The internal principle and structure of oscilloscope worthy of your understanding

What is an oscilloscope? Do you really know an oscilloscope? An oscilloscope is a very widely used and relatively complex instrument. This chapter introduces the principle and method of use of oscilloscopes from the perspective of use.

01 Working principle of oscilloscope

An oscilloscope is an Electronic measuring instrument that uses the characteristics of an electronic oscilloscope to convert an alternating electrical signal that cannot be directly observed by the human eye into an image and Display it on a fluorescent screen for measurement. It is an essential instrument for observing experimental phenomena of digital circuits, analyzing problems in experiments, and measuring experimental results. The oscilloscope is composed of oscilloscope and power supply system, synchronization system, X-axis deflection system, Y-axis deflection system, delayed scan system, and standard signal source.

1. Oscilloscope tube

The cathode ray tube (CRT) is abbreviated as the oscilloscope tube, which is the core of the oscilloscope. It converts electrical signals into optical signals. As shown in Figure 1, the electron gun, deflection system and phosphor screen are sealed in a vacuum glass shell to form a complete oscilloscope tube.

Figure 1 The internal structure and power supply diagram of the oscilloscope

(1) Fluorescent screen

The screen surface of the current oscilloscope is usually a rectangular plane, and a layer of phosphorescent material is deposited on the inner surface to form a fluorescent film. A layer of evaporated aluminum film is often added on the fluorescent film. High-speed electrons pass through the aluminum film, hit the phosphors and emit light to form bright spots. The aluminum film has an internal reflection effect, which is beneficial to improve the brightness of the bright spot. Aluminum film also has other functions such as heat dissipation.

When the electrons stop bombarding, the bright spot cannot disappear immediately but remains for a period of time. The elapsed time for the brightness of the bright spot to drop to 10% of the original value is called “afterglow time”. Afterglow time shorter than 10μs is extremely short afterglow, 10μs-1ms is short afterglow, 1ms-0.1s is medium afterglow, 0.1s-1s is long afterglow, and greater than 1s is extremely long afterglow. General oscilloscopes are equipped with medium persistence oscilloscopes, high-frequency oscilloscopes use short persistence, and low-frequency oscilloscopes use long persistence.

Due to the different phosphorescent materials used, different colors of light can be emitted on the phosphor screen. Generally, oscilloscopes use green light-emitting oscilloscope tubes to protect people’s eyes.

(2) Electron gun and focus

The electron gun consists of a filament (F), a cathode (K), a grid (G1), a front accelerating electrode (G2) (or second grid), a first anode (A1) and a second anode (A2). Its function is to emit electrons and form a very thin high-speed electron beam. The filament is energized to heat the cathode, and the cathode emits electrons when heated.

The grid is a metal cylinder with a small hole on the top, which is sleeved outside the cathode. Since the grid potential is lower than that of the cathode, it controls the electrons emitted by the cathode. Generally, only a small amount of electrons with a large initial velocity can pass through the small holes of the grid under the action of the anode voltage and run towards the phosphor screen. The electrons with low initial velocity still return to the cathode.

If the grid potential is too low, all electrons return to the cathode, that is, the tube is cut off. The W1 potentiometer in the adjustment circuit can change the grid potential and control the electron flow density to the phosphor screen, so as to adjust the brightness of the bright spot. The first anode, the second anode and the front accelerator are all three metal cylinders on the same axis as the cathode. The front acceleration pole G2 is connected to A2, and the applied potential is higher than A1. The positive potential of G2 accelerates the cathode electrons towards the phosphor screen.

When the electron beam rushes from the cathode to the phosphor screen, it undergoes two focusing processes. The first focusing is done by K, G1, G2. K, K, G1, G2 are called the first electron lens of the oscilloscope. The second focusing occurs in the areas G2, A1, and A2. Adjusting the potential of the second anode A2 can make the electron beam converge at a point on the phosphor screen. This is the second focusing. The voltage on A1 is called the focus voltage, and A1 is also called the focus electrode. Sometimes adjusting the voltage of A1 still fails to meet good focusing, and the voltage of the second anode A2 needs to be fine-tuned. A2 is also called the auxiliary focusing electrode.

