VERTICAL-DEFLECTION PLATES. - We will use figure 6-10 to explain the action of the vertical-deflection plates in signal amplitude measurements. As this discussion begins, remember that vertical-deflection plates are used to show amplitude of a signal, and horizontal-deflection plates are used to show time and/or frequency relationships.

Figure 6-10. - Amplitude versus time.

1. From T0 to T1, the vertical plates maintain their static difference in potential and the beam stays at 0 units; the T0 to T1 change causes an increasing potential difference in the horizontal plates, and the beam moves 1 unit to the right.

2. At T1, a positive potential difference change in the vertical plates occurs, which causes the beam to move up (instantaneously) 2 units.

This vertical (amplitude) beam location is maintained from T1 to T4; horizontal beam movement continues moving to the right as 3 units of time pass.

3. At T4, an instantaneous negative change in potential of 4 units in amplitude occurs, and the beam moves from +2 to -2 units.

4. From T4 to T7, the beam remains at -2 units. During this time period, the beam continues moving horizontally to the right, indicating the passage of time.

5. At T7, a positive increase of amplitude occurs, and the beam moves vertically from -2 to 0 units. From T7 to T8, no change occurs in vertical beam movement; however, horizontal movement continues with time.

The vertical-plate potential difference follows the voltage of the waveform. The horizontal-plate potential follows the passage of time. Together, they produce the image (trace) produced on the screen by the moving beam.

Q.10 The vertical-deflection plates are used to reproduce what function?
Q.11 The horizontal-deflection plates are used to produce what function?

HORIZONTAL-DEFLECTION PLATES. - Now let's look at horizontal-deflection action. Assume that the resistance of the potentiometer shown in figure 6-11 is spread evenly along its length. When the arm of the potentiometer is at the middle position, the same potential exists on each plate. Since there is zero potential difference between the plates, an electrostatic field is not moved downward at a uniform rate; the right plate will become more positive than the left (you are looking down through the top of the CRT). The electron beam will move to the right from screen point 0 through points 1, 2, 3, and 4 in equal time intervals.

Figure 6-11. - Horizontal plates (top view).

If the potentiometer arm is moved at the same rate in the opposite direction, the right plate will decrease in positive potential until the beam returns to the 0 position. At that point, the potential difference between the plates is again zero. Moving the arm toward the other end of the resistance causes the left plate to become more positive than the right, and the beam moves from screen points 0 through 4. If the movement of the potentiometer arm is at a uniform (linear) rate, the beam moves at a uniform rate.

Notice that the ends of the deflection plates are bent outward to permit wide-angle deflection of the beam. The vertical plates are bent up and down in the same manner.

Q.12 Why are the ends of the deflection plates bent outward?

For ease of explanation, the manual movement of the potentiometer arm is satisfactory to introduce you to horizontal beam movement. However, in the oscilloscope this is not how horizontal deflection is accomplished. Beam movement voltages are produced that are much faster by sawtooth circuitry. You may want to review the sawtooth generation section in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuitry before continuing. Nearly all oscilloscopes with electrostatic deflection apply a sawtooth voltage to the horizontal plates to produce horizontal deflection of the beam, as shown in figure 6-12.

Figure 6-12. - Sawtooth generator.

In the figure, the sawtooth generator replaces the potentiometer and is connected to both horizontal plates of the CRT. At the reference line, the potential on both plates is equal. Below the line, the left plate is more positive and the right plate is less positive. This causes the beam to move left. Above the line, the right plate is made more positive than the left and the beam moves to the right. The waveform amplitude causes a uniform movement of the beam across the screen (called TRACE). RETRACE time, shown at the trailing edge of the waveform, quickly deflects the beam back to the starting point.

CRT GRATICULE

A GRATICULE was used in our previous discussion in figure 6-10 . It is simply a calibrated scale (made of clear plastic) of amplitude versus time that is placed on the face of the CRT.

The graticule can be used to determine the voltage of waveforms because the DEFLECTION SENSITIVITY of a CRT is uniform throughout the vertical plane of the screen. Deflection sensitivity states the number of inches, centimeters, or millimeters a beam will be deflected for each volt of potential difference applied to the deflection plates. It is directly proportional to the physical length of the deflection plates and their distance from the screen and inversely proportional to the distance between the plates and to the second-anode voltage. Deflection sensitivity is a constant that is dependent on the construction of the tube.

Deflection sensitivity for a given CRT might typically be 0.2 millimeters per volt. This means the spot on the screen will be deflected 0.2 millimeters (about 0.008 inch) when a difference of 1 volt exists between the plates. Sometimes the reciprocal of deflection sensitivity (called DEFLECTION FACTOR) is given. The deflection factor for the example given would be 125 volts per inch (1/0.008).

Q.13 What term is used to describe the reciprocal of deflection sensitivity of a scope?