The basic principles and basic operating steps of oscilloscope signal capture

很多初学示波器的工程师最关心的是“怎么让波形出来”，这时候我们一般都被教会了要用“AutoSet”键。 但如果AutoSet之后波形还是出不来，我们往往不知所措了; 或者是即使Auto Set能使波形出来，就可以往下进行测量和分析了吗？ 只有很初级的工程师会用AutoSet，所以我们很低端示波器WaveJet系列设计的AutoSet反应速度全世界最快，按一下Auto Set，1秒左右就有波形出来。但AutoSet不能保证信号被准确地高保真地捕获。 高保真地捕获信号是操作示波器的第一要著，否则再继续一些测量和分析就没有什么意义了。为实现高保真地捕获信号，我们需要掌握设置示波器的一些基本原则。 捕获信号的基本原则是：第一，最小化量化误差; 第二，时刻警惕采样率; 第三，至少捕获感兴趣的一个周期的低频成分; 第四，在有些时候使用一些特别的获取模式或处理方法。 首先，我们要了解示波器的屏幕显示。示波器是人机交互的工具，每一个操作会带来屏幕上显示的变化，这变化代表什么含义？这是基础之基础呵。如图一所示，示波器的水平轴有十大格，捕获时间=10 x [Time/Div]，调节面板上的水平时基旋钮，就会相应增加或减小捕获的时间。展开波形可以看到波形有一个个的点组成，这相邻两点之间的时间间隔就是采样周期，是采样率的倒数。屏幕上显示的全部点的个数就表示为示波器的存储深度。 采样率x 采样时间= 存储深度。这是示波器的第一关系式，非常重要。如图一右下边显示的是力科示波器的一次菜单Timebase，上面显示的三个数值，右边的两个数相乘再乘以10就等于左边的数。在调节时基的时候我们要“keep an eye on the sample rate”——时刻警惕采样率。示波器的垂直轴有8大格，垂直范围=8 x [Volts/Div]?? 256二进制码，对应8位的ADC。示波器的ADC只有8位，这是数字示波器的第一局限性。 这也就是说，如果我们需要测量5mV的电压用256个0和1来表征，测量 1000V的电压也只能用256个0和1来表征。 测5mV电压时可以设置为2mV/div，那么最小步进，即最后一位由0跳变到1代表的电压大小是多少？(8 x 2mV)/256=62.5uV，62.5uV代表的是最小步进（量化误差）。但如果是测量1000V的电压，垂直灵敏度设置为125V/div,那么最小步进是(8 x 125V/div)/256=3.9V,量化误差很大！ 如果用这个量程去测试1V的电压带来的误差就如用一把米刻度去测量头发丝的直径！关于量化误差，大家可以参考《力科第一季》第20集谈到的关于电源纹波的测量。

Figure 1 Oscilloscope screen display and Timebase menu display

Figure 2 The physical meaning of 8-bit ADC [page]

Next, we will emphasize the above four basic principles according to the operation steps of capturing waveforms with an oscilloscope. Many times, when we turn on the oscilloscope, we see many waveforms and measurement parameters displayed on the screen, such as shown in Figure 3. At this time, the first operation step I recommend is to restore the factory settings to clear all previous settings. It is more efficient to clear them and start over. The menu for restoring factory settings is the Recall Default button under File, as shown in Figure 4. When doing this



Figure 3 Display screen with multiple waveforms and multiple parameters

Figure 4 Restore factory default settings

After the 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. In Figure 4, C2 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 example of this article, my experimental environment is a LeCroy DEMO board connected to channel 1 through 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 the BNC line is connected, the coupling mode needs to be set to DC 50Ω, as shown in Figure 5. The second step of vertical channel setting is to adjust the vertical offset and vertical sensitivity to make the waveform fill the screen as much as possible 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, 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 below are used to change the range. In order to make the waveform fill 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 by pressing this knob. Figure 7 shows the comparison of the peak-to-peak results of testing the same signal at different ranges. The result at the 200mV/div range is 608.81mV (average value), and the result at the 80mV/div range is 569.67mV. In the test specification related to amplitude, it should be defined at what range the test is performed. Otherwise, the test results are not comparable. The meanings of other items in the vertical channel setting menu are also clear at a glance, so they 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 pay attention to 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. 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, and the signal has been seriously undersampled and the waveform is seriously distorted. For oscilloscopes above the LeCroy WaveRunner series, a fixed sampling rate can be set. This way, after knowing the characteristics of the measured signal, you can first fix it at an oversampling sampling rate, and then adjust the sampling time base. This will only change the capture time, 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.



Figure 5 Vertical channel setting menu [page]

Figure 6 Vertical channel setting panel



Figure 7 Test results under different ranges, 608.81mV / 569.67mV



Figure 8 Time base setting panel

Figure 9 Time base setting menu [page]

For the signal in Figure 9, the runt occurs regularly and the interval is not long. However, if the runt occurs only once in a long time, we need 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 the sequential mode to locate the occurrence pattern of the runt. (For the sequential mode, you can refer to the first episode of "LeCroy Season 2") After completing the previous steps, enter the fifth step of the signal capture operation and set the appropriate trigger mode. For the triggering of the oscilloscope, please refer to the 17th and 18th episodes of "LeCroy Season 1", which will not be repeated here. 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 the trigger modes such as runt trigger, width trigger, time interval trigger, etc. Figure 13 uses the runt trigger to isolate the runt. Through the above five steps, the high-fidelity capture of the signal can be achieved, and the subsequent measurement and analysis steps are relatively simple. Modern oscilloscopes are based on PC platforms. Operating an oscilloscope is like operating Office software. You can become 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 becoming more and more complex, involving a lot of knowledge background. Even if I study every day, I only have a superficial understanding of many applications. Being in such an industry makes me often sigh that "learning never ends"! This is an era of learning. The oscilloscope is the eyes of engineers. Familiarity 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 throwing out bricks and jade, and I hope everyone can communicate more.



Figure 10 Capture long enough to observe the full picture of the signal



Figure 11 WaveStream mode



Figure 12 Sequential mode



Figure 13 Set the appropriate trigger mode to isolate the event of interest