Basic Principle Of Oscilloscope

Waveform display
According to the principle of oscilloscope tubes, when a DC voltage is applied to a pair of deflection plates, a fixed displacement of the light spot on the fluorescent screen will be generated, and the magnitude of the displacement is proportional to the applied DC voltage. If two DC voltages are applied to both vertical and horizontal deflection plates simultaneously, the position of the light spot on the fluorescent screen is determined by the displacement 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 as the voltage changes. When a sinusoidal AC voltage is applied to the vertical deflection plate, at the moment of time t=0, the voltage is Vo (zero value), the position of the light spot on the fluorescent screen is at the coordinate origin 0, and at the moment of time t=1, the voltage is V1 (positive value). The light spot on the fluorescent screen is at 1 above the coordinate origin 0, and the displacement is proportional to the voltage V1; At the moment of time t=2, the voltage is V2 (the maximum positive value), and the light point on the fluorescent screen is at 2 points above the coordinate origin point 0. The displacement distance is proportional to the voltage V2; By analogy, at each instant of time t=3, t=4,..., t=8, the positions of the light spots on the fluorescent screen are respectively 3, 4,..., and 8 points. In the second and third cycles of AC voltage, the first cycle will be repeated. If the frequency of the sinusoidal AC voltage applied to the vertical deflection plate at this time is very low, only between 1Hz and 2Hz, then a moving light spot will be seen on the fluorescent screen. The instantaneous deflection value of this light point from the coordinate origin will be proportional to the instantaneous value of the voltage applied to the vertical deflection plate. If the AC voltage frequency applied to the vertical deflection plate is above 10 Hz to 20 Hz, 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 is not a point that moves up and down, but a vertical bright line. The length of the bright line depends on the magnitude of the peak to peak sinusoidal AC voltage when the vertical amplification gain of the oscilloscope is constant. If a sinusoidal AC voltage is applied to the horizontal deflection plate, a similar situation will occur, except that the light spot moves on the horizontal axis.
If a voltage that varies linearly with time, such as a sawtooth voltage, is applied to a pair of deflection plates, how will the light spot move on the fluorescent screen? When there is a sawtooth wave voltage on the horizontal deflection plate, at the moment of time t=0, the voltage is Vo (the maximum negative value), and the light spot on the fluorescent screen is at the starting position (on the zero point) on the left side of the coordinate origin, and the displacement distance is proportional to the voltage Vo; At the moment of time t=1, the voltage is V1 (negative), and the light spot on the fluorescent screen is at a point to the left of the coordinate origin. The displacement distance is proportional to the voltage V1; By analogy, at time t=2, t=3, At each instant of t=8, the corresponding positions of the light spots on the fluorescent screen are points 2, 3,..., and 8. At the moment of t=8, the sawtooth wave voltage jumps from the maximum positive value V8 to the maximum negative value Vo, and the light spot on the fluorescent screen moves extremely quickly from 8 to the left at the starting position zero. If the sawtooth voltage is periodic, the first cycle will be repeated in the second, third,... cycle of the sawtooth voltage. If the frequency of the sawtooth wave voltage applied to the horizontal deflection plate at this time is very low, only 1 Hz to 2 Hz, the light spot will be seen moving uniformly from the left starting position zero point to the right starting position zero point on the fluorescent screen, and then the light spot will move extremely quickly from the right starting position zero point to the left starting position zero point. This process is called scanning. When a periodic sawtooth voltage is applied to the horizontal axis, the scan will continue in a cycle. The instantaneous value of the distance from the light point to the zero point of the starting position will be proportional to the instantaneous value of the voltage applied to the deflection plate. If the frequency of the sawtooth wave voltage applied to the deflection plate is above 10Hz to 20Hz, due to the afterglow phenomenon of the fluorescent screen and the visual persistence phenomenon of the human eye, a horizontal bright line can be seen. The length of the horizontal bright line depends on the sawtooth wave voltage value when the horizontal amplification gain of the oscilloscope is fixed. The sawtooth wave voltage value is proportional to the change in time, and the displacement of the light spot on the fluorescent screen is proportional to the voltage value, Therefore, the horizontal bright lines on the fluorescent screen can represent the time axis. Any equal line segment on this bright line represents an equal period of time.
If the measured signal voltage is applied to the vertical deflection plate and the sawtooth wave scanning voltage is applied to the horizontal deflection plate, and the frequency of the measured signal voltage is equal to the frequency of the sawtooth wave scanning voltage, a periodic waveform curve of the measured signal voltage over time will be displayed on the fluorescent screen. In the case where the second and third cycles of the measured periodic signal all repeat the first cycle, the tracks traced by the light spot on the fluorescent screen also overlap the tracks traced for the first time. Therefore, the measured signal voltage displayed on the fluorescent screen is a stable waveform curve that changes with time.
In order to stabilize the graphics on the fluorescent screen, the frequency of the measured signal voltage should maintain an integer ratio relationship with the frequency of the sawtooth wave voltage, that is, a synchronization relationship. In order to achieve this, it is necessary to continuously adjust the frequency of the sawtooth voltage in order to adapt to observing various periodic signals at different frequencies. Secondly, due to the relative instability of the frequency of the measured signal and the frequency of the sawtooth oscillation signal, even if the frequency of the sawtooth voltage is temporarily adjusted to be an integral multiple of the frequency of the measured signal, it cannot keep the graph stable all the time. Therefore, the oscilloscope is equipped with a synchronization device. That is, adding a synchronization signal to a certain part of the sawtooth wave circuit to promote the synchronization of scanning. For simple oscilloscopes (such as domestic SB-10 oscilloscopes, etc.) that can only generate continuous scanning (i.e., generating continuous sawtooth waves from cycle to cycle), it is necessary to input a synchronization signal related to the frequency of the observed signal into their scanning circuit, When the frequency of the added synchronization signal approaches the autonomous oscillation frequency of the sawtooth wave frequency (or an integer multiple thereof), the sawtooth wave frequency can be "dragged into synchronization" or "locked". For oscilloscopes (such as domestic ST-16 oscilloscopes, SBT-5 synchronous oscilloscopes, SR-8 dual trace oscilloscopes, etc.) that have the function of waiting for scanning (that is, they do not generate sawtooth waves at ordinary times, but only generate a sawtooth wave for a single scan when the signal under test arrives), it is necessary to input a trigger signal related to the signal under test on their scanning circuit to closely coordinate the scanning process with the signal under test. In this way, as long as the appropriate synchronization signal or trigger signal is selected as needed, any process to be studied can be synchronized with the sawtooth wave scanning frequency.