button, turn this knob” because the definition of the buttons and knobs will undoubtedly change with time. Instead, this guide is a supplement or reference which provides the reader with a background in electronic instruments. Variations and improvements in instrument design cause each meter, oscilloscope, or function generator to be somewhat unique. However, they all have in common the fundamental measurement principles discussed in this book.

Section 1 covers the basic measurement theory and fundamentals. Sections 2 through 6 cover the mainstream instruments that the typical user will encounter (meters, signal sources, oscilloscopes, etc.). Some circuit concepts and how they relate to electronic measurements are covered in Section 7. Section 8 introduces frequency domain instruments, with the emphasis on spectrum analyzers.

The original motivation for this guide was the author’s experience teaching undergraduate courses in electrical engineering. Even students with a good background in electrical theory seem to have trouble relating the textbook concepts to what is observed in the laboratory. The concepts of the loading effect, grounding, and bandwidth are particularly trouble some. Hence, they are emphasized throughout the book. Although this guide is not a textbook, it certainly could be used as supplemental reading for an engineering or technology laboratory course.

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