Hengaode instrument manufacturer analyzes the principle of oscilloscope use steps

An oscilloscope is a very widely used electronic measuring instrument. It can transform electrical signals that are invisible to the naked eye into visible images, which is convenient for people to study the changing process of various electrical phenomena. The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit a screen coated with a fluorescent substance to produce a small light spot (this is the working principle of a traditional analog oscilloscope). Under the effect of the measured signal, the electron beam is like the tip of a pen, and the curve of the instantaneous value of the measured signal can be drawn on the screen. Using an oscilloscope, you can observe the waveform curves of various signal amplitudes changing with time, and you can also use it to test various electrical quantities, such as voltage, current, frequency, phase difference, amplitude adjustment, and so on.

Introduction

An oscilloscope is a very widely used electronic measuring instrument. It can transform electrical signals that are invisible to the naked eye into visible images, which is convenient for people to study the changing process of various electrical phenomena. The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit a screen coated with a fluorescent substance to produce a small light spot. Under the effect of the measured signal, the electron beam is like the tip of a pen, and the curve of the instantaneous value of the measured signal can be drawn on the screen. Using an oscilloscope, you can observe the waveform curves of various signal amplitudes changing with time, and you can also use it to test various electrical quantities, such as voltage, current, frequency, phase difference, amplitude adjustment, and so on.

Steps for usage

(1) First preset: turn the brightness knob counterclockwise to the end, the vertical and horizontal displacements turn to the middle, the attenuation is placed at the highest level, and the scan is placed in the "external X file";

(2) Turn on the power again, wait for one or two minutes after the indicator light is on, and then perform related operations after warming up; [1]

(3) First adjust the brightness, then adjust the focus, and then adjust the horizontal and vertical displacement so that the bright spot is in the appropriate area in the center;

(4) Adjust the scan, scan fine-tuning and X gain, observe the scan;

(5) Pull out the external X file to the appropriate position in the scanning range, and observe the voltage waveform in the vertical direction provided by the machine according to the law of sine and cosine;

(6) Connect the applied voltage to be studied from the Y input and the ground to the oscilloscope, adjust each gear to the appropriate position, and observe the waveform of this voltage (image with time change) (adjust the synchronization polarity switch to make the image The starting point of is from positive half week or negative half week;

(7) If you want to observe the vertical deviation of bright spots (such as when applying a DC voltage), you can adjust the scan to the "external X" position.

(Different oscilloscopes may operate differently)

Fundamental

Display circuit

The display circuit includes two parts: the oscilloscope and its control circuit. The oscilloscope is a special kind of electron tube and an important part of an oscilloscope. The oscilloscope consists of three parts: electron gun, deflection system and fluorescent screen.

(1) Electron gun

The electron gun is used to generate and form a high-speed, focused electron flow to bombard the fluorescent screen to make it glow. It is mainly composed of filament F, cathode K, control pole G, first anode A1 and second anode A2. Except for the filament, the structure of the other electrodes is a metal cylinder, and their axes are kept on the same axis. After the cathode is heated, it can emit electrons in the axial direction; the control electrode has a negative potential relative to the cathode. Changing the potential can change the number of electrons passing through the control pinhole, that is, the brightness of the light spot on the fluorescent screen. In order to improve the brightness of the light spot on the screen without reducing the sensitivity to electron beam deflection, a modern accelerating tube also includes a rear acceleration electrode A3 between the deflection system and the fluorescent screen.

The first anode applies a positive voltage of about several hundred volts to the cathode. A higher positive voltage is applied to the second anode than the first anode. The electron beam passing through the control tiny hole is accelerated by the high potential of the first anode and the second anode, and moves at a high speed in the direction of the phosphor screen. Due to the repulsion of the same charge, the electron beam will gradually spread out. Through the focusing effect of the electric field between the first anode and the second anode, the electrons are re-assembled and converge at one point. Properly controlling the size of the potential difference between the first anode and the second anode can make the focus just fall on the fluorescent screen, showing a bright and small dot. Changing the potential difference between the first anode and the second anode can play the role of adjusting the focus of the light spot. This is the principle of "focus" and "auxiliary focus" adjustment of the oscilloscope. The third anode is formed by coating a layer of graphite inside the cone of the oscilloscope, usually with a very high voltage, it has three functions: â‘ to further accelerate the electrons after passing through the deflection system, so that the electrons have enough energy to go Bomb the phosphor screen to obtain sufficient brightness; â‘¡The graphite layer is coated on the entire cone, which can play a shielding role; â‘¢The electron beam bombards the phosphor screen to generate secondary electrons, and A3 at a high potential can absorb these electrons.

