Oscilloscope Method

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Basic Measurements

Test Methods and Practices

Multimeter Method

Oscilloscope Method

A dc voltage measurement can be made with an oscilloscope, as shown in figure 1-3, that has a direct-coupled deflection amplifier or terminals for connection directly to the deflection plates of the cathode-ray tube. Measuring a dc voltage with an oscilloscope is convenient only under certain circumstances; for example, when other measurements are being made on the same equipment with the oscilloscope or when a vacuum tube voltmeter is not available and a high-impedance measuring device is required.

Figure 1-3. - Dual-trace oscilloscope.

Oscilloscopes have a high input impedance and normally will not load down the circuit under test. However, oscilloscopes are primarily designed for waveform observation and are typically less accurate than other pieces of test equipment used to measure dc voltages. A distinct advantage of the oscilloscope is its ability to monitor the level of ac ripple voltage riding the dc voltage. This feature makes the oscilloscope an indispensable aid in troubleshooting dc power supplies with excessive ripple caused by component failure.

Digital Multimeter Method

Most analog voltmeters (that use D'Arsonval meter movements) in common use today are accurate to approximately 2% of full-scale reading. Most digital multimeters, as shown in figure 1-4, have a high input impedance and are not likely to disturb the circuit being tested. The digital multimeter in most cases provides an accuracy of at least 0.1%.

Figure 1-4. - Digital multimeter.

Digital multimeters display the reading numerically. These direct-reading displays, along with automatic range- and polarity-changing features, eliminate the problem of parallax, reduce error and tedium, and increase measurement speed. Data from these meters in digital format can also be processed by computers, printers, tape and card punches, and magnetic-tape equipment. Digital multimeters are typically compact and lightweight; many come with rechargeable batteries, making them ideal for portable field use. The disadvantages are that they are not rugged and will not tolerate abuse and that some models do not produce sufficient bias voltage to test a diode or transistor junction. The John Fluke Model 77 A/N digital multimeter is presently being purchased by the Navy and will eventually phase out the older and less accurate analog meters.

Differential Voltmeter Method

Using the differential voltmeter, as shown in figure 1-5, provides one of the most accurate methods of measuring dc voltage. Typical accuracies attained by this method are 0.005%. These extremely high accuracies are achieved by the design of the voltmeter with precision internal reference voltages and precision resistors. As discussed earlier in NEETS, module 6, Introduction to Electronic Emission, Tubes, and Power Supplies, most differential voltmeters can be operated as transistor voltmeters (tvm) or as differential null voltmeters. The tvm mode is used to measure the approximate voltage and polarity of the unknown voltage being measured. The approximate voltage, as measured in the tvm mode, is then used to make the initial range and mode switch selections for nulling the input voltage.

Figure 1-5. - Ac-dc differential voltmeter.

The advantages of using a differential voltmeter for measuring dc voltages are the extreme accuracy and minimal circuit loading made possible by the high input impedance of the meter. However, differential voltmeters are less portable, heavier, and require greater skill and time when performing measurements than other types of voltmeters. Additionally, they require long warm-up periods and are susceptible to variations in temperature and humidity.

Q.5 What are the advantages of using a differential voltmeter?

AC VOLTAGE MEASUREMENTS

When ac voltage measurements are performed, the input impedance of the selected test equipment determines the amount of energy removed from the circuit under test. If an ac meter is placed across a high-impedance circuit, the meter may load the high-impedance circuit and disturb circuit conditions, possibly to the point of causing the circuit to cease functioning. A dc electronic voltmeter, used in conjunction with a rectifying probe, extracts only a small amount of energy from the circuit under test. Another advantage of an electronic voltmeter over the analog voltmeter is that voltages of low values can be accurately measured.

If the circuit being measured is a relatively high-frequency circuit, the internal capacitance of an analog voltmeter rectifier could produce a disturbance by detuning the circuit. Figure 1-6 depicts the Frequency response of a Simpson 260. Note the percent of error introduced at different frequencies. For high-frequency voltage measurements, an electronic voltmeter or an oscilloscope should be used. The sensitivity of the meter (or oscilloscope) determines the lowest voltage it can measure accurately, and the shunt capacitance of its input determines the upper frequency limits. It should be clear that the Frequency response of a piece of test equipment is just as important as its range limitations. If you exceed the range limitations of a meter, it will either "peg" the meter or belch out the smell of smoke that many of us are intimately acquainted with. This, however, is not the case when you exceed the frequency limitations of your test equipment. Your test equipment will normally show a response, but that response will be grossly inaccurate. The lesson to be learned here is that you should be fully aware of the limitations of your test equipment and adhere to them.

Figure 1-6. - Simpson 260 Frequency response for ac voltage ranges.

Q.6 The Frequency response of test equipment refers to what aspect of ac voltage measurements?