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Oscilloscope probe



Overview

1.1 Definition of oscilloscope probe

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 certain type of equipment or network that connects the signal source to the oscilloscope input. It must provide a convenient and high-quality connection between the signal source and the oscilloscope input. There are three key issues regarding the sufficiency of the connection: physical connection, impact on circuit operation, and signal transmission.

1.2 The development process of oscilloscope probes

In the past 50 years, various oscilloscope probe interface designs have been constantly evolving to meet the requirements of increased instrument bandwidth speed and measurement performance. In the earliest days, banana plugs and UHF-type connectors were usually used. In the 1960s, ordinary BNC connectors became the most commonly used probe interface type because of the smaller size and higher frequency of BNC. The BNC probe interface is still used in the design of test and measurement instruments, and the current higher-quality BNC-type connectors provide the maximum available bandwidth function close to 4GHz.

Later, some manufacturers proposed a design alternative to the common BNC probe interface. While using the BNC connector, an additional analog coded scale factor detection pin was provided as a mechanical and electronic interface design. Part of the oscilloscope enables compatible oscilloscopes to automatically detect and change the vertical attenuation range displayed by the oscilloscope.

1.3 The structure of oscilloscope probe

Most probes are composed of probe head, probe cable, compensation equipment or other signal conditioning network and probe connector.

In order to make oscilloscope measurements, you must first be able to physically connect the probe to the test point. To achieve this, most probes have at least one or two meters of associated cable. But the probe cable reduces the probe bandwidth: the longer the cable, the greater the drop. In addition to the one or two meters long cable, most probes also have a probe head or a handle with a probe. The probe head can fix the probe, and the user can move the probe to contact the test point. Usually this probe adopts a spring-supported hook form, which can actually connect the probe to the test point.

In order to obtain usable measurement results, the signal on the probe must be transmitted to the input of the oscilloscope with sufficient fidelity through the probe head and cable.

Classification features

There are hundreds or even thousands of different oscilloscope probes on the market. One of the technical indicators of oscilloscope probes is frequency characteristics. It is convenient to divide the types of probes by frequency, but the frequency coverage of oscilloscope probes is limited and it is difficult to divide them according to radio frequency bands such as LF, HF, VHF, UHF, and RF. Oscilloscope probes are one of all probes. The most commonly used probes are voltage and current probes, and probes are usually classified according to measurement objects.

The first category

2.1 Passive voltage probe 2.1.1 Passive probe

The passive probe is made of wires and connectors. When attenuating, resistors and capacitors are also included. 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 strongest and most economical probes. They are not only easy to use, but also widely used.

The second category

2.1.2 High-impedance passive voltage probes

Starting from actual needs, the most used voltage probes are high-impedance passive voltage probes. The voltage probe occupies the largest part. Passive voltage probes provide various attenuation coefficients 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, a 1× probe 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), the switchable 1×/10× probe is much more convenient. However, the switchable 1×/10× probe is essentially two different probes in one product. Not only are their attenuation coefficients different, but also their bandwidth, rise time, and impedance (R and C) characteristics are also different. Therefore, these probes cannot fully match the input of the oscilloscope and cannot provide the best performance achieved by standard 10× probes.

The third category

2.1.3 Low-impedance passive voltage probes

The bandwidth of most high-impedance passive probes is less than 100MHz to 500MHz or more Between high bandwidth. The low-impedance passive voltage probes (also called 50 ohm probes, Zo probes, voltage divider probes) have very good frequency characteristics. Using probes that match coaxial cables, the bandwidth can reach 10 GHz and 100 picoseconds or faster. Time. This probe is designed for use in 50 ohm environments, which are generally high-speed equipment verification, microwave communications, and time domain reflectometry (TDR).

The fourth category

2.1.4 Passive high voltage probe

"High voltage" is a relative concept. From the perspective of the probe, 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 require good insulation strength to ensure the safety of users and oscilloscopes.

The fifth category

2.2 Active voltage probes 2.2.1 Active probes

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), which provides a very low input capacitance. Low capacitance results in high input impedance over a wider frequency band. It can be seen from the Xc formula below:

The sixth category

2.2.2 Active FET probe

The specified bandwidth of active FET probe is generally in Between 500MHz and 4GHz. In addition to the higher bandwidth, the high input impedance of active FET probes allows measurements at test points where the impedance is unknown, and the risk of loading effects is much lower. In addition, since the low capacitance reduces the influence of the ground wire, a longer ground wire can be used.

Active FET probes do not have the voltage range of passive probes. The linear dynamic range of an active probe is generally between ±0.6V to ±10V.

