Oscilloscope probe classification and selection

The probe is the first link in observing signals. Its main function is to carry the signal transmission link, completely and reliably transmit the signal to be measured to the oscilloscope for measurement and analysis. But do you know how to achieve the best matching of probes? 1. Probe classification Probes are usually classified according to measurement objects, and the classification is shown in Figure 1. Among them, high-resistance passive probes, high-voltage differential probes and current probes are the most familiar to us. Next, we will give a brief introduction.　　  　　

Figure 1 Probe classification　　

1.11.1 High-impedance passive probes From the actual demand, the proportion of high-impedance passive voltage probes with compensation is the largest, which can meet the needs of most low-speed digital signals, power supplies and other typical oscilloscopes. The following table shows the specific parameters of common high-impedance passive probes:

Table 1 Specifications and parameters of common high-impedance passive probes This type of probe has a high input resistance (generally above 1MΩ) and an adjustable compensation capacitor. When connecting to an oscilloscope for the first time, it is generally necessary to adjust the capacitance value with an adjusting rod to match the oscilloscope. Input capacitor to eliminate low or high frequency gain. Figure 2 shows the overcompensated waveform, Figure 3 shows the normal waveform, and Figure 4 shows the undercompensated waveform.　　            Figure 2 Overcompensated waveform         Figure 3 Normal waveform         Figure 4 Undercompensated waveform 1.21.2 High voltage differential probe　　

First, let’s introduce the concept of differential: Differential signals are referenced to each other, not to signals referenced to ground. The high-voltage differential probe is essentially composed of two symmetrical voltage probes, each with good insulation and high impedance to ground. It can provide a high common-mode rejection ratio in a wider frequency range and can connect any two points. The floating signal is converted into a signal to ground, which is mainly used for testing in industries such as switching power supplies. The schematic diagram is shown in Figure 5.　　  　　

Figure 5 Principle of high-voltage differential probe The following table shows the specific parameters of common high-voltage differential probes:　　

Table 2 Specifications and parameters of common high-voltage differential probes　　

1.31.3 Current probe　　

The current probe uses Hall sensors and induction coils to measure DC and AC currents. Its working principle is to convert the current signal into a voltage signal. The oscilloscope collects the voltage signal and then displays it as a current signal.　   　　

Figure 6 Current probe picture　　

The advantage of the current probe is that it can measure the current without disconnecting the power supply line. Typical applications are system power measurement, power factor measurement, switch machine inrush current waveform measurement, etc. The main disadvantage is that its small current measurement capability is limited by the noise floor of the oscilloscope, so its small current measurement capability is limited.　　

2. How to choose the appropriate probe?　　

The above has given a brief introduction to some common probes, so how to choose a suitable probe? What parameters of the probe should we pay attention to?　　

Impedance matching: The input impedance of the probe must match the input impedance of the oscilloscope used to reduce the load on the circuit under test. For low input impedance oscilloscopes, active probes or probes with 50Ω input impedance should be selected; for high input impedance oscilloscopes, ×10 probes should be selected. For example, the input impedance of the oscilloscope is 1MΩ/10pF, and the input impedance of the probe is preferably 10MΩ/1pF. Such a probe has 10 times the signal attenuation, has a small load on the measured signal, and can match the input impedance of the oscilloscope.　　

Bandwidth: The bandwidth of the probe should be equal to or greater than the bandwidth of the oscilloscope. If a pure sinusoidal signal is observed, the probe bandwidth should be equal to the highest value of the frequency of the signal being measured; if a non-sinusoidal signal is observed, the probe bandwidth should accommodate the fundamental wave and the most important harmonic components of the signal being measured.　　

Rise time: In order to accurately measure the rise time and fall time of the pulse, the rise time of the system (that is, the sum of the rise times of the oscilloscope and the probe) should be 3-5 times faster than the rise time of the signal under test.　　

3. Summary　　

In order to ensure the accuracy of test results, the probe is required to have minimal impact on the circuit under test and maintain maximum signal fidelity when transmitted to the oscilloscope. If the probe changes the signal or the operation of the circuit under test in any way, the oscilloscope will see a distortion of the actual signal, which may lead to erroneous or misleading measurement results. Only a probe that is a good match for both the oscilloscope and the circuit under test is your best choice.