Chapter 54. Oscilloscope Techniques – Electrical Technology, Vol2: Machines and Measurements, 1/e

54

Oscilloscope Techniques

OBJECTIVES

In this chapter you will learn about:

  • Preliminary checks on the oscilloscope
  • Screen patterns obtained with deflection voltages
  • Lissajous figures in the range from 0° to 360°
  • Frequency ratios
  • Voltage and current measurement

The cathode ray tube

54.1  INTRODUCTION

The most glamorous and important electrical/electronic test and measuring instrument is the cathode ray oscilloscope. The scope is widely used for the visual observation of electrical work forces. In addition, the oscilloscope is finding diversified applications in many nonelectronic industrial and scientific uses where physical effects and phenomena are converted into electrical signals.

Oscilloscopes range from general purpose to elaborate special purpose types. A modern cathodes ray tube (CRT) is shown in Figure 54.1. The measurement capabilities of the oscilloscope are limited only by the skill of the operator. The oscilloscope must also be in good working condition. Otherwise, a defect in the electrical system may cause a misleading pattern. To avoid this, periodic checks should be conducted on: intensity and focus, positioning, synchronizing, deflection, deflection polarity, equalizing X and Y deflection, voltage calibration, and deflection sensitivity (see Figures 54.2 through 54.11).

Figure 54.1 Simplified Diagram of a Modern Cathode Ray Tube

54.2  PRELIMINARY CHECKS

The CRT beam should be controlled to produce a small luminous dot by adjusting simultaneously the intensity and focus controls. A point (dot) is said to have position, but no magnitude (area). Therefore, the first thing in checking the operating condition of an oscilloscope is to adjust the cathode ray beam for correct intensity and to focus it to produce a very fine luminous spot (dot; see Figure 54.2). If this is not possible then trouble may exist in the electron gun or power supply.

Figure 54.2 Adjust the Cathode Ray Beam for Correct Intensity and Focus it to Produce a Very Fine Luminous Spot

For adjusting the intensity and focus, power must be applied to the oscilloscope and the intensity and focus controls must be turned fully clockwise. Both the horizontal and vertical gain controls must be turned fully counter-clockwise (no deflection). The positioning controls must be adjusted so that the fluorescent dot is in the centre of the screen (See Figure 54.3). The intensity and focus control must be adjusted simultaneously to obtain a very fine dot of light. It should be possible to reduce the dot to a point still visible. This will allow one to check the intensity and focus controls individually.

The intensity control must be rotated through its entire range. When the controls are turned fully counter-clockwise the beam should be cut off and when turned fully clockwise the spot will offer a very bright brilliance. The spot should be cut off for one-third of the rotation of the control, after a fine dot of light should appear.

The focus control, as shown in Figure 54.4, causes the dot to increase in size when rotated on each side of its fine dot position. About one-third of its rotation from the fully counter-clockwise position should produce the correct focus or the smallest dot area. However, with the correct intensity and focus adjustment, there should be at least one-third in rotation on the control in either direction. In different oscilloscopes, these fine dot control positions will vary but the adjustments should come well within the range of intensity and focus controls.

Figure 54.3 A Typical Cathode Ray Oscilloscope

Figure 54.4 Beam Focus

When the spot is motionless, the screen is subjected to a concentrated electron beam bombardment, causing the fluorescent material to become permanently desensitized in that area. In view of this, it is necessary to make a rapid observation. In addition, high brightness patterns when stationary for long periods might burn themselves into the screen materials; therefore, it is a good practice to reduce brightness (intensity) to a usable minimum level.

With the vertical and horizontal controls turned fully clockwise, the vertical positioning control must be rotated through its entire range and the displacement of the spot on the Y axis observed. Erratic movement of the spot during this test will indicate a defective control or component in the positioning circuit. The test should be repeated using the horizontal positioning control and the displacement of the spot on the X axis noted (see Figure 54.5).

The oscilloscope must be switched on, the sync selector turned to the initial sync position and the sync amplitude turned control fully counter-clockwise. The coarse frequency control is turned to a frequency range that includes 50 Hz. A 50-Hz test signal is applied to the vertical input terminals and vertical gain control is turned up for normal viewing. Fine frequency (vernier) control should be adjusted (see Figure 54.6) until one complete cycle appears and is almost stationary. The sync amplitude control is turned slowly clockwise until the pattern becomes stationary. This adjustment is important as too much sync voltage will distort the pattern. Fine frequency control is readjusted to obtain two complete cycles; the adjustment is continued to obtain five cycles.

