Module 16—Introduction to Test Equipment
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Figure 6-24.—Period time of the waveform (TIME/DIV).
The potentiometer is labeled VAR, and the panel has an UNCAL indicator that lights when the VAR control is in the variable position. When you desire to accurately measure the time of one cycle of an input signal, turn the VAR control to the CAL position and turn the TIME/DIV switch to select an appropriate time base. Suppose you choose the 10-microsecond position to display two cycles of an input signal, as shown in figure 6-25. One cycle occupies 3 centimeters (small divisions) along the horizontal axis. Each cm has a value of 10 microseconds. Therefore, the time for one cycle equals 30 microseconds (3 x 10). Recall that the frequency for a signal may be found by using the following procedure:
Figure 6-25.—Time measurement of a waveform (TIME/DIV).
In selecting a time base, you should select one that is lower in frequency than the input signal. If the input signal requires 5 milliseconds to complete one cycle and the sawtooth is set for 0.5 milliseconds per centimeter with a 10-centimeter-wide graticule, then approximately one cycle will be displayed. If the time base is set for 1 millisecond per centimeter, approximately two cycles will be displayed. If the time base is set at a frequency higher than the input frequency, only a portion of the input signal will be displayed.
In the basic oscilloscope, the sweep generator runs continuously (FREE-RUNNING); in more elaborate oscilloscopes, it is normally turned off. In the oscilloscope we’re using as an example, the sweep generator can be triggered by the input signal or by a signal from some other source. (Triggering will be discussed later in this chapter.) This type of oscilloscope is called a triggered oscilloscope. The triggered oscilloscope permits more accurate time measurements to be made and provides a more stable presentation than the nontriggered-type oscilloscope.
On some oscilloscopes, you will find a 10 times (10X) magnification control. As previously mentioned, this allows the displayed sweep to be magnified by a factor of 10.
Q-15. When you select the time base to display a signal, should the time base be the same, higher, or lower than the input signal?
COMPONENTS USED TO PROVIDE A STABLE DISPLAY
The triggering and level controls are used to synchronize the sweep generator with the input signal. This provides a stationary waveform display. If the input signal and horizontal sweep generator are unsynchronized, the pattern tends to jitter, making observations difficult.
The A TRIGGER controls at the lower right of the scope (figure 6-26) are used to control the stability of the oscilloscope CRT display. They are provided to permit you to select the source, polarity, and amplitude of the trigger signal. These controls, labeled A TRIGGER, LEVEL, SOURCE, and SLOPE, are described in the following paragraphs.
Figure 6-26.—Components that control stability.
The SOURCE control allows you to select the appropriate source of triggering. You can select input signals from channel 1 or 2, the line (60 hertz), or an external input.
TRIGGER LEVEL/SLOPE Controls
The LEVEL control allows you to select the amplitude point of the trigger signal at which the sweep is triggered. The SLOPE lets you select the negative or positive slope of the trigger signal at which the sweep is triggered.
The TRIGGER LEVEL (mounted with the TRIGGER SLOPE on some scopes) determines the voltage level required to trigger the sweep. For example, in the TRIGGER modes, the trigger is obtained from the signal to be displayed. The setting of the LEVEL control determines the amplitude point of the input waveform that will be displayed at the start of the sweep.
Figure 6-27 shows some of the displays for a channel that can be obtained for different TRIGGER LEVEL and TRIGGER SLOPE settings. The level is zero and the slope is positive in view A; view B also shows a zero level but a negative slope selection. View C shows the effects of a positive trigger level setting and positive trigger slope setting; view D displays a negative trigger level setting with a positive trigger slope setting. Views E and F have negative slope settings. The difference is that view E has a positive trigger level setting, whereas F has a negative trigger level setting.
Figure 6-27.—Effects of SLOPE and TRIGGER LEVEL controls.
In most scopes, an automatic function of the trigger circuitry allows a free-running trace without a trigger signal. However, when a trigger signal is applied, the circuit reverts to the triggered mode of operation and the sweep is no longer free running. This action provides a trace when no signal is applied.
Synchronization is also used to cause a free-running condition without a trigger signal. Synchronization is not the same as triggering. TRIGGERING refers to a specific action or event that initiates an operation. Without this event, the operation would not occur. In the case of the triggered sweep, the sweep will not be started until a trigger is applied. Each succeeding sweep must have a trigger before a sweep commences. SYNCHRONIZATION, however, means that an operation or event is brought into step with a second operation.
A sweep circuit that uses synchronization instead of triggering will cause a previously free-running sweep to be locked in step with the synchronizing signal. The TRIGGER LEVEL control setting can be increased until synchronization occurs; but, until that time, an unstable pattern will appear on the CRT face.
The COUPLING section allows you to select from four positions: AC, LF REJ, HF REJ, and DC. The AC position incorporates a coupling capacitor to block any dc component. The LF and HF REJ positions reject low- and high-frequency components, respectively. The DC position provides direct
coupling to the trigger circuits. This is useful when you wish to view only the LF or HF component of a signal.