(3) Deflection system

The deflection system controls the direction of the electron rays, so that the light spot on the phosphor screen depicts the waveform of the measured signal with the change of the external signal. In Figure 8.1, two pairs of vertical deflection plates Y1, Y2 and Xl, X2 form a deflection system. The Y-axis deflection plate is in front and the X-axis deflection plate is behind, so the Y-axis sensitivity is high (the measured signal is added to the Y-axis after processing). Voltages are applied to the two pairs of deflection plates, so that an electric field is formed between the two pairs of deflection plates, and the electron beams are respectively controlled to deflect in the vertical direction and the horizontal direction.

(4) Power supply of oscilloscope

In order for the oscilloscope to work normally, there are certain requirements for the power supply. It is stipulated that the potential between the second anode and the deflection plate is similar, and the average potential of the deflection plate is zero or close to zero. The cathode must work at a negative potential. The grid G1 has a negative potential (-30V~-100V) relative to the cathode, and it is adjustable to achieve brightness adjustment. The first anode has a positive potential (approximately +100V~+600V), which should also be adjusted for focus adjustment.

The second anode is connected to the front accelerating electrode, and it has a positive high voltage (about +1000V) to the cathode, and the adjustable range of the potential relative to the ground is ±50V. Since the current of each electrode of the oscilloscope is very small, it can be powered by a common high voltage via a resistor divider.

02 The basic composition of an oscilloscope

As can be seen from the previous section, as long as you control the voltage on the X-axis deflection plate and the Y-axis deflection plate, you can control the shape of the graph displayed by the oscilloscope. We know that an electronic signal is a function of time f

The basic block diagram of the oscilloscope is shown in Figure 2. It consists of five parts: oscilloscope, Y-axis system, X-axis system, Z-axis system and power supply.

Figure 2 Block diagram of the basic composition of an oscilloscope

The measured signal ① is connected to the “Y” input terminal, and is appropriately attenuated by the Y-axis attenuator, and then sent to the Y1 amplifier (pre-amplification), and push-pull output signals ② and ③. The delay stage delays Г1 time to the Y2 amplifier. After amplification, large enough signals ④ and ⑤ are generated and added to the Y-axis deflection plate of the oscilloscope. In order to display a complete and stable waveform on the screen, the measured signal ③ of the Y-axis is introduced into the trigger circuit of the X-axis system, and a trigger pulse ⑥ is generated at a certain level of the positive (or negative) polarity of the introduced signal, and start Sawtooth wave scanning circuit (time base generator) to generate scanning voltage ⑦.

Since there is a time delay Г2 from trigger to start of scanning, in order to ensure that the X-axis starts to scan before the Y-axis signal reaches the fluorescent screen, the delay time Г1 of the Y-axis should be slightly greater than the delay time Г2 of the X-axis. The scanning voltage ⑦ is amplified by the X-axis amplifier to generate push-pull outputs ⑨ and ⑩, which are applied to the X-axis deflection plate of the oscilloscope. The z-axis system is used to amplify the forward sweep of the scanning voltage and turn it into a positive rectangular wave, which is sent to the oscilloscope grid. This makes the waveform displayed in the forward scanning process have a certain brightness, and the wiping is performed in the backward scanning process.

The above is the basic working principle of an oscilloscope. The dual-track display uses an electronic switch to display two different measured signals input from the Y axis on the fluorescent screen. Due to the visual persistence of the human eye, when the switching frequency is high to a certain level, two stable and clear signal waveforms are seen.

There is often an accurate and stable square wave signal generator in the oscilloscope for calibrating the oscilloscope.

03 Use of oscilloscope

This section introduces how to use the oscilloscope. There are many types and models of oscilloscopes, and their functions are also different. In digital circuit experiments, a 20MHz or 40MHz dual-trace oscilloscope is more commonly used. The usage of these oscilloscopes is similar. This section is not aimed at a certain model of oscilloscope, but introduces the common functions of oscilloscopes in digital circuit experiments conceptually.