Figure 54.5 Vertical and Horizontal Positioning

Figure 54.6 Sweep Frequencies

One cycle will appear on the screen if the sweep frequency is equal to the frequency of the test signal (50 Hz). The horizontal time base excursions will then be 1/50th of a second.

To check the deflection linearity, a sine wave signal is applied to the vertical input and the time base is synchronized to produce one cycle. Although the sine wave of the power frequency is fixed at 50 Hz, it provides a good standard signal as a starter.

To check the horizontal linearity, the oscilloscope is switched on, the horizontal time base adjusted to produce one sine wave, and the normal amount of sync is applied to lock in the pattern. Horizontal gain control is turned fully counter-clockwise. Vertical gain control is adjusted to a produce an ~75-mm vertical line. The horizontal gain control is turned up gradually and the horizontal expansion of the sine wave pattern is noted. There should be an even expansion of the sine waveform on each side of the centre as shown in Figure 54.7(a).

To check the vertical linearity, the vertical gain control is turned fully counter clockwise. The horizontal gain control is adjusted to produce an ~75-mm horizontal line. The vertical gain control is turned up gradually and the peak-to-peak expansion of the sine wave pattern noted. There should be an even expansion of the sine wave on each side of the baseline as shown in Figure 54.7(b).

Figure 54.7 Horizontal and Vertical Linearity

Terms such as positive going and negative going must not be confused with positive or negative half sine waves. As shown in Figure 54.8, sine wave contains both positive going and negative going cycles. A half cosine wave is either positive going or negative going depending on the direction of wave motion as shown in Figure 54.9.

Figure 54.8 Positive and Negative Swings: (a) Positive and Negative Peaks of Sine Wave (b) Positive and Negative Swings in a Half Sine Wave

Figure 54.9 Positive and Negative Swings: (a) Positive and Negative Peaks of Cosine Wave (b) Positive or Negative Peaks of a Cosine Wave

To check the deflection polarity, the oscilloscope is switched on and the necessary control is adjusted to produce an image. Horizontal gain control is turned fully counter-clockwise and the vertical input attenuator, if used, is adjusted to X1. This will provide maximum deflection sensitivity. A 0.25-μF capacitor is connected across the test leads or input binding posts. This short circuits the random noise pulse and holds the spot steady. The vertical positioning control is adjusted until the spots are set at the bottom of the screen.

The test leads are connected across a 1-V cell observing polarity, positive to vertical input lead and negative to ground lead. On contact with the cell, the spot should be deflected up and return. The spot movement is only momentary, but sufficiently long enough to observe the direction. The capacitor is discharged and the test is repeated. If the oscilloscope is provided with a polarity reversal switch, it should be in the normal position. Deflection polarity is illustrated in Figure 54.10.

When measuring the phase shift or making other tests requiring equal X and Y traces, it is necessary to equalize both deflection traces. This is important as the vertical deflection sensitivity is slightly greater than the horizontal deflection sensitivity and X and Y amplifiers will show unequal traces for equal gain of a standard input signal connected to the X and Y amplifiers.

When the vertical and horizontal forces are equal, their combined force is represented by a 45° diagonal trace (see Figure 54.11).

Figure 54.10 Deflection Polarity

Figure 54.11 Equal Vertical and Horizontal Forces

A 50-Hz test signal is connected to both vertical and horizontal input terminals and sweep control is switched to horizontal amplifier and oscilloscope turned on.

Vertical gain control is turned fully counter-clockwise and horizontal gain control adjusted to provide a 75-mm trace. Horizontal input lead is removed and vertical gain control is adjusted to provide a 75-mm trace. Horizontal gain control setting should not be disturbed. Now, the horizontal input lead is reconnected. A diagonal trace should appear that is 45° off horizontal as shown in Figure 54.11. This indicates that the X and Y traces are equal and the resultant trace represents the vector sum. If an elliptical pattern appears on the screen, 50-Hz phase shift between the vertical and horizontal amplifiers should be corrected.

54.3  SCREEN PATTERN OBTAINED WITH DEFLECTION VOLTAGES

With no external voltage applied to either plate, the spot rests on the centre of the screen (see Figure 54.12(a)). If we apply an a.c. voltage between the vertical input to vertical and ground, a vertical line is displayed, as shown in Figure 54.12(b). If the signal voltage is applied to the horizontal input terminal and earth, a horizontal line will be displayed on the screen (see Figure 54.12(c)).

