Module 16 - Introduction to Test Equipment
Navy Electricity and Electronics Training Series (NEETS)
Chapter 6:  Pages 6-11 through 6-20

divided into the basic sections shown in figure 6-13: (1) a CRT, (2) a group of control circuits that control the waveform fed to the CRT, (3) a power supply, (4) sweep circuitry, and (5) deflection circuitry.

Figure 6-13. - Block diagram of an oscilloscope.

Q-14.   List the circuits that all oscilloscopes have in common.

Figure 6-14 is a drawing of the front panel of a dual-trace, general-purpose oscilloscope. Oscilloscopes vary greatly in the number of controls and connectors. Usually, the more controls and connectors, the more versatile the instrument. Regardless of the number, all oscilloscopes have similar controls and connectors. Once you learn the fundamental operation of these common controls, you can move with relative ease from one model of oscilloscope to another. Occasionally, controls that serve similar functions will be labeled differently from one model to another. However, you will find that most controls are logically grouped and that their names usually indicate their function.

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Figure 6-14. - Dual-trace oscilloscope.

The oscilloscope in figure 6-14 is called DUAL-TRACE because it can accept and display two vertical signal inputs at the same time - usually for comparison of the two signals or one signal and a reference signal. This scope can also accept just one input. In this case, it is used as a Single-TRACE OSCILLOSCOPE. For the following discussion, we will consider this to be a single-trace oscilloscope. The oscilloscope in the figure is commonly used in the fleet. You are likely to use this one (model AN/USM-425) or one very similar to it. Let's now look at the front panel controls.

COMPONENTS useD to DIsPLAY The WAVEforM

The CRT DIsPLAY SCREEN is used to display the signal (figure 6-15). It allows you to make accurate measurements using the vertical and horizontal graticules, as discussed earlier.

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Figure 6-15. - CRT display and graticule.

COMPONENTS useD to ADJUST CRT DIsPLAY QUALITY

The controls in figure 6-16 allow you to adjust for a clear signal display. They also allow you to adjust the display position and magnify the horizontal trace by a factor of 10 (X10). Keep in mind that the controls may be labeled differently from one model to another, depending on the manufacturer. Refer to figure 6-16 as you study the control descriptions in the next paragraphs.

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Figure 6-16. - Quality adjustment for CRT display.

INTEN (Intensity) Control

The INTEN (intensity) control (sometimes called BRIGHTNESS) adjusts the brightness of the beam on the CRT. The control is rotated in a clockwise direction to increase the intensity of the beam and should be adjusted to a minimum brightness level that is comfortable for viewing.

FOCUS and ASTIG (Astigmatism) Controls

The FOCUS control adjusts the beam size. The ASTIG (astigmatism) control adjusts the beam shape. The FOCUS and ASTIG controls are adjusted together to produce a small, clearly defined circular dot. When displaying a line trace, you will use these same controls to produce a well-defined line. Figure 6-17, view A, shows an out-of-focus beam dot. View B shows the beam in focus. Views C and D show out-of-focus and in-focus traces, respectively.

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Figure 6-17. - Effects of FOCUS and ASTIG (astigmatism) controls.

TRACE ROTATION Control

The TRACE ROTATION control (figure 6-16) allows for minor adjustments of the horizontal portion of the trace so that you can align it with the horizontal lines on the graticule.

BEAM FINDER Control

Occasionally, the trace will actually be located off the CRT (up or down or to the left or right) because of the orientation of the deflection plates. When pushed, the BEAM FINDER (figure 6-16) pulls the beam onto the screen so that you can use the horizontal and vertical POSITION controls to center the spot.

Horizontal and Vertical POSITION Controls

The horizontal and vertical POSITION controls (figure 6-16) are used to position the trace. Because the graticule is often drawn to represent a graph, some oscilloscopes have the positioning controls labeled to correspond to the X and Y axes of the graph. The X axis represents horizontal movement; the Y axis represents the vertical movement. Figure 6-18 shows the effects of positioning controls on the trace.

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Figure 6-18. - Effects of horizontal and vertical controls.

In view A, the horizontal control has been adjusted to move the trace too far to the right; in view B, the trace has been moved too far to the left. In view C, the vertical POSITION control (discussed later) has been adjusted to move the trace too close to the top; in view D, the trace has been moved too close to the bottom. View E (figure 6-18) shows the trace properly positioned.

10X MAG (Magnifier) Switch

The 10X MAG (magnifier) switch (figure 6-16) allows you to magnify the displayed signal by a factor of 10 in the horizontal direction. This ability is important when you need to expand the signal to evaluate it carefully.

COMPONENTS useD to DETERMINE The Amplitude of a Signal

We will now discuss the dual-trace components of the scope. You will use these components to determine the amplitude of a signal. Notice in figure 6-19 that the highlighted section at the upper left of the scope looks just the same as the section at the lower left of the scope. This reveals the dual-trace capability section of the scope. The upper left section is the CH (channel) 1 input and is the same as the CH 2 input at the lower left. An input to both inputs at the same time will produce two independent traces on the CRT and use the dual-trace capability of the scope.