1. Fluorescent screen

The phosphor screen is the display part of the oscilloscope. There are multiple scale lines in the horizontal and vertical directions on the screen, indicating the relationship between the voltage and time of the signal waveform. The horizontal direction indicates the time, and the vertical direction indicates the voltage. The horizontal direction is divided into 10 grids, the vertical direction is divided into 8 grids, and each grid is divided into 5 parts. The vertical direction is marked with 0%, 10%, 90%, 100% and other signs, and the horizontal direction is marked with 10%, 90% signs for measuring DC level, AC signal amplitude, delay time and other parameters. The voltage value and time value can be obtained by multiplying the number of grids occupied by the measured signal on the screen by an appropriate proportional constant (V/DIV, TIME/DIV).

2. Oscilloscope tube and power system

(1) Power

The main power switch of the oscilloscope. When this switch is pressed, the power indicator light is on, indicating that the power is on.

(2) Intensity

Turn this knob to change the brightness of the light spot and scan line. It can be smaller when observing low-frequency signals, and larger when observing high-frequency signals.

Generally, it should not be too bright to protect the fluorescent screen.

(3) Focus

The focus knob adjusts the size of the electron beam section to focus the scan line into the clearest state.

(4) Ruler brightness (Illuminance)

This knob adjusts the brightness of the light behind the fluorescent screen. Under normal indoor light, the lighting should be darker. In an environment with insufficient indoor light, the lighting can be adjusted appropriately.

3. Vertical deflection factor and horizontal deflection factor

(1) Vertical deflection factor selection (VOLTS/DIV) and fine adjustment

Under the action of the unit input signal, the distance the light spot shifts on the screen is called the shift sensitivity. This definition is applicable to both the X axis and the Y axis. The reciprocal of the sensitivity is called the deflection factor. The unit of vertical sensitivity is cm/V, cm/mV or DIV/mV, DIV/V, and the unit of vertical deflection factor is V/cm, mV/cm or V/DIV, mV/DIV. In fact, due to customary usage and the convenience of measuring voltage readings, the deflection factor is sometimes regarded as the sensitivity.

Each channel in the tracking oscilloscope has a vertical deflection factor selection band switch. Generally, there are 10 levels from 5mV/DIV to 5V/DIV according to 1, 2, and 5. The value indicated by the band switch represents the voltage value of one grid in the vertical direction on the phosphor screen. For example, when the band switch is set to 1V/DIV, if the signal light spot on the screen moves one division, it means that the input signal voltage changes by 1V.

There is often a small knob on each band switch to fine-tune the vertical deflection factor of each gear. Rotate it clockwise to the end, and it is in the “calibration” position. At this time, the vertical deflection factor value is consistent with the value indicated by the band switch. Turn this knob counterclockwise to fine-tune the vertical deflection factor. After fine-tuning the vertical deflection factor, it will cause inconsistency with the indicated value of the band switch. This should be paid attention to. Many oscilloscopes have a vertical expansion function. When the fine-tuning knob is pulled out, the vertical sensitivity is expanded by several times (the deflection factor is reduced by several times). For example, if the deflection factor indicated by the band switch is 1V/DIV, the vertical deflection factor is 0.2V/DIV when the ×5 expansion state is used.

When doing digital circuit experiments, the ratio of the vertical movement distance of the measured signal on the screen to the vertical movement distance of the +5V signal is often used to determine the voltage value of the measured signal.

(2) Time base selection (TIME/DIV) and fine adjustment

The use of time base selection and fine-tuning is similar to the vertical deflection factor selection and fine-tuning. The time base selection is also realized by a band switch, and the time base is divided into several gears according to 1, 2, and 5. The indication value of the band switch represents the time value for the light spot to move one division in the horizontal direction. For example, in the 1μS/DIV file, the light spot moving one grid on the screen represents a time value of 1μS.

