Oscilloscope probes

1 Overview
Oscilloscope probes are essential to the accuracy and correctness of measurement results. They are electronic components that connect the circuit under test to the input of the oscilloscope. The simplest probe is a wire that connects the circuit under test to the input of the electronic oscilloscope. Complex probes are composed of resistors and capacitors and active devices. Simple probes are not shielded and are easily interfered by external electromagnetic fields. In addition, their equivalent capacitance is large, which increases the load on the circuit under test and distorts the measured signal.
1.1 Definition of oscilloscope probes
In essence, an oscilloscope probe establishes a physical and electronic connection between the test point or signal source and the oscilloscope; in fact, an oscilloscope probe is a type of device or network that connects the signal source to the oscilloscope input. It must provide a sufficiently convenient and high-quality connection between the signal source and the oscilloscope input. There are three key issues in the adequacy of the connection: physical connection, impact on circuit operation, and signal transmission.
1.2 The development of oscilloscope probes
Over the past 50 years, various oscilloscope probe interface designs have been evolving to meet the requirements of increased instrument bandwidth speed and measurement performance. In the earliest days, banana plugs and UHF connectors were commonly used. In the 1960s, ordinary BNC connectors became the common probe interface type because BNC was smaller and had a higher frequency. At present, BNC probe interfaces are still used in test and measurement instrument design, and the current higher-quality BNC connectors provide a maximum available bandwidth function close to 4GHz.
Later, some manufacturers proposed a workaround for the ordinary BNC probe interface design. While using the BNC connector, an analog coded scale factor detection pin was additionally provided as part of the mechanical and electronic interface design, allowing compatible oscilloscopes to automatically detect and change the vertical attenuation range displayed by the oscilloscope.
1.3 Structural form of oscilloscope probes
Most probes consist of a probe head, a probe cable, a compensation device or other signal conditioning network, and a probe connector. As shown in Figure 1.

Figure 1 Probe structure
To make oscilloscope measurements, you must first be able to physically connect the probe to the test point. To achieve this, most probes have an associated cable of at least one or two meters long, as shown in Figure 1. However, the probe cable reduces the probe bandwidth: the longer the cable, the greater the reduction. In addition to the one or two meter long cable, most probes also have a probe head or handle with a probe that can hold the probe and the user can move the probe to contact the test point. Usually this probe is in the form of a spring-loaded hook that can actually connect the probe to the test point.
In order to obtain a usable measurement result, the signal on the probe must be transmitted to the oscilloscope input with sufficient fidelity through the probe head and cable.
2 Main categories of oscilloscope probes and characteristics of each type of probe
Hundreds, even thousands of different oscilloscope probes are available on the market. One of the technical indicators of oscilloscope probes is frequency characteristics. It is convenient to classify the types of probes by frequency, but the frequency coverage of oscilloscope probes is limited and it is difficult to classify them according to radio frequency bands such as LF, HF, VHF, UHF, RF, etc. Oscilloscope probe is one of all probes. The most commonly used probe is voltage and current probe. Probes are usually classified according to the measurement object. The specific classification is shown in Figure 2:

2.1 Passive voltage probes
2.1.1 Passive probes
Passive probes are made of wires and connectors, and also include resistors and capacitors when compensation or attenuation is required. There are no active devices (transistors or amplifiers) in the probe, so the probe does not need to be powered. Passive probes are generally the most rugged and economical probes. They are not only easy to use, but also widely used.
2.1.2 High-impedance passive voltage probes
From a practical point of view, the most commonly used are voltage probes, of which high-impedance passive voltage probes account for the largest part. Passive voltage probes provide various attenuation factors of 1×, 10× and 100× for different voltage ranges. Among these passive probes, 10× passive voltage probes are the most commonly used probes. For applications where the signal amplitude is 1V peak-to-peak or lower, 1× probes may be more suitable or even indispensable. In applications where low-amplitude and medium-amplitude signals are mixed (tens of millivolts to tens of volts), switchable 1×/10× probes are much more convenient. However, switchable 1×/10× probes are essentially two different probes in one product, with different attenuation factors and different bandwidth, rise time, and impedance (R and C) characteristics. Therefore, these probes cannot fully match the input of the oscilloscope and cannot provide the optimal performance achieved by standard 10× probes.
2.1.3 Low-impedance passive voltage probes
Most high-impedance passive probes have bandwidths ranging from less than 100MHz to 500MHz or higher. Low-impedance passive voltage probes (also called 50-ohm probes, Zo probes, and voltage divider probes) have very good frequency characteristics and can reach bandwidths of 10GHz and rise times of 100 picoseconds or faster using probes matched to coaxial cables. Such probes are designed for use in 50-ohm environments, which are generally high-speed device characterization, microwave communications, and time domain reflectometry (TDR).
2.1.4 Passive high-voltage probes
"High voltage" is a relative concept. From a probe perspective, we can define high voltage as any voltage that exceeds the voltage that a typical general-purpose 10× passive probe can safely handle. High voltage probes are required to have good insulation strength to ensure the safety of the user and the oscilloscope.
2.2 Active voltage probes

