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Appendix E: Tektronix TDS 1002B Oscilloscope

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Appendix E

Tektronix TDS 1002B Oscilloscope

Introduction

Digital multimeters (DMMs) are extremely useful devices to measure and characterize electrical

parameters; however, they have a number of limitations. Whether in ammeter or voltmeter mode,

they give a one-number summary of the electricity. Their AC frequency range is limited to audio

frequencies: 20 Hz to 20 kHz. And perhaps most significantly, they do not show to the engineer

a picture of the electrical signal’s shape and variation.

Oscilloscopes are useful at frequencies well above 1 MHz and some even beyond 1 GHz. Unlike

the DMM, the oscilloscope (a.k.a., scope) usually measures voltage, not current. Also unlike the

DMM, the oscilloscope’s “common” lead is always internally connected to ground. Effectively,

then, the oscilloscope displays the signal lead’s voltage relative to ground. So, while a freely

floating DMM can measure the 1 V potential difference between two terminals at 110 V and

111 V above ground, an oscilloscope typically cannot. (See Appendix B for potential grounding

errors with oscilloscopes.) However, while a DMM converts every waveform of any shape to a

single number, V

rms

, an oscilloscope displays a graph of voltage versus time. (See Figure E.1.)

Figure E.1: An oscilloscope displays one wave section after another, making an

apparently steady display. Determining when to start a new wave section is called

triggering, and the trigger point is set by specifying the signal’s level and slope

(rising or falling). The trigger point is placed in the center of the display, but it

can be moved using the horizontal POSITION knob. The holdoff is an adjustable

dead time following a triggered wave section.

Oscilloscopes are generally used to display periodic signals. Every fraction of a second, a new

section of the wave is displayed. If these successively displayed wave-sections match, the dis-

play will appear unchanging. Thus, the precise triggering of successive wave sections is critical

for a stable display. In addition, the scales used for the HORIZONTAL and VERTICAL axes should

be appropriate for the signal. That is, the signal should be neither off-scale large nor so small as

to be indistinguishable from zero. A too large time (horizontal) scale will result in displaying

hundreds of cycles as a blur; a too small time scale will cause just a fraction of a cycle to be dis-

played.

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BK Precision 4011A Instruções de Operação Página 68