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CHAPTER 5
INTRODUCTION TO WAVEFORM INTERPRETATION
LEARNING OBJECTIVES
Upon completion of this chapter, you will be able to do the following:
1. Explain the use of waveform interpretation in testing applications.
2. Identify the different types of modulation and methods of measuring modulation.
3. Explain the various uses of spectrum analyzers.
4. Explain the various uses of time-domain reflectometers.
5. Identify the various tests that can be performed with the swept-frequency technique.
INTRODUCTION TO WAVEFORM INTERPRETATION
Measurements performed with oscilloscopes, time-domain reflectometers, and spectrum analyzers
enable you to view the signal produced by the equipment or circuit under test. However, a visual display
is of no value unless you are able to interpret the signal characteristics.
A displayed waveform is a representation of a varying signal related to time. You can graphically
plot an unknown waveform by using a system of coordinates in which the amplitude of the unknown
signal is plotted linearly against time. An analysis of the resultant waveform provides you with valuable
information in determining the characteristics of many electronic (and some mechanical) devices. For
example, the waveform of a signal may indicate the presence of harmonics or parasitic oscillations, or it
may indicate how closely a device is following a desired cycle of operation. As the parts in an amplifier
begin to shift in value or deteriorate, waveform distortion often occurs and indicates abnormal operation
of a circuit and often precedes circuit breakdown. Malfunctioning of electrical or electronic circuits
within equipment can usually be traced, by waveform inspection, to a specific part or parts of the circuit
responsible for the distorted signal. On the basis of these facts, it is apparent that there is an important
need for test equipment that can provide a visual presentation of a waveform at the instant of its
occurrence in a circuit.
DISTORTION is a term used by technicians and engineers alike that generally signifies
dissatisfaction with the shape of the wave processed by an amplifier. Distortion of a waveform is the
undesired change or deviation in the shape of the observed signal with respect to a reference waveform.
Classifying any waveform as a distorted wave without reference to the electronic circuitry involved is
meaningless. A waveform that can be validly termed distorted with respect to a specific amplifier circuit
may be the normal waveform to be expected from another amplifier circuit. One of the most important
steps in waveform analysis, the one that usually proves the most difficult for the maintenance personnel,
is the interpretation of patterns viewed on the test equipment.
This chapter will cover some of the basic test methods and practices associated with waveform
interpretation.
