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This article will introduce the various types and uses of probes. What is a probe? An oscilloscope is the most commonly used measuring instrument by electronic engineers, and the oscilloscope probe is undoubtedly the most frequently used accessory for it. An oscilloscope probe is an electronic component that connects the circuit under test to the input of the oscilloscope. Without a probe, the oscilloscope becomes just a decorative item, usable only as an ornament. Before choosing an oscilloscope probe, it’s best to check the oscilloscope’s manual to find out what type of probe is suitable for the oscilloscope we are using. The following points are, in our opinion, important when selecting a probe: ensure that the probe’s interface matches that of our oscilloscope. The probe interface of most oscilloscopes is a BNC interface. Some oscilloscopes may have an SMA interface. Check whether the input impedance and capacitance of the selected probe match those of the oscilloscope. Because we all want to minimize the impact of the probe on the circuit under test. The degree of matching between the probe’s impedance and capacitance and those of the oscilloscope **affects the accuracy of the measured signal. https://pic3.zhimg.com/80/v2-5bcc1dd26e801a485478a93e375933d6_720w.jpg BNC connector. https://pic2.zhimg.com/80/v2-9b2be07c20c56635b4b4f44e105f3bd9_720w.jpg SMA connector. Some oscilloscopes support switching between 50 Ω and 1 MΩ input impedances. But for most measurements, 1 MΩ is the most common. A 50 Ω input impedance is often used to measure high-speed signals, such as microwaves. There are also signal transmission delays in logic circuits and impedance measurements of circuit boards, among others. The input impedance of an oscilloscope can usually be set to 1 MΩ or 50 Ω, but its input capacitance is influenced by bandwidth and other design factors. Generally, an oscilloscope with a 1 MΩ impedance has an input capacitance of 14 pF. This value may also be between 5pF and 100pF. Therefore, in order to match the probe’s capacitance to the input capacitance of the oscilloscope, it is necessary to know the capacitance range of the probe before selecting it; thereafter, the probe’s capacitance is adjusted using a calibration rod. This is what is meant by probe compensation, and it is the first step that must be taken when using a probe. https://pic3.zhimg.com/80/v2-14b29ac4ba9de82d2878ffa2b6715896_720w.jpg So how many probes do we need, and which ones? Depending on our measurement requirements, the number and type of probes needed also vary. It’s a bit like people who use DSLRs; perhaps they have only one camera, but often they have many lenses. For example, if it is just a simple measurement of DC voltage, then a 1 MΩ passive probe is basically sufficient. However, if it is necessary to measure the relative voltage difference between live wires in a three-phase power supply, or between a live wire and the neutral wire, as is often required in power system testing, then we need to use differential probes. https://pic1.zhimg.com/80/v2-ab82fabf5b83f636af4748700653c0d0_720w.jpg Differential probes, passive probes: Passive probes are the most common type of probe; manufacturers usually include a few of them as standard when selling oscilloscopes. Common passive probes consist of a probe head, a probe cable, compensation devices or other signal conditioning networks, and a probe connector. No active components such as transistors or amplifiers are used in these types of probes, so there is no need to power the probe. Overall, passive probes are more common, easier to use, and cheaper. The common adjustable attenuation ratios for passive probes are as follows: 1×: no attenuation; 10×: 10 times attenuation; 100×: 100 times attenuation; 1000×: 1000 times attenuation. Passive voltage probes offer various attenuation factors for different voltage ranges. Among these passive probes, the 10× passive voltage probe is the most commonly used one. For applications with a signal amplitude of 1V peak-to-peak or lower, a 1× probe may be more suitable, or even essential. In applications with a mixture of low-amplitude and medium-amplitude signals (tens of millivolts to tens of volts), it is much more convenient to use a switchable 1×/10× probe. However, the switchable 1×/10× probe is essentially two different probes within one probe; not only do their attenuation coefficients differ, but their bandwidth, rise time, and impedance (R and C) characteristics also vary. Therefore, these probes cannot fully match the input of an oscilloscope, and they cannot deliver the optimal performance achieved by standard 10× probes. Probe attenuation expands the voltage measurement range of the oscilloscope through internal resistors; when these internal resistors are used together with the oscilloscope’s input resistance, they form a voltage divider. For example, a typical 10x probe is equipped with an internal 9MΩ resistor, and when used with an oscilloscope having an input impedance of 1MΩ, it provides a 10:1 attenuation ratio at the oscilloscope’s input channel. This means that the signal displayed on the oscilloscope will be 1/10 of the actual measured signal amplitude; therefore, we often need to adjust the attenuation factor in the oscilloscope’s channel settings to 10X as well. https://pic1.zhimg.com/80/v2-b1c2775303d05ff02ac85cd6ae76e714_720w.jpg This attenuation feature allows us to measure signals that exceed the voltage limits of the oscilloscope. Moreover, the attenuation circuit results in higher resistance (which is usually a good thing) and lower capacitance, which is important for high-frequency measurements. https://pic1.zhimg.com/80/v2-84e3eaf69fc42a325ca52b2120dacee8_720w.jpg The schematic diagram of the 10X passive probe. An active probe is so named because it contains active components such as transistors and amplifiers, and therefore requires power supply to function. In the most common cases, the active device is a field-effect transistor (FET), which provides a very low input capacitance; this low capacitance results