2.2.1 Active Probes
Active probes contain or rely on active devices, such as transistors. Most commonly, the active device is a field effect transistor (FET), which provides very low input capacitance, which results in high input impedance over a wider frequency band. This can be seen from the Xc formula below:

2.2.2 Active FET probes
The specified bandwidth of active FET probes is generally 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 with unknown impedance with much lower risk of loading effects. In addition, longer ground leads can be used because the low capacitance reduces the effect of the ground lead.
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.
2.2.3 Active differential probes
Differential signals are referenced to each other rather than to ground. Differential probes can measure signals from floating devices. In essence, it is composed of two symmetrical voltage probes, each with good insulation and high impedance to ground. Differential probes can provide high common mode rejection ratio (CMRR) over a wider frequency range.
2.3 Current probes
In principle, the current value can be easily obtained by measuring the voltage value with a voltage probe and dividing it by the impedance value being measured. However, in reality, this kind of measurement introduces a large error, so the method of converting voltage to current is generally not used. The current probe can accurately measure the current waveform by using a current transformer input, converting the signal current flux into voltage through the transformer, and then amplifying it by the amplifier in the probe and sending it to the oscilloscope.
2.3.1 AC
current probe In the transformer, the AC current changes with the change of the current direction, which produces a change in the electric field and induces voltage. The AC current probe is a passive device and does not require an external power supply.
2.3.2 DC current probe
Traditional current probes can only measure AC signals because stable DC current cannot induce current in the transformer. However, using the Hall effect, a semiconductor device with current bias will generate a voltage corresponding to the DC electric field. Therefore, the DC current probe is an active device and requires an 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.
2.4 Logic probe
When using an oscilloscope to observe and analyze the analog characteristics of digital waveforms, a logic probe is required. To isolate the exact cause, digital designers usually need to view 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. This logic trigger function can be added to most oscilloscopes.

