Exploration: Principles and types of laboratory oscilloscope probes

Anyone who has used an oscilloscope will have touched the probe. Usually, the oscilloscope is used to measure voltage signals (there are also photometry or current, which are converted into voltage measurements by corresponding sensors first). The main function of the probe is to Lead the measured voltage signal from the measuring point to the oscilloscope for measurement.

Most people will pay more attention to the use of the oscilloscope itself, but ignore the choice of probe. In fact, the probe is an intermediate link between the signal under test and the oscilloscope. If the signal is already distorted at the probe, then the oscilloscope does not work well. In fact, the design of the probe is much more difficult than the oscilloscope, because the oscilloscope can be well shielded and does not need to be disassembled frequently, and the probe must ensure at least the same bandwidth as the oscilloscope in addition to the convenience of detection. The difficulty is much greater. Therefore, when the earliest high-bandwidth real-time oscilloscope first appeared, there was no corresponding probe, and it took a while for the probe to come out.

To choose a suitable probe, the first point is to understand the influence of the probe on the test, which includes the meaning of 2 parts: 1 / the influence of the probe on the circuit under test; 2 / the signal distortion caused by the probe. The ideal probe should have no effect on the circuit under test and at the same time without any distortion on the signal. Unfortunately, no real probe can meet these two conditions at the same time, usually need to make some trade-offs between these two parameters.

In order to consider the influence of the probe on the measurement, we can usually simply equate the probe model to a model of R, L, and C, and put this model and our circuit under test together for analysis.

First, the probe itself has input resistance. Like the principle of measuring voltage with a multimeter, in order to minimize the impact on the circuit under test, the input resistance Rprobe of the probe itself should be as large as possible. However, since Rprobe cannot achieve infinity, it will generate voltage division with the circuit under test. The actual measured voltage may not be the actual voltage before the probe point, which is often encountered in some power supply or amplifier circuit tests. In order to avoid the influence caused by the resistance load of the probe, Rprobe is generally required to be greater than 10 times of Rsource and Rload. The input impedance of most probes is between tens of k ohms and tens of mega ohms.

Second, the probe itself has input capacitance. This capacitance is not intentionally made, but the parasitic capacitance of the probe. This parasitic capacitance is also the most important factor affecting the bandwidth of the probe, because this capacitance will attenuate high frequency components and slow down the rising edge of the signal. Generally, high-bandwidth probes have relatively small parasitic capacitance. Ideally, Cprobe should be 0, but it can't be done in practice. Generally, the input capacitance of a passive probe is between 10pf and several hundred pf, and the input capacitance of an active probe with a higher bandwidth is generally between 0.2pf and several pf.

Secondly, the probe input will also be affected by the inductance. The input resistance and capacitance of the probe are relatively easy to understand, but the inductance at the input end of the probe is often overlooked, especially when measuring at high frequencies. Where does the inductance come from? We know that there will be an inductance when there is a wire. There must be a wire connection between the probe and the circuit under test. At the same time, the return of the signal must also pass through the ground wire of the probe. Usually the ground wire of a 1mm probe will have an inductance of about 1nH. The longer the signal and ground wire, the greater the inductance value. The parasitic inductance and parasitic capacitance of the probe form a resonant circuit. When the inductance value is too large, high-frequency resonance may occur under the excitation of the input signal, causing signal distortion. Therefore, it is necessary to strictly control the length of the signal and the ground wire during high-frequency testing, otherwise it is easy to produce ringing.

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