Analysis of application types and applicable scope of oscilloscope probes

The oscilloscope is one of the essential tools for electronic engineers. It can convert electrical signals invisible to the naked eye into visible images, and display changes in the measured signals on the screen, making it easier for people to design circuits and locate errors. Among the components that make up an oscilloscope, it goes without saying that the importance of the probe is directly related to the accuracy of the oscilloscope's measurement results.

As the name suggests, the probe plays a detection role. It is an important medium connecting the circuit under test and the input of the oscilloscope. The simplest probe is a wire connecting the circuit under test and the input end of the electronic oscilloscope. Complex probes are composed of resistive and capacitive components and active devices. A simple probe without shielding measures is easily interfered by external electromagnetic fields, and its equivalent capacitance is large, causing an increase in the load of the circuit under test and distortion of the measured signal.

When an inappropriate probe is used, it will have a relatively large impact on the measurement results, which can be roughly divided into two aspects. In the first case, the probe changes the original shape of the waveform, causing a deviation in the observed waveform. The second situation causes the oscilloscope to operate abnormally. Normal equipment becomes inoperable.

Probe selection

To avoid these situations, you must be familiar with the types of oscilloscope probes, and then choose and use them according to the actual situation. There are many types of commonly used probes, including passive probes, active probes, differential probes, current probes, low capacitance probes, high voltage probes, etc. The following is an introduction to the applicable scope of these probes to facilitate your selection.

Active voltage probe: generally suitable for bandwidths greater than 500M? Hz, single-ended signals with amplitudes less than plus or minus 3 volts. Tektronix's active voltage probe models include: P7260, P7240, TAP1500, etc. Some models can measure 3dB bandwidth up to 6G.

Passive probe: Generally used for testing bandwidth less than 500M? Hz single-ended signal. It is a relatively economical type of probe. For example, P6139A, the line length is 1.3 meters and the bandwidth is 500M? Hz, system input impedance 10M ohms, typical input capacitance 8pF, maximum voltage 300Vrms, compensation range 8 to 12pF.

Differential probe: Generally used to test differential signals. Divided into two types: high bandwidth and high voltage. High-voltage differential probes can test differential signals up to 4400 volts; high-bandwidth differential probes can test high-speed signals up to 12.5G. Some products can even reach 16GHz.

Current probe: Generally, the Hall effect is used to test current signals. The test range is generally from DC to 2G. Amplitude from 1mA to 20000A.

Low capacitance probe: This is a probe set up for matching. There are currently relatively few models of this kind of probe on the market. Can it measure up to 9G bandwidth? Hz.

High-voltage probe: suitable for testing high-voltage signals. It can measure voltages up to 4000 volts. It can be divided into single-ended and differential probes.

According to actual experience, the most commonly used probes are passive probes, with high-impedance passive voltage probes accounting for the largest proportion. Passive voltage probes provide various attenuation coefficients for different voltage ranges, such as 1×, 10×, and 100×. Among these passive probes, the 10× passive voltage probe is the most commonly used probe. For applications where the signal amplitude is 1V peak or less, a 1× probe may be suitable or even essential. In applications where low and medium amplitude signals are mixed (tens of millivolts to tens of volts), a switchable 1×/10× probe is much more convenient. However, a switchable 1×/10× probe is essentially two different probes in one product, with not only different attenuation coefficients but also different bandwidth, rise time and impedance (R and C) characteristics. Therefore, these probes cannot perfectly match the oscilloscope's input and provide the optimal performance achieved with standard 10× probes.

In general, the most commonly used probes are passive probes. Depending on the situation, different attenuation coefficients can be selected to match. This article is more basic, explains the importance of oscilloscope probes, and gives a general introduction to the types of probes. Finally, I will explain the most frequently used passive probes. I hope that novices will have a better understanding of oscilloscopes after reading this article.