Figure 3 Schematic diagram of logic probe
2.5 Other probes
Since the application range of oscilloscopes is very wide, in addition to the above-mentioned probe types, there are also various special probes. These professional probes have different functions according to the different front-end sensors. Below we introduce two of them for readers to understand.
In principle, the photoelectric probe is a combination of an ordinary voltage probe and a photoelectric conversion device, which can directly measure the optical signal transmitted by optical devices and optical fibers.
The temperature probe is a combination of an ordinary voltage probe and a temperature sensor, which can directly measure the temperature of an object. The temperature probe is a type of sensor probe. Various sensor probes can be used with an oscilloscope to measure a variety of physical quantities.
3 The influence of oscilloscope probes on measurements
3.1 Load effect
The so-called load effect is that when an oscilloscope is connected to the circuit under test, sometimes the input resistance of the oscilloscope will affect the circuit under test, causing the signal of the circuit under test to change. If the load effect is very large, the waveform cannot be measured accurately. If you want to reduce the load effect, you need to increase the input resistance at one end of the oscilloscope. The larger the input resistance, the smaller the input capacitance, and the smaller the load effect.
In oscilloscope measurement, another load effect refers to the load effect of the probe on the circuit under test. To ensure the accuracy of the measurement, it is necessary to reduce the load effect of the probe on the circuit under test so as not to affect the measured signal. Therefore, a probe with high input impedance should be selected. The input impedance of the probe can be equivalent to the parallel connection of resistance and capacitance. At low frequencies (below 1MHz), the load of the probe is mainly impedance; at high frequencies (above 10MHz), the load of the probe is mainly capacitive reactance. In order to reduce the load effect of the probe on the circuit under test, a probe with high impedance and low capacitive reactance should be selected. For example, a passive probe with a bandwidth of 100MHz has an input resistance of 1~10Ω and an input capacitance of 1~10pF. The load effect of an active probe is better than that of a passive probe, and the frequency characteristics are better.
3.2 Impedance matching
Impedance is the ratio of voltage to current. Ideally, when testing the instrument under test, it should not affect its normal operation, and the measured value should be the same as when the test instrument is not connected. When connecting instruments for measurement, the influence of impedance on measurement accuracy should be considered. In order to ensure that the maximum power can be transmitted between instruments, the impedance should be matched. If the impedance is a pure resistor, the input impedance and output impedance should be equal. If the impedance contains a reactance component, the input impedance of the load should be conjugate matched with the output impedance of the source, so that the maximum power can be transmitted.
The impedance value of impedance matching is usually consistent with the characteristic impedance value of the transmission line used. For RF systems, 50Ω impedance is generally used. For high-impedance instruments, due to the existence of equivalent parallel capacitance, as the frequency increases, the parallel combination impedance gradually decreases, which will load the circuit under test. For example, with a 1MΩ input impedance, when the frequency reaches 100MHz, the equivalent impedance is only about 100Ω. Therefore, high-bandwidth oscilloscopes generally use 50Ω input impedance, which can ensure the matching of the oscilloscope and the source. However, when using a 50Ω input impedance, it must be considered that the load effect of the 50Ω input impedance is more obvious. At this time, it is best to use a low-capacitance active probe.
3.3 Capacitive Load
As the signal frequency or conversion rate increases, the capacitive component of the impedance becomes the main factor. As a result, capacitive loading becomes a major problem, especially capacitive loading affects the rise and fall times of fast switching waveforms and the amplitude of high-frequency components in the waveform.
4 Main technical specifications of oscilloscope probes
4.1 Bandwidth and rise time
The bandwidth of a probe refers to the frequency that causes the probe response output amplitude to drop to 70.7% (-3dB). Rise time refers to the probe's response to 10~90% of the step function, indicating the fast measurement transition that the probe can transmit from the head to the oscilloscope input. For most probes, the product of bandwidth and rise time is close to 0.35. In many cases, bandwidth is verified by pulse rise time to ensure minimum distortion.
4.2 Capacitance
The probe head capacitance indicator refers to the capacitance on the probe probe tip, which is the capacitance equivalent to the probe on the test point of the circuit under test or the device under test. The probe is also equivalent to a capacitor on the oscilloscope side, and this capacitance value should match the oscilloscope capacitance. For 10× and 100× probes, this capacitance is called compensation capacitance, which is different from the probe head capacitance. The compensation capacitance will be described below.

4.3 Aberration
Aberration is any deviation in amplitude from the expected or ideal response of the input signal. In practice, aberration usually occurs immediately between fast waveform transitions, which manifests as so-called "ringing". High-frequency probes that do not specify extreme aberration can provide completely misleading measurements. The presence of aberration can indicate severely distorted bandwidth and roll-off characteristics.
4.4 Attenuation Factor
When properly terminated, the probe should have a constant attenuation factor. The attenuation factor is the ratio of the output signal to the input signal. Some probes may have selectable attenuation factors, typical attenuation factors are 1×, 10×, and 100×. The 1× and 10× range circuits are shown in Figure 4, both of which are composed of resistors and capacitors.

4.5 Probe attenuation compensation
The so-called probe attenuation compensation refers to adjusting the variable capacitor in the probe to make the frequency relatively stable when the oscilloscope and the probe are used together. Probe compensation means frequency compensation between the end of the probe and the input of the oscilloscope. The relationship between the end of the probe and the input of the oscilloscope is shown in Figure 5. By adjusting C2, the following relationship can be obtained:

Figure 5 Capacitance probe compensation circuit

Although the input resistance of the oscilloscope is only 1MΩ, the input capacitance connected in parallel with it varies according to the model. Even for the same model, the input capacitance on each channel is different. Therefore, if the combination of the oscilloscope and the probe is changed, the phase compensation of the probe must be changed accordingly.
The method of probe calibration is as follows: Connect the probe to the square wave signal output terminal of the probe calibration. When the characteristics of the probe are in the best state, as shown in (a) in Figure 6, if the situation shown in (b) or (c) occurs, please use a screwdriver to adjust the frequency compensation fine-tuning capacitor on the probe for calibration.

