We know as long as we control the voltage which on X-axis and Y-axis deflection boards on digital oscilloscopes, we can control the graphic shapes on CRT display. An electronic signal is the function of time f（t）, which changes with time. Therefore, we can plus a voltage which is proportional with the time variable of the X-axis on CRT, and plus test signals on Y-axis, the graphics which are produced by the measured signals change with time. Among the electronic signals, the signal is proportional with the time variable in a period of time which is called sawtooth wave.
Digital oscilloscope is consisted of CRT, X-axis, Y-axis, Z-axis system and power supply.
Test signal ①was accessed to "Y" input terminal, and appropriate weakened by attenuator of Y-axis, then was sent to the Y1 amplifier (preamp), then push-pull output signal ②, ③. Through delay class to Y2 amplifier will be have a delay timeГ1. After amplifying it generate amplified signal ④, ⑤, added to the Y-axis deflection board on CRT of digital oscilloscope. In order to display the signal stable and completely, we can make the test signal③ access to the trigger circuit of the X-axis system, when we lead into positive or negative of the signal, and produce trigger pulse, then switch on sawtooth wave scanning circuit(time base generator) and produce scanning voltage⑦. As there is a period of time for delayГ2 from trigger to switch on scanning, ensure X-axis have started scanning before the signals of Y-axis are sent to the screen, so the delay timeГ1 which on X-axis should slightly larger than the delay timeГ2 which on X-axis. The scanning voltage⑦ through the amplifier of X-axis and amplified by the amplifier, push-pull output ⑨, ⑩ then adding ⑨, ⑩ to the X-axis deflection board on the CRT. Z-axis system is for amplifying scanning voltage on the way, and changing it to positive rectangular wave, at last send them to the gate of CRT on digital oscilloscope. This makes the waveform has a fixed luminance and wiping traces scan in return.
These are the basic parts of digital oscilloscopes. Dual trace show is using the electronic switch to display the two different signals which are input from Y-axis. As the function for human’s eyes have persistence of vision, when converting high frequency to a certain extent, we will see two stable and clear signal waveforms.
Digital oscilloscopes always have a square-wave generator of accuracy and stability, just for verifying the oscilloscopes.