The “fine adjustment” knob is used for time base calibration and fine adjustment. When turning it clockwise until it is in the calibration position, the time base value displayed on the screen is consistent with the nominal value shown by the band switch. Turn the knob counterclockwise to fine-tune the time base. After the knob is pulled out, it is in the scanning expansion state. Usually ×10 expansion, that is, the horizontal sensitivity is expanded 10 times, and the time base is reduced to 1/10.For example, in the 2μS/DIV file, the time value represented by a horizontal grid on the phosphor screen in the extended scan state is equal to

2μS×(1/10)=0.2μS

There are 10MHz, 1MHz, 500kHz, 100kHz clock signals on the TDS test bench, which are generated by a quartz crystal oscillator and a frequency divider, with high accuracy, and can be used to calibrate the time base of the oscilloscope.

The standard signal source CAL of the oscilloscope is specially used to calibrate the time base and vertical deflection factor of the oscilloscope. For example, the standard signal source of COS5041 oscilloscope provides a square wave signal with VP-P=2V, f=1kHz.

The Position knob on the front panel of the oscilloscope adjusts the position of the signal waveform on the fluorescent screen. Rotate the horizontal displacement knob (marked with a horizontal two-way arrow) to move the signal waveform left and right, and rotate the vertical displacement knob (mark with a vertical two-way arrow) to move the signal waveform up and down.

4. Input channel and input coupling selection

(1) Input channel selection

There are at least three options for input channels: channel 1 (CH1), channel 2 (CH2), and dual channel (DUAL). When channel 1 is selected, the oscilloscope only displays the signal of channel 1. When channel 2 is selected, the oscilloscope only displays the signal of channel 2. When dual channels are selected, the oscilloscope displays the channel 1 signal and channel 2 signal at the same time. When testing a signal, first connect the ground of the oscilloscope with the ground of the circuit under test.

According to the selection of the input channel, insert the oscilloscope probe into the corresponding channel socket, connect the ground on the oscilloscope probe to the ground of the circuit under test, and the oscilloscope probe touches the point to be measured. There is a two-position switch on the oscilloscope probe. When this switch is set to the “×1” position, the measured signal is sent to the oscilloscope without attenuation, and the voltage value read from the phosphor screen is the actual voltage value of the signal. When the switch is set to the “×10” position, the measured signal is attenuated to 1/10, and then sent to the oscilloscope. The voltage value read from the phosphor screen is multiplied by 10 to get the actual voltage value of the signal.

(2) Input coupling method

There are three options for input coupling: alternating current (AC), ground (GND), and direct current (DC). When “ground” is selected, the scan line shows the position of “oscilloscope ground” on the fluorescent screen. DC coupling is used to determine the absolute value of the DC signal and observe extremely low frequency signals. AC coupling is used to observe AC and AC signals containing DC components. In digital circuit experiments, the “DC” method is generally selected to observe the absolute voltage value of the signal.

5. Trigger

The first section points out that after the measured signal is input from the Y-axis, a part of it is sent to the Y-axis deflection plate of the oscilloscope, and the light spot is driven to move proportionally in the vertical direction on the phosphor screen; the other part is shunted to the x-axis deflection system to generate a trigger Pulse, trigger the scan generator, generate a repetitive sawtooth wave voltage and apply it to the X deflection plate of the oscilloscope to move the light spot in the horizontal direction. The two are combined. The pattern drawn by the light spot on the phosphor screen is the measured signal Graphics.

It can be seen that the correct trigger mode directly affects the effective operation of the oscilloscope. In order to obtain a stable and clear signal waveform on the fluorescent screen, it is very important to master the basic trigger function and its operation method.

(1) Trigger source (Source) selection

To display a stable waveform on the screen, you need to add the measured signal itself or a trigger signal that has a certain time relationship with the measured signal to the trigger circuit. The trigger source selection determines where the trigger signal is supplied. There are usually three trigger sources: internal trigger (INT), power trigger (LINE), and external trigger EXT).

The internal trigger uses the signal under test as the trigger signal, which is a frequently used trigger method. Since the trigger signal itself is a part of the signal under test, a very stable waveform can be displayed on the screen. Either channel 1 or channel 2 in the dual trace oscilloscope can be selected as the trigger signal.

Power trigger uses AC power frequency signal as trigger signal. This method is effective when measuring signals related to AC power frequency. It is especially effective when measuring low-level AC noise of audio circuits and thyratrons.

