如何选择正确的示波器探针

以.pdf格式下载本文 此文件类型在适用时包括高分辨率图形和原理图。

Keysight Technologies示波器产品经理Daniel Bogdanoff

从字面上看，有数百种不同的示波器探针可用(Fig. 1)，那么您如何选择合适的呢？没有一个答案，因为所有设计都不同。为了帮助决策过程，以下是您需要考虑的一些不同的探针特征。

带宽

探针的带宽通常是其横幅规范。它本质上描述了探针能够传递到示波器的频率的高度。在投资新调查之前，您应该考虑信号频率的速度。根据经验，您的探针应至少比您想看到的最快信号快三到五倍。

衰减比率

1. A probe at work on a chip.

Probes have different (sometimes switchable) attenuation ratios that change how the signals are fed into your oscilloscope. For example, a 10:1 probe connected to a 1-V signal will pass 100 mV to the scope’s input. The scope will read the probe’s attenuation ratio and display the correct signal values on the screen so that you needn’t do the calculations in your head.

具有较高的衰减率（100：1，1000：1）将使您可以查看更高的电压，但它也将使范围的内部放大器噪声更加明显。衰减比率越高，您会看到的范围越大。例如，10：1探针将显示10倍噪声。这可能会导致您相信您应该始终使用1：1探针。不对！较低的衰减探针通常在您的系统上负载较高，这使我们进入了下一个探针特征。

Probe Loading

将任何内容连接到正在测试的设备（DUT）将改变系统的固有电气特性。尽管我们最好的研发努力，但探针仍遵守物理定律。（如果我们解决这个讨厌的障碍，我们一定会让您知道！）因此，每当您探测系统时，都会影响您的设备。不必要地加载系统会导致不准确的测量值，甚至会在示波器屏幕上改变波形的形状！

探测负载也不仅是电阻的；它们具有固有的电容和感应效应。电容效应通常是最难处理的，并且对频率的响应极高。过多的电容载荷将减慢系统的带宽，从而有效地减慢信号的上升和下降时间。如果上升/秋季时间太放慢了，您将开始看到系统中的位错误。

通常，探针的衰减比越高，电容载荷越低。因此，1：1探针可能不是最佳选择。通常，1：1探针的电容载荷约为100 pf，而10：1探针通常在10 pf的范围内。因此，即使1：1探针有助于降低示波器放大器的噪声，但仍存在权衡。

高端示波器通常使用数字信号处理（DSP）来帮助补偿探针加载，但是完全消除探针加载根本是不可能的。因此，确保您选择的探针具有适合您设计参数的阻抗值。

帮助减少探针加载的另一种方法是使用活动探针。

主动与无源探针

Many engineers will only consider an active probe if they need to look at high bandwidths. However, the probe loading from active probes is significantly lower than that of passive probes. By designing a high-frequency amplifier into the probe head, the probe cable and oscilloscope front end are isolated from your DUT. This can bring the probe loading down to 1 pF or better.

What does this mean for your measurements, though? Let’s look at a system with a 1.1-ns rise time. Conventional wisdom would tell you that you only need around 300 MHz of bandwidth. Use the formula:

必需带宽= 0.34/rise_time

So we set up our system and measure our edge with an active probe, and see a 1.1-ns rise time. We then disconnect the active probe and connect the free-with-the-scope 500-MHz passive probe. Subsequently, we measure the rise time and see it has changed to 1.5 ns. What happened? The passive probe’s capacitance is adding more load to our system, and it’s having trouble driving the extra load. The extra load from the probe is distorting the device’s signals! The difference in measured edge speeds can be clearly seen inFig. 2.

2.显示的是使用活性探针与被动探针的测量边缘速度的差异。

So, should I ever use the passive probes that come with my oscilloscope? Yes! Passive probes are great forqualitativemeasurements (i.e., checking clock frequencies, browsing for bugs, etc.). But, active probes excel in定量诸如输出波纹或上升时间之类的测量。尽管主动探针的成本比被动探针高，但它们可以在测量准确性方面产生很大的影响。

If you want to know how much your probes are loading your system, you can double-probe your DUT. First, connect one probe to a known step signal on your DUT (or as close as you can get). Observe and/or save the waveform on the screen. Then, leaving the first probe connected, connect a second probe to the exact same spot and observe the change in your edge speed.

Probe Compensation Range

Probe compensation makes it possible to adjust the impedance of your probes to fine-tune your acquisition system. The input impedance of an oscilloscope can vary from model to model, and even from channel to channel, due to differing component values.

Passive probes typically come with an adjustable input impedance that allows you to impedance match your probe and scope. This ensures that your probe will perform as expected for its rated bandwidth. The probe’s impedance can usually be adjusted on your probe using a small (included) screwdriver.Figs. 3,4， 和5give examples of what an under-, over-, and properly compensated probe looks like:

3.此屏幕截图表示过度补偿的探针；信号应为方波。

4.这是一个适当补偿的探针的样子。

5.此屏幕截图表示补偿较低的探针；在这种情况下，信号也应该是方波。

All of these screenshots are using an identical probe and signal; only the probe compensation has changed.

Your oscilloscope will have a “typical” input capacitance value, and you want to make sure that your probe’s compensation range can match. If your probe can’t fully compensate for your scope, you will get sub-par measurements, especially when measuring higher frequencies.

Connecting to Your Device

评估探针时的另一个考虑因素是您将如何物理连接到系统。对于低频测量，被动探针上的抓帽可能足够好。但是，随着系统带宽的增加，您需要查看其他选项。

Can you use a ZIF tip, an SMA probe head, a solder-in probe head, or my favorite: the N2848A magnetic QuickTip head? Such questions are beyond the scope of this article, but essential to engineers needing to look at fast signals. Make sure you are designing boards that can be probed, and selecting probes that will provide a good connection to your board. The length of your ground lead is also an important part of connecting to your device, because it can act as an antenna.

信号依赖性探测

根据您的探测信号，可以提供许多不同样式的探针。信号的类型可以显着缩小您的探针选择。您的信号是差异吗？使用差分探针并消除共同模式噪声。测量电流？使用电流探针代替感官电阻和电压探针。测量极高的电压？不要冒险生命，并使用高压探测器以其额外的高衰减比例。

其他考虑因素

值得注意的是，您的整个收购系统的带宽很重要，而不仅仅是调查。示波器的带宽和探针的带宽都根据以下等式影响总系统带宽：

假设您有200 MHz的范围，并使用了200 MHz探测器。您的系统带宽约为141 MHz。

此外，探针通常带有探针配件套件。在订购之前，重要的是要知道什么或不包括在内。

结束思想

您要测量的几乎所有信号都可以使用许多类型的探针。为避免混乱，请坚持基础：带宽，衰减比和加载。充分了解这些规格将帮助您完成许多选项，并帮助您选择正确的探测。