A detailed introduction to the types and basic components of oscilloscopes

Oscilloscopes can be classified according to their internal structure, field of use, measurement range, etc. There are also some oscilloscopes used in special environments.

Types of Oscilloscopes

According to the range of the measured signal, it can be divided into the following types:

①Ultra-low frequency oscilloscope, suitable for measuring ultra-low frequency signals;

② Ordinary oscilloscope, suitable for measuring intermediate frequency signals;

③ High frequency oscilloscope and ultra-high frequency oscilloscope, suitable for measuring high frequency (100MHz) and ultra-high frequency (1000MHz) signals.

According to the number of displayed signals, there are single-trace oscilloscopes (only display one signal), dual-trace oscilloscopes (can display two signals at the same time), and multi-trace oscilloscopes (can display the waveforms of multiple signals at the same time).

Based on the circuit structure, there are tube oscilloscopes, transistor oscilloscopes and integrated circuit oscilloscopes.

In terms of measurement function, there are analog oscilloscopes and digital memory oscilloscopes. Digital memory oscilloscopes digitize the measured signal and temporarily store it in a memory, then read it out from the memory and display it on the oscilloscope. It is often used in situations where digital signals are measured to facilitate observation of the waveform and signal content of the digital signal.

Oscilloscopes can be divided into cathode ray tube (CRT) oscilloscopes, color LCD displays, and oscilloscopes made with computer color monitors based on the waveform display device.

In order to adapt to the characteristics of measuring TV signals, oscilloscope manufacturers have specially produced synchronous oscilloscopes. The oscilloscope circuit is equipped with a circuit that is synchronized with the TV line and field signals, and a key for selecting TV lines or TV fields is specially set on the control panel so that the signal waveform is stable when observing TV signals. Figure 1-27 shows the panel diagram of the DS-5220A dual-trace oscilloscope, and Figure 1-28 shows the panel diagram of the ST16A single-trace oscilloscope.

The components of an oscilloscope

The basic composition and principle block diagram of a general oscilloscope are shown in Figure 1-29. It mainly consists of four parts: display system, vertical deflection (Y-axis) signal amplification system, horizontal deflection (X-axis) sawtooth wave generation system and synchronous trigger system.

(1) Display system. The display system consists of an oscilloscope (cathode ray tube CRT) and a power supply.

Inside the oscilloscope, the electron gun emits a large number of electrons, which are controlled by the control grid, focused by the focusing electrode, accelerated by the accelerating electrode, and deflected by the deflection electrode or deflection coil. Finally, an electron beam is formed and hits the fluorescent screen, making the corresponding part of the fluorescent screen glow.

In the display system, the power supply system is responsible for providing the voltages required by the oscilloscope's control electrode, focusing electrode, accelerating electrode, etc. Adjusting these voltages can ensure that the light spots appearing on the fluorescent screen are well focused and have moderate brightness. The focus and brightness potentiometers installed on the oscilloscope panel are used to adjust these voltages.

The deflection electrode inside the oscilloscope (or the external deflection coil) is responsible for the deflection of the electron beam. Taking the deflection electrode as an example, there is a pair of X deflection plates and Y deflection plates in the oscilloscope. Each pair of deflection plates is composed of basically parallel metal plates. The voltage change on each pair of deflection plates will inevitably affect the trajectory of electron movement. The voltage change on the Y deflection plate can only affect the vertical position of the light spot on the display screen, and the X deflection plate only affects the horizontal position of the light spot. The two pairs of deflection plates work together to determine the coordinates of the light spot on the display screen at any moment.

When no voltage is applied to either pair of deflection plates, the electron beam should hit the center of the fluorescent screen.

(2) Y-axis signal amplification system. The Y-axis signal amplification system amplifies the observed signal and converts it into a voltage and applies it to the Y deflection plate, so that the electron beam will change in the Y-axis (vertical) direction according to the law of the signal.

Generally, the amplification factor of the Y-axis signal amplification system can be calculated by rotating the Y-axis deflection sensitivity.

The center position of the oscilloscope's Y-axis direction can also be adjusted using the Y-axis displacement knob.

For example, adjusting the y-axis displacement knob to the center line of the y-axis direction of the oscilloscope represents a signal of 0v, and adjusting the y-axis deflection sensitivity knob of the y-axis signal amplification system to 1v/div, a continuous signal of 0~2V will appear within a range of 2 grids above the center line of the Y-axis direction.

(3) X-axis sawtooth wave generation system. Generally, a sawtooth wave signal is added to the X-axis of the oscilloscope. When the sawtooth wave voltage is the minimum, the electron beam is at the leftmost end of the oscilloscope, and when the sawtooth wave voltage is the maximum, the electron beam is at the rightmost end of the oscilloscope. In this way, in the positive direction of the sawtooth wave, the electron c in the oscilloscope will move evenly from left to right over time, so the X-axis of the oscilloscope can be treated as the time axis. For example, the time used for the positive direction of the sawtooth wave is 1ms, and the X-axis on the oscilloscope screen is divided into 10 scales, then the time represented by each X scale is 0.1ms.

As for the return time of the sawtooth wave, we hope it is as short as possible, so as to minimize the impact of the retrace line on the oscilloscope waveform. But in fact, retrace requires a certain amount of time. In order to prevent the waveform generated by the retrace from being displayed on the fluorescent screen, we can try to reduce the brightness of the scan return to reduce the impact of the retrace line on the waveform.

The X-axis sawtooth wave generation system generates the sawtooth wave voltage required for oscilloscope scanning, also known as the scanning voltage. According to the scanning requirements, the sawtooth wave should have good linearity, short return path, and adjustable frequency. The frequency adjustment of the sawtooth wave is also called the adjustment of the scanning period.

(4) Synchronous trigger system. When observing a periodic signal, if the period of the scanning voltage is an integer multiple of the period of the measured signal, the waveform depicted in the next scanning period is exactly the same as the waveform depicted in the previous period. In this way, a clear and stable waveform can be obtained on the fluorescent screen. This is called synchronization of the signal and the scanning voltage.

But in fact, the scanning voltage is generated by the X-axis sawtooth wave generation system of the oscilloscope itself, and it is not related to the measured signal input on the Y-axis. Therefore, it is rare to encounter a situation where the scanning voltage cycle is exactly an integer multiple of the measured signal cycle. When the scanning voltage cycle is not an integer multiple of the measured signal cycle, the Y-axis signal starting from the left end of the X-axis is not equal each time, which will cause confusion in the observation results.

To solve this problem, a synchronous trigger signal is often generated using the measured signal, and the synchronous signal is used to control the X-axis sawtooth wave generation system, forcing the period of the X-axis sawtooth wave generator to become an integer multiple of the period of the measured signal. This is the synchronous trigger system. The synchronous trigger system can synchronize the X-axis sawtooth wave generation system with the external measured signal, thereby stably displaying the external signal.