COMPONENTS USED TO SELECT SCOPE TRIGGERING
The TRIG MODE section in figure 6-28 allows for automatic triggering or normal triggering. In AUTO (automatic), the triggering will be free-running in the absence of a proper trigger input or will trigger on the input signal at frequencies above 20 hertz. In NORM (normal), the vertical channel input will trigger the sweep.
Figure 6-28.—Components to select triggering.
COMPONENTS USED TO SELECT HORIZONTAL-DEFLECTION MODE
For the present, notice only that the HORIZ DISPLAY (horizontal display) in figure 6-29 can be controlled by the TIME/DIV switch. Other switches in this section will be explained later in this chapter.
Figure 6-29.—Components to select mode of horizontal deflection.
COMPONENTS USED TO CALIBRATE THE PROBE OF THE SCOPE
In figure 6-30, you can see the components used to calibrate the test probe on the scope. A 1-volt, 2-kilohertz square wave signal is provided for you to adjust the probe for an accurate square wave and to check the vertical gain of the scope. You adjust the probe with a screwdriver, as shown in the figure.
Figure 6-30.—Components to calibrate probe.
SIMILARITIES AMONG OSCILLOSCOPES
The oscilloscope you use may differ in some respects from the one just covered. Controls and circuits may be identified by different names. Many of the circuits will be designed differently. However, all the functions will be fundamentally the same. Before using an oscilloscope, you should carefully study the operator’s manual that comes with it.
USING THE OSCILLOSCOPE
An oscilloscope can be used for several different types of measurements, such as time, phase, frequency, and amplitude of observed waveforms. Earlier in this chapter, you learned that the oscilloscope is most often used to study the shapes of waveforms when the performance of equipment is being checked. The patterns on the scope are compared with the signals that should appear at test points (according to the technical manual for the equipment under test). You can then determine if the equipment is operating according to peak performance standards.
Q-16. Oscilloscopes are used to measure what quantities?
TURNING ON THE SCOPE
Before turning on the scope, make sure it is plugged into the proper power source. This may seem obvious, but many technicians have turned all knobs on the front panel out of adjustment before they noticed that the power cord was not plugged in. On some scopes, the POWER switch is part of the INTEN (intensity) control. Turn or pull the knob until you hear a click or a panel light comes on (figure 6-31). Let the scope warm up for a few minutes so that voltages in all of the circuits become stabilized.
Figure 6-31.—Components to energize scope.
OBTAINING A PATTERN ON THE SCREEN
When adjusting a pattern onto the screen, adjust the INTEN (intensity) and FOCUS controls for a bright, sharp line. If other control settings are such that a dot instead of a line appears, turn down the intensity to prevent burning a hole in the screen coating. Because of the different speeds at which the beam travels across the screen, brightness and sharpness will vary at various frequency settings. For this reason, you may have to adjust the INTEN and FOCUS controls occasionally while taking readings.
NUMBER OF CYCLES ON THE SCREEN
Because distortion may exist at the beginning and end of a sweep, it is better to place two or three cycles of the waveform on the screen instead of just one, as shown in figure 6-32.
Figure 6-32.—Proper signal presentation.
The center cycle of three cycles provides you with an undistorted waveform in its correct phase. The center of a two-cycle presentation will appear inverted, but will be undistorted. To place waveforms on the CRT in this manner, you must understand the relationship between horizontal and vertical frequencies. The relationship between the frequencies of the waveform on the vertical plates and the sawtooth on the horizontal plates determines the number of cycles on the screen, as shown in figure 6-33.
Figure 6-33.—Vertical versus horizontal relationship.
The horizontal sweep frequency of the scope should always be kept lower than, or equal to, the waveform frequency; it should never be higher. If the sweep frequency were higher, only a portion of the waveform would be presented on the screen.
If, for example, three cycles of the waveform were to be displayed on the screen, the sweep frequency would be set to one-third the frequency of the input signal. If the input frequency were 12,000 hertz, the sweep frequency would be set at 4,000 hertz for a three-cycle scope presentation. For two cycles, the sweep frequency would be set at 6,000 hertz. If a single cycle were desired, the setting would be the same as the input frequency, 12,000 hertz.
The information presented in the previous sections served as a general overview of basic single-trace oscilloscope operation using one channel and operating controls. Now, you will be introduced to DUAL- TRACE operation.
Dual-trace operation allows you to view two independent signal sources as a dual display on a single CRT. This operation allows an accurate means of making amplitude, phase, time displacement, or frequency comparisons and measurements between two signals.
A dual-trace oscilloscope should not be confused with a dual-beam oscilloscope. Dual-beam oscilloscopes produce two separate electron beams on a single scope, which can be individually or jointly controlled. Dual-trace refers to a single beam in a CRT that is shared by two channels.
Q-17. Scopes that produce two channels on a single CRT with a single beam are referred to as what types of scopes?
Components Used to Select Vertical-Deflection Operating Mode
The VERT MODE controls (figure 6-34) allow you to select the operating mode of the scope for vertical deflection.
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics