Electrical Indicating and Test Instruments 1 Introduction 161

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Electrical Indicating and Test Instruments

7.1 Introduction 161

7.2 Digital Meters 162
7.2.1 Voltage-to-Time Conversion Digital Voltmeter 163
7.2.2 Potentiometric Digital Voltmeter 163
7.2.3 Dual-Slope Integration Digital Voltmeter 164
7.2.4 Voltage-to-Frequency Conversion Digital Voltmeter 164
7.2.5 Digital Multimeter 164
7.3 Analogue Meters 165
7.3.1 Moving Coil Meter 165
7.3.2 Moving Iron Meter 167
7.3.3 Clamp-on Meters 168
7.3.4 Analogue Multimeter 169
7.3.5 Measuring High-Frequency Signals with Analogue Meters 169
7.3.6 Calculation of Meter Outputs for Nonstandard Waveforms 170
7.4 Oscilloscopes 172
7.4.1 Analogue Oscilloscope (Cathode Ray Oscilloscope) 173
Channel 174
Single-ended input 175
Differential input 175
Time base circuit 176
Vertical sensitivity control 176
Display position control 176
7.4.2 Digital Storage Oscilloscopes 177
7.4.3 Digital Phosphor Oscilloscope 178
7.4.4 Digital Sampling Oscilloscope 179
7.4.5 Personal Computer-Based Oscilloscope 180
7.5 Summary 180

    1. Problems 181


The mode of operation of most measuring instruments is to convert the measured quantity into an electrical signal. Usually, this output quantity is in the form of voltage, although other forms of output, such as signal frequency or phase changes, are sometimes found.
We shall learn in this chapter that the magnitude of voltage signals can be measured by various electrical indicating and test instruments. These can be divided broadly into electrica meters (in both analogue and digital forms) and various types of oscilloscopes. As well as signal level voltages, many of these instruments can also measure higher magnitude voltages, and this is indicated where appropriate. The oscilloscope is particularly useful for interpreting instrument outputs that exist in the form of a varying phase or frequency of an electrical signal.
Electrical meters exist in both digital and analogue forms, although use of an analogue form now tends to be restricted to panel meters, where the analogue form of the output display means that abnormal conditions of monitored systems are identified more readily than is the case with the numeric form of output given by digital meters. The various forms of digital and analogue meters found commonly are presented in Sections 7.2 and 7.3.
The oscilloscope is a very versatile measuring instrument widely used for signal measurement, despite the measurement accuracy provided being inferior to that of most meters. Although existing in both analogue and digital forms, most instruments used professionally are now digital, with analogue versions being limited to inexpensive, low-specification instruments intended for use in educational establishments. Although of little use to professional users, the features of analogue instruments are covered in this chapter because students are quite likely to meet these when doing practical work associated with their course. As far as digital oscilloscopes are concerned, the basic type of instrument used is known as a digital storage oscilloscope. More recently, digital phosphor oscilloscopes have been introduced, which have a capability of detecting and recording rapid transients in voltage signals. A third type is the digital sampling oscilloscope, which is able to measure very high-frequency signals. A fourth and final type is a personal computer (PC)-based oscilloscope, which is effectively an add-on unit to a standard PC. All of these different types of oscilloscopes are discussed in Section 7.4.

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