An oscilloscope is a widely used electronic measuring instrument. It can convert electrical signals that are invisible to the naked eye into visible images, making it easier for people to study the changes in various electrical phenomena. The oscilloscope uses a narrow beam of high-speed electrons, which strikes a screen coated with a fluorescent material, producing a small bright spot. Under the influence of the measured signal, the electron beam acts like the tip of a pen, capable of drawing curves representing the instantaneous value changes of the measured signal on the screen. Using an oscilloscope, one can observe the waveform curves of amplitude variations over time for different signals and also use it to test various electrical quantities such as voltage, current, frequency, phase difference, modulation depth, mobile phone repair, etc.
I. Working Principle of Oscilloscopes
(a) Composition of Oscilloscopes
A regular oscilloscope has five basic components: display circuit, vertical (Y-axis) amplifier circuit, horizontal (X-axis) amplifier circuit, scanning and synchronization circuit, and power supply circuit. The functional block diagram of a common oscilloscope is shown in Figure 5-1.
1. Display Circuit
The display circuit includes two parts: the oscilloscope tube and its control circuit. The oscilloscope tube is a special type of electron tube and is an important component of the oscilloscope. The basic principle diagram of the oscilloscope tube is shown in Figure 5-2. As seen from the figure, the oscilloscope tube consists of three parts: the electron gun, deflection system, and fluorescent screen.
(1) Electron Gun
The electron gun generates and forms a high-speed, focused beam of electrons that strikes the fluorescent screen to make it glow. It mainly consists of the filament F, cathode K, control grid G, first anode A1, and second anode A2. Except for the filament, the structure of the other electrodes is metallic cylindrical, and their axes are aligned on the same axis. When the cathode is heated, it can emit electrons along the axial direction; the control grid is negatively charged relative to the cathode, and changing the potential can change the number of electrons passing through the small hole of the control grid, thereby controlling the brightness of the light spot on the fluorescent screen. To improve the brightness of the light spot on the screen without reducing the sensitivity of the electron beam deflection, modern oscilloscope tubes add a post-acceleration electrode A3 between the deflection system and the fluorescent screen.
Figure 5-2 shows a schematic diagram of the internal structure of the oscilloscope tube.
The first anode is connected to a positive voltage of about several hundred volts relative to the cathode. A higher positive voltage is applied to the second anode. The electron beam passing through the control grid's small hole is accelerated under the high potential of the first anode and the second anode, moving at high speed toward the fluorescent screen. Due to the repulsion of charges of the same nature, the electron beam gradually spreads out. Through the focusing effect of the electric field between the first anode and the second anode, the electrons re-converge and intersect at a point. Appropriately controlling the potential difference between the first anode and the second anode allows the focus to fall exactly on the fluorescent screen, showing a bright, small dot. Changing the potential difference between the first anode and the second anode can adjust the focus of the light spot, which is the principle behind the "focus" and "auxiliary focus" adjustments of the oscilloscope. The third anode is formed by coating a layer of graphite inside the cone of the oscilloscope tube, usually with a very high voltage applied. It serves three purposes: ① accelerating the electrons further after they pass through the deflection system so that the electrons have enough energy to strike the fluorescent screen for sufficient brightness; ② the graphite layer covering the entire cone can serve as a shielding function; ③ the electron beam striking the fluorescent screen generates secondary electrons, which can be absorbed by A3 at a high potential.
(2) Deflection System
The deflection system of the oscilloscope tube is mostly electrostatic deflection and consists of two pairs of mutually perpendicular parallel metal plates, known as horizontal deflection plates and vertical deflection plates respectively. They control the movement of the electron beam in the horizontal and vertical directions. When the electrons move between the deflection plates, if there is no voltage applied to the deflection plates, there will be no electric field between the deflection plates. Electrons entering the deflection system after leaving the second anode will move along the axial direction and hit the center of the screen. If there is a voltage on the deflection plates, an electric field will form between the deflection plates. Electrons entering the deflection system will be deflected toward the designated position on 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 equals zero, then the electron beam with velocity υ passing through the deflection plate space will move along the original direction (set as the axis direction) and strike the origin of the fluorescent screen. If there is a constant potential difference between the two deflection plates, then an electric field will form between the deflection plates, which is perpendicular to the motion direction of the electrons. Thus, the electrons will deflect towards the deflection plate with a higher potential. In this case, the electrons will follow a parabolic trajectory in the space between the two deflection plates, making a tangent motion at this point. Finally, the electrons land on point A on the fluorescent screen. The distance between point A and the origin (0) of the fluorescent screen is called the deflection amount, denoted by y. The deflection amount y is proportional to the voltage Vy applied to the deflection plates. Similarly, when a DC voltage is applied to the horizontal deflection plates, a similar situation occurs, but the light spot deflects horizontally.