(2) Deflection system

The deflection system of the oscilloscope is mostly electrostatic deflection type, which is composed of two pairs of parallel metal plates perpendicular to each other, called horizontal deflection plate and vertical deflection plate. Control the movement of the electron beam in the horizontal and vertical directions respectively. When the electrons move between the deflection plates, if there is no voltage applied to the deflection plates, there is no electric field between the deflection plates, and the electrons entering the deflection system after leaving the second anode will move in the axial direction and shoot toward the center of the screen. If there is voltage on the deflection plate and there is an electric field between the deflection plates, the electrons entering the deflection system will be directed to the specified position of the fluorescent screen under the action of the deflection electric field.

If the two deflection plates are parallel to each other, and their potential difference is equal to zero, then the electron beam with velocity Ï… passing through the deflection plate space will move in the original direction (set as the axis direction) and hit the coordinate origin of the fluorescent screen . If there is a constant potential difference between the two deflection plates, an electric field is formed between the deflection plates. This electric field is perpendicular to the moving direction of the electrons, and the electrons are deflected toward the deflection plate with a higher potential. In this way, in the space between the two deflection plates, the electron moves tangentially at this point along the parabola. Finally, the electrons land on point A on the phosphor screen. This point A is a distance away from the origin (0) of the phosphor screen. This distance is called the deflection and is represented by y. The deflection amount y is proportional to the voltage Vy applied to the deflection plate. Similarly, when a DC voltage is applied to the horizontal deflection plate, a similar situation occurs, except that the light spot is deflected in the horizontal direction.

(3) Physical picture of fluorescent screen oscilloscope

The fluorescent screen is located at the terminal of the oscilloscope. Its function is to display the deflected electron beam for easy observation. A layer of luminescent substance is coated on the inner wall of the oscilloscope's phosphor screen. Therefore, the location on the phosphor screen exposed to high-speed electrons shows fluorescence. The brightness of the light spot at this time depends on the number, density and speed of the electron beam. When the voltage of the control electrode is changed, the number of electrons in the electron beam will change accordingly, and the brightness of the light spot will also change. When using an oscilloscope, it is not advisable to allow a very bright light spot to be fixed at a position on the oscilloscope fluorescent screen, otherwise the fluorescent substance at that point will be burned out due to long-term impact of electrons, thus losing its luminous ability.

Fluorescent screens coated with different fluorescent substances will show different colors and different afterglow time when impacted by electrons. Usually used for observing the general signal waveform is green light, which is a medium afterglow oscilloscope for observation of non-periodic Sex and low-frequency signals are used to emit orange-yellow light, which is a long afterglow oscilloscope; oscilloscopes used for photography generally use short afterglow oscilloscopes that emit blue.

Vertical (Y axis) amplifier circuit

Due to the low deflection sensitivity of the oscilloscope of the student oscilloscope, for example, the commonly used oscilloscope 13SJ38J type, its vertical deflection sensitivity is 0.86mm / V (about 12V voltage produces 1cm of deflection), so the general signal voltage to be measured must be Firstly, it is amplified by the vertical amplifying circuit, and then added to the vertical deflection plate of the oscilloscope, so as to obtain a figure of appropriate size in the vertical direction.

Horizontal (X axis) amplifier circuit

Since the deflection sensitivity of the oscilloscope in the horizontal direction is also very low, the voltage (sawtooth wave voltage or other voltage) connected to the horizontal deflection plate of the oscilloscope must also be amplified by the horizontal amplification circuit before being added to the oscilloscope â€™s Horizontally deflect the board to get a figure of appropriate size in the horizontal direction.

Scanning and synchronization circuit

The scanning circuit generates a sawtooth wave voltage. The frequency of the sawtooth wave voltage can be continuously adjusted within a certain range. The function of the sawtooth voltage is to cause the electron beam emitted by the cathode of the oscilloscope to form a periodic horizontal displacement on the phosphor screen that is proportional to time, that is, to form a time baseline. In this way, the time-varying waveform of the measured signal applied in the vertical direction can be displayed on the fluorescent screen.

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