(1) Understand the working principle of oscilloscope.

(2) Master the basic adjustment method and working mode of oscilloscope.

(3) Master the method of observing signals with an oscilloscope.

Laboratory instrument

1 dual trace oscilloscope, 1 function signal generator and coaxial cable.

Oscilloscope principle

Oscilloscope is an instrument for displaying two-dimensional images, which reflects the transient process of voltage by changing the trajectory of electrons through electric field. Two-dimensional images need two coordinates y and x for mathematical description. The two-dimensional image on the oscilloscope is formed by two electric fields, namely, Y electric field (Y deflection) and X electric field (X deflection) * * * which affect the electron trajectory.

For the two-dimensional function of voltage signal V=F(t), two coordinates, V and T, are needed to describe it. Mathematical drawing is very simple. Oscilloscope forms Y electric field by adding voltage V=F(t) to deflection Y, which affects the trajectory or displacement of electrons in Y direction, thus displaying two-dimensional graphics. This reflects the v value. (If V=F(t) changes slowly, what is the trajectory of electrons in the Y direction). But this does not depict a two-dimensional graph with V=F(t), and t is not represented. How can I express t? Time cannot be "added" to X deflection, only the concept of time can be "transferred" to the concept of voltage. If the linear relationship of V=Kt is established, time will "become" voltage, but with the increase of t, the voltage will be very large and will exceed the display screen, which is impossible to achieve. Finally, sawtooth wave is selected to realize both. When V=Kt, the X electric field is added to the X deflection, which affects the electron trajectory (orthogonal superposition). V=F(t) is described by Y electric field * * *.

V=F(t) and V=Kt are actually two completely unrelated voltage signals, and their time t is also irrelevant. In order to establish contact, the oscilloscope has set up an auxiliary function trigger synchronization system.

In a word, around the establishment of two-dimensional graphics, the oscilloscope panel is equipped with vertical Y direction adjustment function, horizontal X direction adjustment function and auxiliary function of triggering synchronization system. Press the three function areas to be familiar with the function of each button, which is easy to understand and remember.

The structure of 1 oscilloscope

Oscilloscope consists of oscilloscope tube, attenuation and amplification input system, scanning signal generator, trigger synchronization system and power supply system.

2 The principle of oscilloscope display waveform

The function of X deflection plate is to make the light spot move horizontally, and the function of Y deflection plate is to make the light spot move vertically. Therefore, when no voltage is applied to the X deflection plate and only sinusoidal signals are applied to the Y deflection plate, we can only see a vertical bright line on the screen. When the frequency of the signal is small enough, we can clearly see the motion process of the light spot-sinusoidal vibration.

When the scanning signal on the X deflection plate completes M cycles and the sinusoidal signal on the Y deflection plate just completes N cycles, then the light spot on the screen will repeat the previous trajectory movement, and we can see a stable figure.

3 Synchronization and trigger scanning

Oscilloscope can adopt the method of "trigger scanning" to stabilize the waveform. It uses the measured signal to control the generation time of scanning voltage. When the trigger level is adjusted to make the measured signal reach the positioning phase, the scanning circuit starts to work, generating sawtooth waves and displaying the measured signal. Because the scanning circuit only works when the measured signal reaches a positioning phase, the waveform displayed in each scan is the same, so the waveform seen on the fluorescent screen is stable.

4 Lissajous figure

When the X-axis input scanning sawtooth voltage signal, the sawtooth voltage signal "simulates" the concept of time, and the oscilloscope shows the transient process of the Y-axis input signal. When a sinusoidal signal is input on the X-axis and another sinusoidal signal is input on the Y-axis, and the frequencies of these two signals are simple integer multiples, a composite pattern of electron beams undergoing two mutually perpendicular resonant motions is observed, which is called Lissajous pattern.