USE OF THE OSCILLOSCOPE

The CATHODE-RAY OSCILLOSCOPE (CRO or O-SCOPE) is commonly used for the analysis of waveforms generated by electronic equipment. Several types of cathode-ray oscilloscopes are available for making waveform analysis. The oscilloscope required for a particular test is determined by characteristics, such as input-Frequency response, input impedance, sensitivity, sweep rate, and the methods of sweep control. The SYNCHROSCOPE is an adaptation of the cathode-ray oscilloscope. It features a wide-band amplifier, triggered sweep, and retrace blanking circuits. These circuits are desirable for the analysis of pulse waveforms.

Oscilloscopes are also part of some harmonic analysis test equipments that display harmonic energy levels. To effectively analyze waveform displays, you must know the correct wave shape. The maintenance instructions manual for each piece of equipment illustrates what waveforms you should observe at the various test points throughout the equipment. Waveforms that will be observed at any one selected test point will differ; each waveform will depend on whether the operation of the equipment is normal or abnormal.

The display observed on a cathode-ray oscilloscope is ordinarily one similar to those shown in figure 2-13. Views A and B show the instantaneous voltage of the wave plotted against time. Elapsed time (view A) is indicated by horizontal distance, from left to right, across the etched grid (graticule) placed over the face of the tube. The amplitude (view B) of the wave is measured vertically on the graph.

Figure 2-13. - Typical waveform displays.

The oscilloscope is also used to picture changes in quantities other than simply the voltages in electric circuits. For example, if you need to see the changes in waveform of an electric current, you must first send the current through a small resistor. You can then use the oscilloscope to view the voltage wave across the resistor. Other quantities, such as temperatures, pressures, speeds, and accelerations, can be translated into voltages by means of suitable transducers and then viewed on the oscilloscope. A detailed discussion of the oscilloscope is presented in chapter 6 of this module.

USE OF THE SPECTRUM ANALYZER

The SPECTRUM ANALYZER is a device that sweeps over a band of frequencies to determine (1) what frequencies are being produced by a specific circuit under test and (2) the amplitude of each frequency component. To accomplish this, the spectrum analyzer first presents a pattern on a display. Then the relative amplitudes of the various frequencies of the spectrum of the pattern are plotted (see figure 2-14). On the vertical, or Y axis, the amplitudes are plotted; on the horizontal, or X axis, the frequencies (time base) are plotted. The overall pattern of this display indicates the proportion of power present at the various frequencies within the SPECTRUM (fundamental frequency with sideband frequencies).

Figure 2-14. - Spectrum analyzer pattern.

Q.10 What device sweeps a band of frequencies to determine frequencies and amplitudes of each frequency component?

The spectrum analyzer is used to examine the frequency spectrum of radar transmissions, local oscillators, test sets, and other equipment operating within its frequency range. Proper interpretation of the displayed frequency spectrum enables you to determine the degree of efficiency of the equipment under test. With experience, you will be able to determine definite areas of malfunctioning components within equipment. In any event, successful spectrum analysis depends on the proper operation of a spectrum analyzer and your ability to correctly interpret the displayed frequencies. Later, in chapter 6, we will discuss the various controls, indicators, and connectors contained on the spectrum analyzer.

TESTING SEMICONDUCTOR DEVICES

Because of the reliability of semiconductor devices, servicing techniques developed for transistorized equipment differ from those normally used for electron-tube circuits. Electron tubes are usually considered to be the circuit component most susceptible to failure and are normally the first components to be tested. Transistors, however, are capable of operating in excess of 30,000 hours at maximum rating without failure. They are often soldered in the circuit in much the same manner as resistors and capacitors. Therefore, they are NOT so quickly removed for testing as tubes.

Substitution of a semiconductor diode or transistor known to be in good condition is one method of determining the quality of a questionable semiconductor device. This method should be used only after you have made voltage and resistance measurements. This ensures the circuit has no defect that might damage the substitute semiconductor device. If more than one defective semiconductor is present in the equipment section where trouble has been localized, the semiconductor replacement method becomes cumbersome. Several semiconductors may have to be replaced before the trouble is corrected. To determine which stage(s) failed and which semiconductors are not defective, you must test all the removed semiconductors. You can do this by observing whether the equipment operates correctly as you reinsert each of the removed semiconductor devices into the equipment.

TESTING DIODES

Semiconductor diodes, such as general-purpose germanium and silicon diodes, power silicon diodes, and microwave silicon diodes, can be tested effectively under actual operating conditions. However, crystal-rectifier testers are available to determine dc characteristics that provide an indication of crystal-diode quality.

A common type of crystal-diode test set is a combination ohmmeter-ammeter. Measurements of forward resistance, back resistance, and reverse current can be made with this equipment. Using the results of these measurements, you can determine the relative condition of these components by comparing their measured values with typical values obtained from test information furnished with the test set or from the manufacturer's data sheets. A check that provides a rough indication of the rectifying property of a diode is the comparison of the back-and-forward resistance of the diode at a specified voltage. A typical back-to-forward-resistance ratio is on the order of 10 to 1, and a forward-resistance value of 50 to 80 ohms is common.

Q.11 What is the typical back-to-forward resistance ratio of a good-quality diode?