(3) Fluorescent Screen
The fluorescent screen is located at the terminal end of the oscilloscope tube. Its function is to display the deflected electron beam so that it can be observed. The inner wall of the oscilloscope's fluorescent screen is coated with a layer of luminescent material, so the location on the fluorescent screen impacted by high-speed electrons will glow. At this moment, the brightness of the light spot depends on the number, density, and speed of the electrons in the electron beam. When the voltage of the control grid is changed, the number of electrons in the electron beam will also change, altering the brightness of the light spot. While using the oscilloscope, it is not advisable to let a very bright light spot remain fixed at one position on the fluorescent screen of the oscilloscope tube, otherwise, the phosphor material at that point may burn out due to long-term electron impact, thus losing its ability to glow.
Fluorescent screens coated with different phosphor materials will display different colors and different afterglow times when struck by electrons. Usually, for observing general signal waveforms, green light-emitting ones with medium afterglow oscilloscope tubes are used. For observing non-periodic and low-frequency signals, orange-yellow light-emitting ones with long afterglow oscilloscope tubes are used. In oscilloscopes used for photography, short afterglow blue-emitting oscilloscope tubes are generally adopted.
2. Vertical (Y-axis) Amplifier Circuit
Since the deflection sensitivity of the oscilloscope tube is very low, for example, the commonly used oscilloscope tube model 13SJ38J has a vertical deflection sensitivity of 0.86mm/V (approximately a deflection of 1cm caused by 12V voltage), most of the measured signal voltages need to be amplified by the vertical amplifier circuit before being added to the vertical deflection plate of the oscilloscope tube, to obtain appropriately sized graphs in the vertical direction.
3. Horizontal (X-axis) Amplifier Circuit
Since the deflection sensitivity in the horizontal direction of the oscilloscope tube is also very low, the voltage (sawtooth wave voltage or other voltage) connected to the horizontal deflection plate of the oscilloscope needs to be amplified by the horizontal amplifier circuit first, then added to the horizontal deflection plate of the oscilloscope tube, to obtain appropriately sized graphs in the horizontal direction.
4. Scanning and Synchronization Circuit
The scanning circuit generates a sawtooth wave voltage. The frequency of this sawtooth wave voltage can be continuously adjustable within a certain range. The role of the sawtooth wave voltage is to make the electron beam emitted by the cathode of the oscilloscope tube form a periodic, time-proportional horizontal displacement on the fluorescent screen, forming a time baseline. In this way, the variation waveform of the measured signal in the vertical direction over time can be displayed on the fluorescent screen.
5. Power Supply Circuit
The power supply circuit provides the necessary negative high voltage, filament voltage, etc., for the vertical and horizontal amplifier circuits, the scanning and synchronization circuits, as well as the oscilloscope tube and control circuits.
From the functional block diagram of the oscilloscope's principle, it can be seen that the measured signal voltage is added to the Y-axis input terminal of the oscilloscope, then amplified by the vertical amplifier circuit and applied to the vertical deflection plate of the oscilloscope tube. The horizontal deflection voltage of the oscilloscope tube, although often adopting a sawtooth voltage (used for observing waveforms), sometimes also adopts other external voltages (used for measuring frequency, phase difference, etc.), hence there is a horizontal signal selection switch at the input end of the horizontal amplifier circuit, allowing the choice between the internal sawtooth wave voltage of the oscilloscope or other external voltages added to the X-axis input terminal as the horizontal deflection voltage.
In addition, to keep the pattern displayed on the fluorescent screen stable, it is required that the frequency of the sawtooth wave voltage signal be synchronized with the frequency of the measured signal. Therefore, not only must the frequency of the sawtooth wave voltage be continuously adjustable, but also a synchronization signal must be input into the circuit generating the sawtooth wave. For simple oscilloscopes that can only produce continuous scanning (i.e., generate continuous, endless sawtooth waves), a synchronization signal related to the frequency of the observed signal needs to be input into the scanning circuit to constrain the oscillation frequency of the sawtooth wave. For oscilloscopes with waiting scanning functionality (i.e., do not generate sawtooth waves normally, but generate a sawtooth wave and perform a scan when the measured signal arrives), a trigger signal related to the measured signal needs to be input into the scanning circuit, making the scanning process closely coordinated with the measured signal. To meet various needs, the synchronization (or trigger) signal can be selected via a synchronization or trigger signal selection switch. Typically, there are three sources: ① the measured signal is used as the synchronization (or trigger) signal, led from the vertical amplifier circuit, referred to as "internal synchronization" (or "internal trigger") signal; ② an externally added signal is used as the synchronization (or trigger) signal, referred to as "external synchronization" (or "external trigger") signal, added to the external synchronization (or external trigger) input terminal; ③ some oscilloscopes' synchronization signal selection switches have a "power synchronization" option, where the 220V, 50Hz power voltage is reduced by the secondary winding of a transformer and used as the synchronization signal.
(b) Basic Principle of Waveform Display
According to the principle of the oscilloscope tube, when a DC voltage is applied to a pair of deflection plates, it will cause a fixed displacement of the light spot on the fluorescent screen. This displacement is proportional to the applied DC voltage. If two DC voltages are simultaneously applied to the vertical and horizontal pairs of deflection plates, the position of the light spot on the fluorescent screen will be determined by displacements in both directions.
If a sinusoidal AC voltage is applied to a pair of deflection plates, the light spot on the fluorescent screen will move according to the voltage changes. As shown in Figure 5-4, when a sinusoidal AC voltage is applied to the vertical deflection plates, at time t=0, the voltage is Vo (zero value), and the position of the light spot on the fluorescent screen is at the coordinate origin 0. At time t=1, the voltage is V1 (positive value), and the light spot on the fluorescent screen is at position 1 above the coordinate origin 0, with the displacement size proportional to the voltage V1. At time t=2, the voltage is V2 (maximum positive value), and the light spot on the fluorescent screen is at position 2 above the coordinate origin 0, with the displacement distance proportional to the voltage V2. By analogy, at times t=3, t=4, ..., t=8, the positions of the light spot on the fluorescent screen are points 3, 4, ..., 8 respectively. In the second cycle, third cycle, ... of the AC voltage, the situation will repeat that of the first cycle. If at this time the frequency of the sinusoidal AC voltage applied to the vertical deflection plates is very low, merely 1Hz~2Hz, then a moving light spot will be seen on the fluorescent screen. The instantaneous deflection value of this light spot from the coordinate origin will be proportional to the instantaneous value of the voltage applied to the vertical deflection plates. If the frequency of the AC voltage applied to the vertical deflection plates is above 10Hz~20Hz, then due to the afterglow phenomenon of the fluorescent screen and the visual persistence phenomenon of the human eye, what is seen on the fluorescent screen will not be a moving point up and down, but a vertical bright line instead. The length of this bright line, given a fixed vertical amplification gain of the oscilloscope, depends on the peak-to-peak value of the sinusoidal AC voltage. Similarly, if a sinusoidal AC voltage is applied to the horizontal deflection plates, a similar situation will occur, just with the light spot moving along the horizontal axis.