The external trigger uses an external signal as the trigger signal, and the external signal is input from the external trigger input terminal. There should be a periodic relationship between the external trigger signal and the measured signal. Since the signal under test is not used as a trigger signal, when to start scanning has nothing to do with the signal under test.

Correct selection of the trigger signal has a lot to do with the stability and clarity of the waveform display. For example, in the measurement of digital circuits, it may be better to select the internal trigger for a simple periodic signal, but for a signal with a complex period and there is a signal with a period relationship with it, it may be better to choose an external trigger. good.

(2) Trigger coupling (Coupling) mode selection

There are many ways to couple the trigger signal to the trigger circuit, the purpose is to stabilize and reliable the trigger signal. Here are some commonly used ones.

AC coupling is also called capacitive coupling. It only allows triggering with the AC component of the trigger signal, and the DC component of the trigger signal is blocked. This coupling method is usually used when the DC component is not considered to form a stable trigger. However, if the frequency of the trigger signal is less than 10 Hz, it will cause difficulty in triggering.

DC coupling (DC) does not isolate the DC component of the trigger signal. When the frequency of the trigger signal is low or the duty cycle of the trigger signal is large, it is better to use DC coupling.

When low frequency suppression (LFR) is triggered, the trigger signal is added to the trigger circuit through a high-pass filter, and the low-frequency component of the trigger signal is suppressed; when high frequency suppression (HFR) is triggered, the trigger signal is added to the trigger circuit through a low-pass filter, and the trigger signal is High frequency components are suppressed. There is also a TV sync (TV) trigger for TV repair. Each of these trigger coupling methods has its own scope of application, and needs to be experienced in use.

(3) Trigger level (Level) and trigger polarity (Slope)

Trigger level adjustment is also called synchronization adjustment, which synchronizes the scan with the measured signal. The level adjustment knob adjusts the trigger level of the trigger signal. Once the trigger signal exceeds the trigger level set by the knob, the sweep is triggered. Turn the knob clockwise to increase the trigger level; turn the knob counterclockwise to decrease the trigger level. When the level knob is adjusted to the level lock position, the trigger level is automatically maintained within the amplitude of the trigger signal, and a stable trigger can be generated without level adjustment. When the signal waveform is complex and the level knob cannot be used for stable triggering, use the HoldOff knob to adjust the holdoff time (scan pause time) of the waveform, so that the scan can be synchronized with the waveform stably.

The polarity switch is used to select the polarity of the trigger signal. When it is set to the “+” position, in the direction of signal increase, when the trigger signal exceeds the trigger level, the trigger will be generated. When it is set to the “-” position, in the direction of signal decrease, when the trigger signal exceeds the trigger level, the trigger will be generated. Trigger polarity and trigger level jointly determine the trigger point of the trigger signal.

6. Sweep Mode (SweepMode)

There are three scanning modes: Auto, Normal and Single.

Auto: When there is no trigger signal input, or when the trigger signal frequency is lower than 50Hz, the sweep is self-excited.

Normal state: When there is no trigger signal input, the scan is in the ready state and there is no scan line. After the trigger signal arrives, the sweep is triggered.

Single: The single button is similar to a reset switch. In the single scan mode, the scan circuit is reset when the single button is pressed, and the Ready light is on at this time. A scan is generated after the trigger signal arrives. After the single scan is over, the ready light goes out. A single scan is used to observe aperiodic signals or single transient signals, and it is often necessary to take a picture of the waveform.

The basic functions and operations of the oscilloscope are briefly introduced above. The oscilloscope also has some more complicated functions, such as delayed sweep, trigger delay, XY working mode, etc., which are not introduced here. Getting started with oscilloscopes is easy, but the real proficiency needs to be mastered in the application. It is worth pointing out that although the oscilloscope has more functions, it is better to use other instruments and meters in many cases. For example, in digital circuit experiments, it is much simpler to use a logic pen to determine whether a single pulse with a narrow pulse width occurs; when measuring the pulse width of a single pulse, it is better to use a logic analyzer.

