An oscilloscope is a commonly used electronic measuring instrument that converts electrical signals that are invisible to the naked eye into visible images and is widely used in many industries. One of the most indispensable components in an oscilloscope is the probe of an oscilloscope. What role does the probe play in the oscilloscope? The following is a brief introduction to the application of the oscilloscope probe. I hope it can help you.
1. Introduction to oscilloscope probe
An oscilloscope probe establishes a physical and electrical connection between a test point or source and an oscilloscope; in fact, an oscilloscope probe is a type of device or network that connects a signal source to an oscilloscope input. It must be input at the source and oscilloscope. Provide a convenient connection between the quality and quality. There are three key issues with the degree of connectivity: physical connectivity, impact on circuit operation, and signal transmission.
2. Classification of oscilloscope probes
Hundreds, or even thousands, of different oscilloscope probes are available on the market. One technical indicator of the oscilloscope probe is the frequency characteristic. It is convenient to divide the type of the probe by frequency, but the frequency coverage of the oscilloscope probe is limited and it is difficult to divide according to the LF, HF, VHF, UHF, RF and Other bands of the radio frequency. An oscilloscope probe is one of all probes. The most commonly used probes are voltage-current probes, which are usually categorized by measurement object.
1 passive voltage probe
1.1 Passive probe
Passive probes are made of wires and connectors that also include resistors and capacitors when compensation or attenuation is required. There are no active devices (transistors or amplifiers) in the probe, so there is no need to power the probe. Passive probes are generally the most rugged and economical probes that are easy to use and widely used.
1.2 High resistance passive voltage probe
From the actual needs, the most used is the voltage probe, of which the high-resistance passive voltage probe accounts for the largest part. Passive voltage probes provide various attenuation factors of 1×, 10× and 100× for different voltage ranges. Among these passive probes, the 10× passive voltage probe is the most commonly used probe. For applications where the signal amplitude is 1V peak-to-peak or lower, a 1× probe may be suitable or even essential. In applications where low and medium amplitude signal mixing (several tens of millivolts to tens of volts) is possible, switching 1 x /10 x probes is much more convenient. However, the switchable 1×/10× probe is essentially two different probes in one product, not only with different attenuation coefficients, but also with different bandwidth, rise time and impedance (R and C) characteristics. As a result, these probes do not exactly match the input of the oscilloscope and do not provide the optimal performance of a standard 10× probe.
1.3 low resistance passive voltage probe
Most high-impedance passive probes have bandwidths ranging from less than 100MHz to 500MHz or higher. Low-resistance passive voltage probes (also known as 50-ohm probes, Zo probes, and voltage divider probes) have excellent frequency characteristics, and probes with matching coaxial cables can be used up to 10 GHz and 100 picoseconds or faster. time. These probes are designed for use in 50 ohm environments, typically high speed equipment verification, microwave communication, and time domain reflectometry (TDR).
1.4 Passive high voltage probe
"High pressure" is a relative concept. From a probe perspective, we can define high voltage as any voltage that exceeds the voltage that can be safely handled by a typical universal 10x passive probe. High-voltage probes require good dielectric strength to ensure the safety of the user and the oscilloscope.
2. Active voltage probe
2.1 Active probe
Active probes contain or rely on active devices such as transistors. In the most common case, the active device is a field effect transistor (FET) that provides very low input capacitance, which results in high input impedance over a wider frequency band. Can be seen from the following Xc formula:
2.2 Active FET probe
The specified bandwidth of active FET probes is typically between 500MHz and 4GHz. In addition to higher bandwidth, the high input impedance of active FET probes allows measurements to be made at test points where impedance is unknown, and the risk of loading effects is much lower. In addition, longer ground lines can be used because low capacitance reduces ground line effects.
Active FET probes do not have a voltage range for passive probes. Active probes typically have a linear dynamic range of ±0.6V to ±10V.
2.3 Active Differential Probe
The differential signals are referenced to each other, not to the grounded signal. The differential probe measures the signal of the floating device. In essence, it consists of two symmetrical voltage probes with good insulation and high impedance for the ground. Differential probes provide high common mode rejection ratio (CMRR) over a wider frequency range.
3 current probe
In principle, it is easy to obtain the current value by measuring the voltage value with a voltage probe and dividing it by the measured impedance value. However, in practice, the error introduced by this measurement is large, so the method of voltage-converted current is generally not used. The current probe can accurately measure the current waveform by using a current transformer input. The signal current flux is converted into a voltage by a mutual inductance transformer, and then amplified by an amplifier in the probe and sent to the oscilloscope.
3.1 AC current probe
The alternating current is in the transformer, and as the direction of the current changes, a change in the electric field is generated and a voltage is induced. The AC current probe is a passive device and requires no external power supply.
3.2 DC current probe
Conventional current probes can only measure AC AC signals because a stable DC current cannot induce current in the transformer. However, with the Hall effect, the semiconductor device with current bias will generate a voltage corresponding to the DC electric field. Therefore, the DC current probe is an active device that requires external power supply.
Therefore, current probes are basically divided into two categories: AC current probes and AC/DC current probes. AC current probes are usually passive probes, and AC/DC current probes are usually active probes.
4 logic probe
When using an oscilloscope to observe the analog characteristics of a digital waveform, a logic probe is required. For the exact cause of isolation, digital designers often need to look at specific data pulses that occur under specific logic conditions, which requires a logic trigger function. Figure 3 shows a schematic diagram of a logic probe that can be added to most oscilloscopes.
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