As shown in Figure 5-5, if a voltage that varies linearly with time (such as a sawtooth wave voltage) is applied to a pair of deflection plates, how will the light spot move on the fluorescent screen? As seen in Figure 5-5, when there is a sawtooth wave voltage on the horizontal deflection plates, at time t=0, the voltage is Vo (maximum negative value), and the light spot on the fluorescent screen is at the starting position (zero point) on the left side of the coordinate origin, with the displacement distance proportional to the voltage Vo. At time t=1, the voltage is V1 (negative value), and the light spot on the fluorescent screen is at position 1 on the left side of the coordinate origin, with the displacement distance proportional to the voltage V1. By analogy, at times t=2, t=3, ..., t=8, the corresponding positions of the light spot on the fluorescent screen are points 2, 3, ..., 8 respectively. At time t=8, the sawtooth wave voltage jumps from the maximum positive value V8 to the maximum negative value Vo, causing the light spot on the fluorescent screen to move extremely quickly from point 8 back to the starting position zero point on the left. If the sawtooth wave voltage applied to the horizontal deflection plates has a very low frequency, merely 1Hz~2Hz, then a light spot will be seen moving uniformly from the starting position zero point on the left to point 8 on the right on the fluorescent screen, followed by the light spot moving extremely quickly from point 8 on the right back to the starting position zero point on the left. This process is called scanning. When a periodic sawtooth wave voltage is applied to the horizontal axis, the scanning will continue indefinitely. The instantaneous value of the distance of the light spot from the starting position zero point will be proportional to the instantaneous value of the voltage applied to the deflection plates. If the frequency of the sawtooth wave voltage applied to the deflection plates is above 10Hz~20Hz, then due to the afterglow phenomenon of the fluorescent screen and the visual persistence phenomenon of the human eye, a horizontal bright line will be seen, whose length, given a fixed horizontal amplification gain of the oscilloscope, depends on the value of the sawtooth wave voltage, which is proportional to the time changes, and the displacement of the light spot on the fluorescent screen is also proportional to the voltage value. Therefore, the horizontal bright line on the fluorescent screen can represent the time axis. Any equal line segment on this bright line represents an equal period of time.
As shown in Figure 5-6, if the measured signal voltage is applied to the vertical deflection plates and the sawtooth wave scanning voltage is applied to the horizontal deflection plates, and the frequency of the measured signal voltage is equal to the frequency of the sawtooth wave scanning voltage, then a curve representing the variation of the measured signal voltage over time for one period will be displayed on the fluorescent screen (as shown in Figure 5-6). From Figure 5-6, it can be seen that at time t=0, the signal voltage is Vo (zero value), and the sawtooth wave voltage is V0' (negative value), so the light spot on the fluorescent screen is on the left side of the coordinate origin, with the displacement distance proportional to the voltage V0'. At time t=1, the AC voltage is V1 (positive value), and the sawtooth wave voltage is V1' (negative value), so the light spot on the fluorescent screen is in the second quadrant of the coordinates. Similarly, at times t=2, t=3, ..., t=8, the light spots on the fluorescent screen are at points 2, 3, ..., 8 respectively. At time t=8, the sawtooth wave voltage jumps from the maximum positive value V8' to the maximum negative value V0', causing the light spot on the fluorescent screen to move extremely quickly from point 8 back to the starting position 0 point. Afterward, during the second period, third period, ... of the measured periodic signal, the situation repeats that of the first period, and the trajectories drawn by the light spot on the fluorescent screen overlap those drawn during the first period. Therefore, the measured signal voltage displayed on the fluorescent screen is a stable waveform curve varying over time.
If the frequency of the measured signal voltage is an integer multiple of the frequency of the sawtooth wave voltage, then a stable waveform with an integer number of periods of the measured signal will be displayed on the fluorescent screen. However, if the frequency of the measured signal voltage is not an integer multiple of the frequency of the sawtooth wave voltage, then a stable waveform cannot be obtained on the fluorescent screen, as shown in Figure 5-7. In Figure 5-7, during the first scan, the waveform curve from 0 to 1 is displayed on the screen; during the second scan, the waveform curve from 1 to 2 is displayed on the screen; during the third scan, the waveform curve from 2 to 3 is displayed on the screen; ... It can be seen that the waveform curve displayed on the screen each time is different, so the image is unstable.
From the above, it can be seen that to make the image on the fluorescent screen stable, the frequency of the measured signal voltage should maintain an integer ratio relationship with the frequency of the sawtooth wave voltage, i.e., a synchronous relationship. To achieve this, the frequency of the sawtooth wave voltage needs to be continuously adjustable to adapt to observing various different frequency periodic signals. Secondly, due to the relative instability of the frequencies of the measured signal and the sawtooth wave oscillation signal, even if the frequency of the sawtooth wave voltage is temporarily adjusted to an integer multiple relationship with the frequency of the measured