04 Attention must be paid to the use of digital oscilloscopes

1 Introduction

The use of digital oscilloscopes has become increasingly popular due to its unique advantages such as waveform triggering, storage, display, measurement, and waveform data analysis and processing. Due to the large performance difference between the digital oscilloscope and the analog oscilloscope, if used improperly, a large measurement error will be generated, which will affect the test task.

2. Distinguish between analog bandwidth and digital real-time bandwidth

Bandwidth is one of the most important indicators of an oscilloscope. The bandwidth of an analog oscilloscope is a fixed value, while the bandwidth of a digital oscilloscope has two types: analog bandwidth and digital real-time bandwidth. The highest bandwidth that a digital oscilloscope can achieve by using sequential sampling or random sampling techniques for repetitive signals is the digital real-time bandwidth of the oscilloscope. The digital real-time bandwidth is related to the highest digitization frequency and waveform reconstruction technology factor K (digital real-time bandwidth = highest digitization rate/K) , Generally not directly given as an indicator.

From the definition of the two bandwidths, it can be seen that the analog bandwidth is only suitable for the measurement of repetitive periodic signals, while the digital real-time bandwidth is suitable for the measurement of repetitive signals and single signals at the same time. The manufacturer claims how many megabytes of bandwidth the oscilloscope can reach, which actually refers to the analog bandwidth, and the digital real-time bandwidth is lower than this value. For example, the bandwidth of TEK’s TES520B is 500MHz, which actually means that its analog bandwidth is 500MHz, and the highest digital real-time bandwidth can only reach 400MHz, which is much lower than the analog bandwidth. Therefore, when measuring a single signal, you must refer to the digital real-time bandwidth of the digital oscilloscope, otherwise it will bring unexpected errors to the measurement.

3. About the sampling rate

The sampling rate is also called the digitization rate, which refers to the number of times the analog input signal is sampled per unit time, often expressed in MS/s. The sampling rate is an important indicator of a digital oscilloscope.

(1) If the sampling rate is not enough, aliasing is prone to occur

If the input signal of the oscilloscope is a 100KHz sine signal, but the frequency of the signal displayed by the oscilloscope is 50KHz, what is going on? This is because the sampling rate of the oscilloscope is too slow, causing aliasing. Aliasing means that the frequency of the waveform displayed on the screen is lower than the actual frequency of the signal, or even if the trigger indicator on the oscilloscope is on, the displayed waveform is still unstable. The generation of aliasing is shown in Figure 1.

So, for a waveform of unknown frequency, how to judge whether the displayed waveform has been aliased? You can slowly change the sweep speed t/div to a faster time base file to see if the frequency parameter of the waveform changes sharply. If yes, it means that the waveform aliasing has occurred; or the shaking waveform has stabilized at a relatively fast time base file, which also indicates that the waveform aliasing has occurred. According to Nyquist’s theorem, the sampling rate is at least 2 times higher than the high-frequency components of the signal to avoid aliasing. For example, a 500MHz signal requires a sampling rate of at least 1GS/s. There are several ways to easily prevent aliasing from happening:

a. Adjust the sweep speed;

b. Use Autoset;

c. Try to switch the collection mode to envelope mode or peak detection mode, because the envelope mode is to find the extreme value in multiple collection records, while the peak detection mode is to find the maximum and minimum values ​​in a single collection record. Both methods can detect faster signal changes.

If the oscilloscope has the InstaVu acquisition method, you can choose it, because this method is fast to acquire the waveform, and the waveform displayed by this method is similar to the waveform displayed by an analog oscilloscope.

(2) The relationship between sampling rate and t/div

The maximum sampling rate of each digital oscilloscope is a fixed value. However, at any scan time t/div, the sampling rate fs is given by:

fs=N/(t/div)N is the sampling point of each grid

When the number of sampling points N is a certain value, fs is inversely proportional to t/div. The larger the scanning speed, the lower the sampling rate. In summary, in order to avoid aliasing when using a digital oscilloscope, it is best to place the sweep speed in a position where the sweep speed is faster. If you want to capture fleeting glitches, the sweep speed is best placed at a slower main sweep speed. The above is the analysis of the oscilloscope, I hope to help you.

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Author: Yoyokuo