Lecture 12: Oscilloscope Basics: Basic Principles and Basic Operation Steps for Capturing Signals

Central topic: How to capture signals with high fidelityOscilloscope operation stepsSolution: Choose the appropriate test sensitivityRestore the oscilloscope to factory settings, set channels, etc. before testing. This is a good habit for testing. Many engineers who are new to oscilloscopes are most concerned about "how to get the waveform out". At this time, we are generally taught to use the AutoSet key. But if the waveform still does not come out after AutoSet, we are often at a loss; even if Auto Set can make the waveform come out, can we continue to measure and analyze? Only very junior engineers know how to use AutoSet, so our low-end oscilloscope WaveJet series has a fast response speed of AutoSet. Press AutoSet and the waveform will come out in about 1 second. But AutoSet cannot guarantee that the signal is accurately captured with high fidelity. 

Capturing the signal with high fidelity is the first priority in operating the oscilloscope, otherwise it will be meaningless to continue some measurements and analysis. In order to achieve high-fidelity signal capture, we need to master some basic principles of setting the oscilloscope. 

The basic principles of signal capture are:
first, minimize quantization error;
second, always be vigilant about the sampling rate;
third, capture at least one cycle of interest in the low-frequency component;
fourth, sometimes use some special acquisition mode or processing method. 

First, we need to understand the screen display of the oscilloscope. The oscilloscope is a tool for human-computer interaction, and each operation will bring about changes in the display on the screen. As shown in Figure 1, the horizontal axis of the oscilloscope has ten grids, and the capture time = 10 x [Time/Div]. Adjusting the horizontal time base knob on the panel will increase or decrease the capture time accordingly. Expanding the waveform can show that the waveform is composed of points. The time interval between two adjacent points is the sampling period, which is the inverse of the sampling rate. The number of all points displayed on the screen represents the storage depth of the oscilloscope. 

Sampling rate x sampling time = storage depth
This is the first relationship of the oscilloscope, which is very important. As shown in the lower right of Figure 1, the Timebase menu of the LeCroy oscilloscope is displayed above. The three values ​​​​shown above, the two numbers on the right multiplied by 10 are equal to the number on the left. When adjusting the time base, we must "keep an eye on the sample rate"-always be vigilant about the sampling rate.

The vertical axis of the oscilloscope has 8 large grids, and the vertical range = 8 x [Volts/Div]?» 256 binary codes correspond to 8-bit ADC.
The ADC of the oscilloscope is only 8 bits, which is the first limitation of the digital oscilloscope. This means that if we need to measure a voltage of 5mV, we can use 256 0s and 1s to represent it, and we can only use 256 0s and 1s to measure a voltage of 1000V. When measuring a voltage of 5mV, we can set it to 2mV/div. Then what is the minimum step, that is, the voltage represented by the last bit jumping from 0 to 1? (8 x 2mV)/256=62.5uV, 62.5uV represents the minimum step (quantization error). But if it is to measure a voltage of 1000V, the vertical sensitivity is set to 125V/div, then the minimum step is (8 x 125V/div)/256=3.9V, and the quantization error is very large! If you use this range to test 1V voltage, the error is like using a meter scale to measure the diameter of a hair!   Figure 1 Oscilloscope screen display and Timebase menu display   Figure 2 Physical meaning of 8-bit ADC Below we follow the steps of capturing waveforms with an oscilloscope to emphasize the above four basic principles. Many times when we turn on the oscilloscope, we see many waveforms and measurement parameters displayed on the screen, as shown in Figure 3.   Figure 3 Display screen with multiple waveforms and multiple parameters
At this time, the first recommended operation step is to restore the factory settings and clear all previous settings. Clearing and setting from scratch is more efficient. The menu for restoring factory settings is the Recall Default button under File, as shown in Figure 4.  Figure 4 Restoring factory default settings
After this operation, if the first and second channels of the oscilloscope are not connected to any probe, two zero-level lines will be seen on the screen, such as C2 in Figure 4 is not connected to a probe, and a zero line is displayed. After this operation, if these two lines are not seen, it means that the channel of the oscilloscope is not working properly, which is also a way to judge whether the oscilloscope is good or bad. The second 

operation step is to connect the probe and select the channel of the oscilloscope. Sometimes, after connecting the probe, the probe calibration and the delay calibration between channels should be performed, but this step is sometimes ignored in non-rigorous measurements. In the illustration of this article, my experimental environment is a LeCroy DEMO board connected to channel 1 via a BNC line, so channel 2 needs to be turned off. Select the channel by pressing the buttons marked 1, 2, 3, and 4 on the oscilloscope panel. The

third step is to set the vertical channel of the oscilloscope. The first step in setting the vertical channel is to select the coupling mode. Low-bandwidth oscilloscopes usually have four coupling modes, DC 50Ω, DC 1MΩ, AC 1MΩ, and Ground. In this example, because it is connected to a BNC line, the coupling mode needs to be set to DC 50Ω, as shown in Figure 5.   Figure 5 Vertical channel setting menu 
The second step in vertical channel setting is to adjust the vertical offset and vertical sensitivity, try to make the waveform fill the screen, and minimize the quantization error. This is the first basic principle of capturing signals.

