Analysis of the Principle Framework of Analog Oscilloscope

The analog oscilloscope is an older type of oscilloscope, first appearing in the 1940s. The one shown here uses a cathode ray tube display and has a bandwidth of only a few MHz.

The trigger of analog oscilloscope is generally simple, usually edge trigger. After setting the corresponding edge trigger conditions, once the effective edge of the measured signal comes, the oscilloscope starts to generate a sawtooth wave to control the horizontal scan, so that the waveform seen on the oscilloscope screen each time is the waveform after the trigger point of the measured signal. If the measured signal is periodic, such as a clock signal, a stable signal waveform can be seen on the oscilloscope.

We can think of an oscilloscope simply as a voltmeter with a graphical display.

An ordinary voltmeter gives a measurement reading of the signal voltage by moving a pointer or digital display on its dial. An oscilloscope is different. An oscilloscope has a screen that can display the change of the signal voltage over time in a graphical way on the screen, that is, the waveform.

The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit the screen coated with fluorescent material to produce tiny light spots (this is the working principle of traditional analog oscilloscopes). Under the action of the measured signal, the electron beam is like the tip of a pen, which can draw a curve of the instantaneous value of the measured signal on the screen. The oscilloscope can be used to observe the waveform curve of various signal amplitudes changing with time, and it can also be used to test various electrical quantities, such as voltage, current, frequency, phase difference, amplitude modulation, etc.

What are the main circuit units of an analog oscilloscope? What are their functions? What are the connections between them?

1. Host and display circuit, which is the display part and power supply, including the brightness, focus, scale, track rotation, etc. on the panel;

2. Vertical part: signal channel, amplification and attenuation circuit of the measured signal;

3. The scanning part, including scanning signal, synchronization circuit, etc., is the circuit that stably and horizontally displays the Y-axis measured signal on the oscilloscope;

4. Brightness enhancement circuit. This part of the circuit can be said to be an image elimination circuit. It is a circuit that does not have retrace, making it easier to observe the signal.

Generally, the measured signal enters the vertical input amplifier circuit, and a signal is output from the emitter follower in the circuit and sent to the scanning circuit for triggering. The horizontal scanning can horizontally scale the waveform of the measured signal at the appropriate gear, and finally send the Y-end and X-axis end signals to the deflection plate of the oscilloscope at the same time.

The composition of an oscilloscope An ordinary oscilloscope has five basic components: display circuit, vertical (Y-axis) amplification circuit, horizontal (X-axis) amplification circuit, scanning and synchronization circuit, and power supply circuit. The principle function block diagram of an ordinary oscilloscope is shown in Figure 5-1.

The display circuit consists of two parts: the oscilloscope and its control circuit. The oscilloscope is a special electron tube and an important part of the oscilloscope. The basic schematic diagram of the oscilloscope is shown in Figure 5-2. As can be seen from the figure, the oscilloscope consists of three parts: the electron gun, the deflection system and the 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 a filament F, a cathode K, a control electrode G, a first anode A1, and a second anode A2. Except for the filament, the structures of the other electrodes are all metal cylinders, and their axes are kept on the same axis. After the cathode is heated, it can emit electrons along the axial direction; the control electrode has a negative potential relative to the cathode, and changing the potential can change the number of electrons passing through the small hole of the control electrode, that is, control the brightness of the light spot on the fluorescent screen. In order to increase the brightness of the light spot on the screen without reducing the sensitivity to the deflection of the electron beam, a post-acceleration electrode A3 is added between the deflection system and the fluorescent screen in modern oscilloscopes.

The first anode has a positive voltage of several hundred volts applied to the cathode. A higher positive voltage than the first anode is applied to the second anode. The electron beam passing through the control pinhole is accelerated by the high potential of the first anode and the second anode, and moves at high speed toward the fluorescent screen. Since like charges repel each other, the electron beam will gradually disperse. Through the focusing effect of the electric field between the first anode and the second anode, the electrons are reassembled and converged at one point. By properly controlling the size of the potential difference between the first anode and the second anode, the focus can be made to fall exactly on the fluorescent screen, showing a bright and small dot. Changing the potential difference between the first anode and the second anode can adjust the focus of the light spot. This is the principle of the "focus" and "auxiliary focus" adjustment of the oscilloscope. The third anode is formed by coating the inside of the cone of the oscilloscope with a layer of graphite, and usually a very high voltage is applied to it. It has three functions: ① It further accelerates the electrons after passing through the deflection system so that the electrons have enough energy to bombard the fluorescent screen to obtain sufficient brightness; ② The graphite layer is coated on the entire cone and can play a shielding role; ③ The electron beam bombarding the fluorescent screen will produce 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 consists of two pairs of mutually perpendicular parallel metal plates, called horizontal deflection plates and vertical deflection plates. They 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 on the deflection plates, there is no electric field between the deflection plates. After leaving the second anode, the electrons entering the deflection system will move axially and shoot toward the center of the screen. If there is voltage on the deflection plates, there is an electric field between the deflection plates. The electrons entering the deflection system will be shot to the designated position of the fluorescent screen under the action of the deflection electric field.

As shown in Figure 5-3. If the two deflection plates are parallel to each other and their potential difference is zero, then the electron beam with a speed υ passing through the deflection plate space will move along 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 will be formed between the deflection plates. This electric field is perpendicular to the direction of electron movement, so the electron will deflect toward the deflection plate with a higher potential. In this way, in the space between the two deflection plates, the electron moves tangentially along the parabola at this point. Finally, the electron lands at point A on the fluorescent screen. This point A is a distance away from the origin of the fluorescent screen (0). This distance is called the deflection amount, 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) Fluorescent screen

The fluorescent screen is located at the terminal of the oscilloscope. Its function is to display the deflected electron beam for observation. A layer of luminescent material is coated on the inner wall of the fluorescent screen of the oscilloscope. Therefore, the location on the fluorescent screen that is impacted by high-speed electrons will show fluorescence. At this time, the brightness of the light spot 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 let a very bright light spot appear fixedly at one position on the fluorescent screen of the oscilloscope tube, otherwise the fluorescent material at that point will burn out due to long-term electron impact, thereby losing its luminous ability.

The fluorescent screen coated with different fluorescent materials will show different colors and different afterglow times when impacted by electrons. The ones usually used to observe general signal waveforms are those that emit green light, which are medium afterglow oscilloscopes, and those used to observe non-periodic and low-frequency signals are those that emit orange-yellow light, which are long afterglow oscilloscopes. Oscilloscopes used for photography generally use blue short afterglow oscilloscopes.

The advantages of analog oscilloscopes are self-explanatory, including good real-time performance, simple principle, and low price. However, the instrument principle itself also contains flaws that will eventually be abandoned by the times. There are roughly the following: