Under normal conditions, the relative displacement of the fluorescent screen light spot is proportional to the voltage input to the X or Y axis of the oscilloscope. For example, a sinusoidal signal voltage Uy = sin omega t is sent to the Y axis deflector plate of the oscilloscope. If no voltage is applied on the X axis, a vertical straight line is seen on the fluorescent screen. When the frequency of the signal is small enough, we can clearly see the motion of the point of light - sinusoidal vibration.
If the horizontal deflection plate of the oscilloscope is added with a linearly varying sawtooth signal voltage.If there is no signal on the Y-axis deflector plate, only a horizontal line can be observed on the fluorescent screen.
If the observed sinusoidal voltage Uy is applied to the Y-axis deflector plate, and the scanning voltage Ux is applied to the X-axis deflector plate at the same time, the frequency of the sinusoidal wave is equal to the repetition frequency of the scanning voltage wave, then a complete sinusoidal wave can be observed on the fluorescent screen.