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How to use an oscilloscope, and a detailed introduction to how to use the oscilloscope

elecfans | 23/06/2021

This article introduces how to use an oscilloscope. There are many types and models of oscilloscopes, and their functions are also different. 20MHz or 40MHz dual-trace oscilloscopes are more commonly used in digital circuit experiments. 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.

Phosphor 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. Multiply the number of grids occupied by the measured signal on the screen by an appropriate proportional constant (V/DIV, TIME/DIV) to get the voltage value and time value.

Oscilloscope and power supply system

1. Power

Oscilloscope main power switch. 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 cross 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, it is better to dim the lighting. In an environment with insufficient indoor light, the lighting can be adjusted appropriately.

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 that the light spot shifts on the screen is called the shift sensitivity. This definition applies 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 the 1V/DIV gear, if the signal light spot on the screen moves one grid, it means that the input signal voltage changes 1V.

There is often a small knob on each band switch to fine-tune the vertical deflection factor of each gear. Rotate it to the end in a clockwise direction and it is in the "calibration" position. At this time, the value of the vertical deflection factor 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 point 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 extended 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-tuning

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 moves one grid on the screen to represent the time value of 1μS.

"Fine adjustment" knob is used for time base calibration and fine adjustment. When it is turned 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 scanning expansion state is equal to 2μS×(1/10)=0.2μS.

There are clock signals of 10MHz, 1MHz, 500kHz, 100kHz 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 COS5041 oscilloscope standard signal source 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 horizontal two-way arrows) to move the signal waveform left and right, and rotate the vertical displacement knob (marked with vertical two-way arrows) to move the signal waveform up and down.

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 signals, 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 fluorescent screen is multiplied by 10 to be 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.

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

In order to display a stable waveform on the screen, it is necessary 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).

Internal trigger uses the measured signal as the trigger signal, which is a frequently used trigger method. Since the trigger signal itself is a part of the measured signal, 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 the 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.

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.

The 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, while 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. it is 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) triggers, the trigger signal is added to the trigger circuit through a high-pass filter, and the low-frequency components of the trigger signal are suppressed; when high-frequency suppression (HFR) triggers, the trigger signal is added to the trigger circuit through a low-pass filter, and the trigger signal High frequency components are suppressed. There is also a TV synchronization (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 makes the scanning synchronized 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. Rotate the knob clockwise to increase the trigger level; rotate 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 trigger cannot be stabilized with the level knob, use the Hold Off knob to adjust the holdoff time (scan pause time) of the waveform, so that the sweep 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.

 SweepMode

There are three scanning modes: automatic (Auto), normal (Norm) and single (Single).

Auto: When there is no trigger signal input, or the frequency of the trigger signal 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: A 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, X-Y working mode, etc., which will not be 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 easier 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.

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