Figure 6 Probe calibration diagram
4.6 Rated maximum voltage
The rated maximum voltage is determined by DC + peak AC, that is, the sum of the DC value and the AC peak value of the output voltage cannot exceed the maximum rated voltage of the oscilloscope. If this rated maximum voltage is exceeded, the probe will be damaged.
4.7 Voltage rating decreases with frequency
The maximum input voltage of the voltage probe at low frequency is clearly specified, and the input voltage will decrease accordingly as the frequency increases. For high-frequency probes, it is necessary to pay attention to the change of input voltage with frequency. When the frequency is higher than 1MHz, the allowable input voltage drops sharply with the increase of frequency.
5 Selection of the best oscilloscope probe
The most important parameters in the characteristics and features of the probe are bandwidth and input impedance. They must match the bandwidth and input impedance of the oscilloscope and minimize the impact on the circuit under test. Therefore, comprehensive considerations should be taken when selecting a probe.
5.1 Bandwidth and rise time
The bandwidth or rise time of the probe should be equal to or better than the bandwidth of the oscilloscope. If a pure sinusoidal signal is observed, the probe bandwidth can be equal to the highest value of the measured signal frequency; if a non-sinusoidal signal is observed, the probe bandwidth should be able to accommodate the fundamental wave and the most important harmonic components of the measured signal. To accurately measure the rise time and fall time of a pulse, the rise time of the system (the sum of the oscilloscope and the probe) should be 3-5 times faster than the fastest rise time to be measured.
5.2 Impedance matching
The input impedance of the probe should match the input impedance of the oscilloscope used, and the load effect on the measured circuit should be minimal. For oscilloscopes with low input impedance, active probes or probes with 50Ω input impedance should be selected; for oscilloscopes with high input impedance, ×10 probes should be selected. For example, if the input impedance of the oscilloscope is 1MΩ/10pF, the probe input impedance should preferably be 10MΩ/1pF. Such a probe has 10 times the signal attenuation, a very light load on the measured signal, and can match the oscilloscope input impedance.
5.3 Load effect
Reduce the load effect of the probe on the measured circuit. In addition to choosing a probe with high input impedance, remember that the probe input impedance decreases inversely with frequency.
5.4 Effect of time delay
Each probe has different delay times for the measured signal. When performing differential measurements and time (or phase) consistency measurements, it is best to use two probes of the same model and equal cable length.
5.5 Good grounding
The rated frequency characteristics of the probe are the results measured in a coaxial system. In actual circuit applications, the probe is often in a non-coaxial matching system, so the ground lead of the probe should be shortened as much as possible to minimize the series inductance. If it is found that the high-resistance probe is poorly grounded, consider using a low-resistance coaxial probe or an adapter, connector, and fixture that matches the probe.
6 Precautions for using oscilloscope probes
Correctly connecting the signal to be tested to the oscilloscope is the first step in the test work. Here we mainly introduce the precautions when connecting the probe to the circuit under test.
1. When the probe is connected to the circuit under test, the ground end of the probe must be connected to the ground wire of the circuit under test. Otherwise, in the suspended state, the potential difference between the oscilloscope and other equipment or the earth may cause electric shock or damage the oscilloscope, probe or other equipment.
2. When measuring pulse signals with short build-up time and high-frequency signals, please try to place the probe's ground wire close to the measured point. If the ground wire is too long, it may cause waveform distortion such as ringing or overshoot. See Figure 7.

Figure 7 Schematic diagram of probe grounding method

3. To prevent the ground wire from affecting the test of high-frequency signals, it is recommended to use the probe's dedicated grounding accessories. Figure 8 shows the standard test accessories that come with a typical general-purpose voltage probe.

Figure 8 Typical universal voltage probe with standard accessories (Image from Tektronix "ABC of Probes")
4. To avoid measurement errors, be sure to check and calibrate the probe before measurement. The calibration principle and method of probe attenuation compensation have been introduced in the previous section and will not be repeated here.
5. For high-voltage testing, a dedicated high-voltage probe should be used. After distinguishing the positive and negative poles, confirm that the connection is correct before powering on and starting the measurement.
6. When both test points are not at ground potential, a "floating" measurement, also known as a differential measurement, should be performed. A professional differential probe should be used.
7 Summary
Probes are critical to oscilloscope measurements, so the probe must have minimal impact on the probed circuit and hope to maintain sufficient signal fidelity for the measured value. If the probe changes the signal in any way or changes the way the circuit operates, the oscilloscope will see a distorted result of the actual signal, which may lead to incorrect measurement results or misleading measurement results. From the above introduction, it can be seen that there are many things to pay attention to when purchasing and using probes correctly. Only probes that match the oscilloscope and the circuit under test well are the probes you should choose and use.
References
[1] "AC and DC Current Measurement", author: Zhou Jiaming, Beijing Marine Instruments.
[2] "Impedance Effect on Measurement", author: Wei Qian, Han Jie, [M] Foreign Electronic Measurement Technology.
[3] "How to Use Oscilloscope Probes", [M] Radio.
[4] "Electronic Oscilloscope Probes", [M] Electronic Instrument Information.
[5] Hitachi Oscilloscope User Manual.
[6] "Probe ABC", published by Tektronix.