Determining the voltage level sensitivity of an oscilloscope

　　J2458 or J2459 oscilloscope and power supply; 0-100V variable DC power supply; high internal resistance voltmeter (0-100V, DC).

　　principle

　　If one plate is at a positive potential relative to the other, the electron beam will be attracted to the plate with positive potential. The deflection of the electron beam causes the light spot on the screen to deflect, and the amount of deflection depends on the potential difference between the plates.

　　If a stable voltage is applied between the two plates, the light spot will move from the center of the screen to a point to the left or right of the center. The specific amount of displacement depends on the voltage applied to the poles. If the voltage is applied to the Y deflection plate, the light spot will move up or down.

　　If an AC voltage is applied, the light spot will draw a straight line, the length of which is the difference between the positive and negative peak values ​​of the applied voltage. The DC voltage corresponding to this straight line is times the effective value of the AC voltage. The effective voltage value of this AC can be displayed by an AC voltmeter.

　　Experimental Procedure

　　Connect the power supply to the AC power supply and the oscilloscope at the same time, and adjust the control knobs ("brightness" and "focus") to obtain a small and clear focused light spot in the center of the fluorescent screen. If adjustments have been made previously, only the voltage applied to the plates needs to be changed. In order to simplify the figure, the power supply and oscilloscope are omitted in Figure 38/2.

　　Assemble the circuit according to Figure 38/2, where H is a variable high voltage power supply, V is a voltmeter, S is a fluorescent screen, X1, X2, Y1 and Y2 are deflection plates, and E is a ground electrode.

　　Y2 is grounded and connected to the negative terminal of the high voltage power supply. X2 is grounded so that the light spot is exactly in the center of the scale. Disconnect X2 from the ground, and then adjust the high voltage control to add a voltage of about 10V between X2 and X1; pay attention to the movement of the light spot in the direction of X2, and record the reading of the voltmeter and the position of the light spot on the scale of the fluorescent screen. Increase the voltage step by step in 10 levels, and record the voltage applied and the deflection caused in each case.

　　Then the high voltage is reversed, the positive electrode is grounded, and X2 is made negative relative to X1, which will cause the light spot to deflect in the opposite direction. A series of observation data is obtained as described above.