SPACING BETWEEN PLATES. - As shown in figure 6-4, the closer together the plates, the more effect the electric force has on the deflection angle of the electron beam.

Figure 6-4. - Spacing between plates.

Q.6 What effect does closer spacing of plates have on the electron beam?

DIFFERENCE OF POTENTIALThe potential on the plates (figure 6-5) can be varied to cause a wider or narrower deflection angle. The greater the potential, the wider the deflection angle.

Figure 6-5. - Differences of potential.

Q.7 Is the deflection angle greater with higher or lower potential on the plates?

BEAM ACCELERATION. - The faster the electrons are moving, the smaller their deflection angle will be, as shown in figure 6-6.

Figure 6-6. - Beam acceleration.

Q.8 Is the deflection angle greater when the beam is moving faster or slower?

Vertical and Horizontal Plates

If two sets of deflection plates are placed at right angles to each other inside a CRT (figure 6-7), the electron beam can be controlled in any direction. By varying the potential of the vertical-deflection plates, you can make the spot (beam) on the face of the tube move vertically. The distance the beam moves will be proportional to the change in potential difference between the plates. Changing the potential difference between the horizontal-deflection plates will cause the beam to move a given distance from one side to the other. Directions other than up-down and left-right are achieved by a combination of horizontal and vertical movement.

Figure 6-7. - Deflection plate arrangement.

As shown in figure 6-8, position X of the beam is in the center. It can be moved to position Y by going up 2 units and then right 2 units. Movement of the beam is the result of the simultaneous action of both sets of deflection plates. The electrostatic field between the vertical plates moves the electrons up an amount proportional to 2 units on the screen. As the beam passes between the horizontal plates, it moves to the right an amount proportional to 2 units on the screen.

Figure 6-8. - Beam movement on the CRT.

If the amount of deflection from the left and down occurred so that each set of plates acted at the same time, the picture would be like the one in view A of figure 6-9. For example, if the vertical plates moved the beam downward (starting from point X) at the rate of 3 units per second and the horizontal plates moved it to the left at the rate of 1 unit per second, both movements would have been completed in 1 second at point Y. The result would be a straight line.

Figure 6-9. - Deflection of the beam.

In view B, the potentials on the vertical and horizontal plates change at the same rate. In the same time period, say 1 second, both plates move the beam 1 unit. The horizontal plates have completed their task at the end of 1 second, but the vertical plates have moved the beam only one-third of the required distance. In this case, the picture in view B would appear on the screen.

Beam-deflection Plate Action

Recall from your study of chapter 2 of this module that waveforms are described in terms of amplitude versus time. You have just seen how the movement of the CRT beam depends on both potential (amplitude) and time.

Q.9 Waveforms are described in terms of what two functions?