in a high input impedance over a wider frequency range. The specified bandwidth for active FET probes is generally between 500 MHz and 4 GHz. In addition to having higher bandwidth, the high input impedance of active FET probes allows measurements to be taken at test points with unknown impedance, with a much lower risk of load effects. Furthermore, since low capacitance reduces the influence of the ground wire, longer ground wires can be used. Active FET probes do not have the voltage range of passive probes. The linear dynamic range of active probes is generally between ±0.6V and ±10V. https://pic2.zhimg.com/80/v2-e96bb886ab825fb325891a67bd2763dd_720w.jpg Active probes, differential probes – differential probes are used to measure differential signals. Differential signals refer to each other, rather than referencing ground. A differential probe can measure the signals of floating devices; it essentially consists of two symmetric voltage probes that provide good insulation and high impedance from the circuit. Differential probes can provide a very high common-mode rejection ratio (CMRR) over a wider frequency range. Compared to ordinary single-ended signal wiring, differential signals have three main advantages: they offer stronger resistance to interference, as the coupling between the two differential wires is excellent; when external noise interferes, it is coupled to both wires almost simultaneously. Since what the receiver cares about is the difference between the two signals, external common-mode noise can be effectively canceled out. It can effectively suppress EMI; by the same principle, since the polarities of the two signals are opposite, the electromagnetic fields they emit can cancel each other out. The tighter the coupling, the less electromagnetic energy is released into the outside environment. The timing positioning is precise; since the switching of differential signals occurs at the intersection of the two signals, unlike ordinary single-ended signals which rely on two threshold voltages to determine the state, it is less affected by manufacturing processes and temperature, which helps reduce timing errors. It is also more suitable for circuits dealing with low-amplitude signals. The LVDS that is popular today refers to this low-amplitude differential signal technology. The principle of differential amplification is that a pair of signals are simultaneously fed into the amplification circuit and then subtracted from each other to obtain the original signal. A differential amplifier is an amplifier composed of two transistors with identical parameter characteristics, connected in direct coupling. If signals of the same amplitude and phase are applied to the two input terminals, the output is zero, thereby overcoming zero drift. https://pic2.zhimg.com/80/v2-5a7ee42e83fcb4fb943faa210f13ca5d_720w.jpg Schematic diagram of a differential probe. Current probe: You might think that by measuring the voltage with a voltage probe and dividing it by the impedance of the circuit being measured, it’s easy to obtain the current value; so why go the extra mile and use a dedicated current probe for this purpose? In fact, the error introduced by such measurements is extremely large, so we generally do not use the method of converting voltage to current. A current probe can accurately measure the current waveform by using a current transformer as the input; the magnetic flux of the signal current is converted into a voltage through the transformer, and then amplified by an amplifier inside the probe before being sent to the oscilloscope. Current probes are basically divided into two categories: AC current probes and AC/DC current probes. AC current probes are usually passive probes that do not require external power supply, while AC/DC current probes are typically active probes. Traditional current probes can only measure AC signals, as a steady DC current cannot induce a current in an instrument transformer. In a current transformer, alternating current causes changes in the electric field as the direction of the current changes, which in turn generates a voltage. However, semiconductor devices that utilize the Hall effect and have current bias will generate a voltage corresponding to a direct current field. Therefore, a DC current probe is an active device that requires external power supply. https://pic1.zhimg.com/80/v2-e2c5d62db7fcb382c6c0dad241da0a4c_720w.jpg AC/DC current probes. Finally, let’s look at some suggestions related to probes: Perform proper compensation of the probes – the input capacitance of different oscilloscopes can vary, and even different channels on the same oscilloscope may differ slightly. To solve this problem, learning how to compensate and adjust the probe is the most fundamental skill that engineers should master. When the probe is connected to the circuit under test, its ground terminal must be connected to the ground wire of the circuit under test. Otherwise, in a suspended state, the potential difference between the oscilloscope and other devices or the ground may cause electric shock or damage to the oscilloscope, probes, or other devices. Try to place the grounding wire of the probe as close as possible to the location of the point being tested. An excessively long ground wire can cause waveform distortions such as ringing or overshoot. When neither of the two test points is at the ground potential, a \"floating\" measurement, also known as a differential measurement, must be performed, requiring the use of specialized differential probes. The probe is crucial for oscilloscope measurements; firstly, it is necessary that the probe have minimal impact on the circuit being measured, while also ensuring sufficient signal fidelity of the measurement values. If the probe alters the signal in any way or changes the way the circuit operates, the actual signal as seen by the oscilloscope will be severely distorted, which can lead to incorrect or misleading measurement results. From the above introduction, it can be seen that there are many aspects regarding the selection and proper use of probes that deserve our attention.