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Figure 6-19. - Components that determine amplitude.

For purposes of this introductory discussion, we will present only CH (channel) 1. You should realize that the information presented also applies to CH 2.

Vertical POSITION Control

The vertical POSITION control allows you to move the beam position up or down, as discussed earlier.

Input Connector

The vertical input (or signal input) jack connects the signal to be examined to the vertical-deflection amplifier. Some oscilloscopes may have two input jacks, one labeled AC and the other labeled DC. Other models may have a single input jack with an associated switch, such as the AC GRD DC switch in figure 6-19. This switch is used to select the ac or dc connection. In the DC position, the signal is connected directly to the vertical-deflection amplifier; in the AC position, the signal is first fed through a capacitor. Figure 6-20 shows the schematic of one arrangement.

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Figure 6-20. - Vertical input arrangement.

The VERTICAL-DEFLECTION Amplifier increases the amplitude of the input signal level required for the deflection of the CRT beam. The deflection amplifier must not have any other effect on the signal, such as changing the shape (called Distortion). Figure 6-21 shows the results of distortion occurring in a deflection amplifier.

Figure 6-21. - Deflection amplifier distortion.

Attenuator Control

An amplifier can handle only a limited range of input amplitudes before it begins to distort the signal. Signal distortion is prevented in oscilloscopes by the incorporation of circuitry that permits adjustment of the input signal amplitude to a level that prevents distortion from occurring. This adjustment is called the ATTENUATOR control in some scopes (VOLTS/DIV and VAR in figure 6-19). This control extends the usefulness of the oscilloscope by enabling it to handle a wide range of signal amplitudes.

The attenuator usually consists of two controls. One is a multiposition (VOLTS/DIV) control, and the other is a variable (VAR) potentiometer. Each position of the control may be marked either as to the amount of voltage required to deflect the beam a unit distance, such as VOLTS/DIV, or as to the amount of attenuation (called the DEFLECTION FACTOR) given to the signal, such as 100, 10, or 1.

Suppose the .5 VOLTS/DIV position were selected. In this position, the beam would deflect vertically 1 division for every 0.5 volts of applied signal. If a sine wave occupied 4 divisions peak-to- peak, its amplitude would be 2 volts peak-to-peak (4 ´ 0.5), as shown in figure 6-22.

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Figure 6-22. - Sine wave attenuation.

The vertical attenuator control (VOLTS/DIV in figure 6-19) provides a means of adjusting the input signal level to the amplifiers by steps. These steps are sequenced from low to high deflection factors. The potentiometer control (VAR in figure 6-19) provides a means of fine, or variable, control between steps. This control may be mounted separately, or it may be mounted on the attenuator control. When the control is mounted separately, it is often marked as FINE Gain or simply Gain. When mounted on the attenuator control, it is usually marked VARIABLE or VAR.

The variable control adds attenuation to the step that is selected. Since accurately calibrating a potentiometer is difficult, the variable control is either left unmarked or the front panel is marked off in some convenient units, such as 1-10 and 1-100. The attenuator control, however, can be accurately calibrated. To do this, you turn off the variable control to remove it from the attenuator circuit. This position is usually marked CAL (calibrate) on the panel, or an associated light indicates if the VAR control is on or off. In figure 6-19, the light called UNCAL indicates the VAR control is in the uncalibrated position.

COMPONENTS useD to SELECT The VERTICAL OPERATING Mode

As we discussed earlier, channel 1 is being used to discuss basic operating procedures for the oscilloscope. Figure 6-23 shows how the vertical mode of operation is selected. The VERT Mode section contains push-button switches that enable you to select channel 1, channel 2, and several other vertical modes of operation. For the present discussion, note only that CH 1 is selected by these switches.

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Figure 6-23. - Vertical-deflection controls.

COMPONENTS useD to DETERMINE PERIOD TIME of The DIsPLAY

The TIME/DIV (figure 6-24) controls on the scope determine the period time of the displayed waveform. As we discussed earlier, the sweep generator develops the sawtooth waveform that is applied to the horizontal-deflection plates of the CRT. This sawtooth voltage causes the beam to move across the screen. This trace (sometimes called SWEEP) sets the frequency of the TIME Base of the oscilloscope. The frequency of the time base is variable, which enables the oscilloscope to accept a wide range of input frequencies. Again, two controls are used (figure 6-24). One is a multiposition switch (TIME/DIV) that changes the frequency of the sweep generator in steps. The second control is a potentiometer (VAR) that varies the frequency between steps. Each step on the TIME/DIV control is calibrated. The front panel has markings that group the numbers into microseconds and milliseconds.

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