Although the vertical offset and vertical sensitivity can be set through the menu once, they can also be quickly operated through the panel. As shown in Figure 6, the four knobs on the top are used to adjust the vertical offset and change the position of the waveform on the screen. Pressing the knob vertically can automatically return the offset to zero. The four knobs on the bottom are used to change the range. In order to make the waveform occupy more than 7.5 grids on the screen, it is sometimes necessary to fine-tune the knob. Select Variable Gain in the vertical channel setting menu to fine-tune it. LeCroy's fourth-generation oscilloscope can directly fine-tune it by pressing this knob.   Figure 6 Vertical channel setting panel 
Figure 7 shows the comparison of the peak-to-peak results of the same signal under different ranges.   Figure 7 Test results under different ranges, 608.81mV / 569.67mV 
in the 200mV/div range is 608.81mV (average value), and in the 80mV/div range is 569.67mV. In the test specifications related to amplitude, it should be defined at what range the test is performed. Otherwise, the test results are not comparable. The meanings of the other items in the vertical channel setting menu are also clear at a glance, and will not be introduced one by one.
After completing the vertical setting, proceed to the fourth operation step, adjusting the time base. There are two points to note when adjusting the time base. The first is to always be vigilant about the sampling rate. The second is to capture at least one cycle of low-frequency components of interest so that the full picture of the signal can be seen. The time base can be adjusted through the panel shown in Figure 8.   Figure 8 Time Base Setting Panel 
Under the factory default settings of the oscilloscope, the storage depth is fixed at 100KS. The longer the capture time is adjusted to the left, the lower the sampling rate. At this time, press the local zoom key on the panel and expand the enlarged waveform to see the details of the rising edge. By observing whether there are more than five sampling points on the rising edge, it can be judged whether the signal is distorted. The

signal capture time shown in Figure 9 is 500us, and the current real-time sampling rate is 200MS/s. The result of multiplying these two numbers is 100KS. At this time, there are only two points on the rising edge. The signal has been seriously undersampled and the waveform is seriously distorted.   Figure 9 Time Base Setting Menu  Figure 10 Capture long enough to observe the full picture of the signal.
For oscilloscopes above the LeCroy WaveRunner series, a fixed sampling rate can be set. In this way, after knowing the characteristics of the measured signal, a fixed oversampling sampling rate is first used, and then the sampling time base is adjusted. Only the capture time will be changed, and the signal will not be distorted. For the signal shown in Figure 9, if the capture time is too short, even the runt cannot be observed, so a longer capture time is required to find the problem.

For the upper illustrated signal in Figure 10, although the capture time has reached 5ms, the waveform we see makes us think that there is a pulse signal every 1.5ms, but in fact, after capturing 20ms, we can see the true characteristics of this signal. At this time, the sampling rate is reduced to 1GS/s, and the waveform is actually a little distorted. This shows the benefit of needing a longer storage depth.
For the signal in Figure 9, the runt appears very regularly, and the interval between them is not long. However, if the runt only appears once in a long time, we have to use some special acquisition modes. As shown in Figure 11, WaveStream mode is used to quickly check whether there is a runt, and Figure 12 uses sequential mode (see) to locate the occurrence pattern of the runt. 
Figure 11 WaveStream mode

Figure 12 Sequential mode After completing the previous steps, enter the fifth step of the signal capture operation and set the appropriate trigger mode (see and ). We need to select the trigger source, trigger point, trigger level, trigger mode, trigger mode, etc. For the runt signal shown in Figure 7, we can isolate it through trigger modes such as runt trigger, width trigger, and time interval trigger. Figure 13 uses the runt trigger to isolate the runt. Figure 13 

Set the appropriate trigger mode to isolate the event of interest

Through the above five steps, you can achieve high-fidelity capture of the signal, and the following measurement and analysis steps are relatively simple.

Modern oscilloscopes are based on PC platforms. Operating an oscilloscope is like operating Office software. You will be familiar with the operation with a few clicks of the mouse, but it takes a long time to understand the physical meaning of many operations, especially as the analysis software packages of oscilloscopes are increasing and becoming more and more complex, involving a lot of knowledge background, and requires diligent study. The

oscilloscope is the eyes of an engineer, and being familiar with the basic operation of capturing signals is the first step for novices to get started. I hope this article can play a role in inspiring others, and I hope that everyone will communicate more.