Figure 54.12 Screen Patterns: (a) Spot at the Centre of the Screen (b) Vertical Line (c) Horizontal Line

A peak-to-peak voltage is equivalent to a d.c. voltage of the same value but a d.c. source does not provide the same sustained up and down motion of the beam unless the d.c. voltage is switched on and off repeatedly. Response of the beam to d.c. voltages is illustrated in Figure 54.13.

Figure 54.13 Response of CRT to d.c. Voltages: (a) Zero Voltage Applied (b) 15 V Positive (c) 15-V Negative

54.3.1  Lissajous Figures

A sine wave voltage is applied to both the plates of the CRT. A diagonal straight line is displayed on the screen (Figure 54.14).

Note: The vertical and horizontal deflection voltages are equal in amplitude and pass through zero at the same instant that are in phase. This is the requirement for displaying a straight set line at 45° angle. If the sine waves applied to the two sets of deflection plates have the same amplitude and the same frequency but are 90° different in phase, a circular pattern is displayed on the screen as illustrated in Figure 54.15.

Figure 54.14 In-phase Equal Amplitude Sine Waves Applied to Both Pairs of Deflecting Plates, the Resulting Pattern is a Straight Line

Figure 54.15 Development of a Circular Pattern by Two Sine Waves with the Same Frequency and Amplitude but 90° Different in Phase

Phase differences in the range from 0 to 90° produce elliptical Lissajous figures as exemplified in Figure 54.16. Any phase angle can be measured as shown in Figure 54.17. The ellipse is carefully centred on the screen and the interval M and N are measured. Then the phase angle between the vertical and horizontal voltages is M/N. Figure 54.18 shows the progress of patterns in this situation for a range of 360° in 45° steps.

Figure 54.16 Elliptical Lissajous Figures Produced by Two Sine Waves with the Same Frequency and Amplitude, but with 30° and 60° Phase Differences

Figure 54.17 Phase Angle Difference of the Deflection Voltages is Equal to Arc Sin M/N

Figure 54.18 Lissajous Figures in the Range from 0° to 360°

Figure 54.19 Ellipses Produced by Unequal Signal Voltages Having a Phase Difference of 90°: (a) Horizontal Voltage>Vertical Voltage (b) Vertical Voltage>Horizontal Voltage

Note: The 45° ellipse leans to the right, whereas the 135° ellipse leans to the left.

The frequency ratio is given by the ratio of number of tangencies to vertical and horizontal boundaries of the pattern as illustrated in Figure 54.20.

Figure 54.20 Lissajous Patterns Produced by Sine Wave Voltages that Have Equal Amplitudes, but that Differ in Frequencies

54.4  VOLTAGE AND CURRENT MEASUREMENTS     

The parameter of voltage that must easily be determined using an oscilloscope for a sine wave is the peak to peak value.

The magnitude is determined using the engraving on the graticule in conjunction with the calibrated ranges of the input amplifier. For example, for the waveform in Figure 54.21, the amplifier sensitivity is set to 20 mV/dv. The peak-to-peak amplitude is 20 × 6 = 120 mV. Should it be the RMS value of voltage that is required, then (assuming a distortion-less sine wave) this peak-to-peak value must be divided by that is

The oscilloscope is a high-input impedance instrument and therefore cannot be directly used for the measurement of current. Current can, of course, be measured as voltage drops across resistors, but care must be taken in connecting the oscilloscope leads to a resistor for this purpose, because unless a differential input amplifier is being used, one side of the voltage dropping resistor will have to be at earth potential.

Figure 54.21 Measurement of Voltage from a CRO Display

SUMMARY
  1. The scope is widely used for the visual observation of electrical waveforms.
  2. The measurement capabilities of the oscilloscope are limited only by the skill of the operator.
  3. With the correct intensity and focus control adjustment, there should be at least one-third in rotation on the control in either direction.
  4. When the spot is motionless, the screen is subjected to a concentrated electron beam bombardment.
  5. Too much sync voltage will distort the pattern.
  6. It is necessary to equalize both deflection traces.
  7. The 45° ellipse leans to the right, whereas the 135° ellipse leans to the left.
  8. Phase angle difference of the deflection voltages is arc sin M/N.
  9. The frequency ratio is given by the ratio of number of tangencies to vertical and horizontal boundaries of the pattern.