The seventh category

2.2.3 Active differential probe

Differential signals refer to each other, not to ground. The differential probe can measure the signal of the floating device. In essence, it is composed of two symmetrical voltage probes, each with good insulation and high impedance to the ground. Differential probes can provide a high common-mode rejection ratio (CMRR) in a wider frequency range.

The eighth category

2.3 Current probe

From the point of view of principle, the voltage value measured by the voltage probe is divided by the measured impedance value. It is easy to The current value can be obtained. However, in fact, the error introduced by this measurement is very large, 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, and the signal current magnetic flux is transformed into a voltage by a transformer, and then amplified by the amplifier in the probe and sent to the oscilloscope.

The ninth category

2.3.1 AC current probe

In a transformer, with the change of the current direction, the electric field changes, and Voltage is induced. The AC current probe is a passive device and does not require an external power supply.

Tenth category

2.3.2 DC current probe

Traditional current probes can only measure AC and AC signals, because stable DC current cannot be in the transformer Induced current. However, using the Hall effect, a semiconductor device with a current bias will generate a voltage corresponding to a DC electric field. Therefore, the DC current probe is an active device and requires an external power supply.

So 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.

The eleventh category

2.4 Logic Probe

When using an oscilloscope to observe and analyze the analog characteristics of digital waveforms, you need to use a logic probe to isolate the exact cause , Digital designers usually need to look at specific data pulses that occur under specific logic conditions, which requires logic trigger functions.

The twelfth category

2.5 Other probes

Because the scope of application of oscilloscopes is very wide, there are various special probes besides the above-mentioned probe types. These professional probes have different functions according to the different front-end sensors. Below we introduce two of them for readers' understanding only.

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 the optical device and optical fiber.

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 kind of sensor probe, and various sensor probes can measure a variety of physical quantities with the oscilloscope.

Measurement influence

3.1 Load effect

The so-called load effect is when the oscilloscope is connected to the circuit under test, sometimes the input resistance of the oscilloscope will affect the circuit under test. Produce influence, cause the signal of the circuit under test to change. If the influence of the load effect is great, the waveform measurement cannot be accurately performed. 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 and the smaller the input capacitance, the smaller the load effect.

In oscilloscope measurement, another kind of 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 it. To the signal to be measured, 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 probe's load is mainly impedance; at high frequencies (above 10MHz), the probe's load 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, such as a passive probe with a bandwidth of 100MHz. Its input resistance is 1-10Ω, and its input capacitance is 1-10pF. The load effect of the active probe is better than that of the passive probe, and the frequency characteristic is better.

3.2 Impedance matching

Impedance is the ratio of voltage to current. Under ideal circumstances, the instrument under test should not affect its normal operation when testing, and the measured value should also be the same It is the same when the test instrument is not connected. When connecting instruments for measurement, the impact of impedance on measurement accuracy should be considered. In order to ensure that the maximum power can be transmitted between the instruments, the impedance should be matched. If the impedance is pure resistance, the value of the input impedance and the output impedance should be equal. If the impedance includes a reactance component, the input impedance of the load should be conjugated to the output impedance of the source, and the maximum power can be transmitted at this time.

The impedance value of impedance matching is usually the same as the characteristic impedance value of the transmission line used. For radio frequency systems, 50Ω impedance is generally used. For high impedance instruments, due to the existence of equivalent parallel capacitance, as the frequency increases, the parallel combined impedance gradually decreases, which will load the circuit under test. Such as 1MΩ input impedance, when the frequency reaches 100MHz, the equivalent impedance is only about 100Ω. Therefore, high-bandwidth oscilloscopes generally use 50Ω input impedance to ensure the matching between 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 slew rate increases, the capacitive component of impedance becomes the main factor. As a result, the capacitive load becomes a major problem, especially the capacitive load will affect the rise time and fall time of the fast switching waveform and the amplitude of the high-frequency components in the waveform.

Technical indicators

4.1 Bandwidth and rise time

The bandwidth of the probe refers to the frequency that causes the output amplitude of the probe's response to drop to 70.7% (-3dB). Rise time refers to the 10~90% response of the probe to the step function, indicating that the probe can be quickly converted from the head to the input of the oscilloscope. For most probes, the product of bandwidth and rise time is close to 0.35. In many cases, the bandwidth is verified by the pulse rise time to ensure minimum distortion.

4.2 Capacitance

The capacitance index of the probe head refers to the capacitance on the probe tip, which is the capacitance of the probe equivalent to the test point of the circuit under test or the device under test. The probe is equivalent to a capacitor at one end of the oscilloscope, and this capacitance value should match the capacitance of the oscilloscope. For 10× and 100× probes, this capacitance is called compensation capacitance, which is different from the probe tip capacitance. The following will continue to introduce the compensation capacitor.

4.3 Aberration

Aberration is any amplitude deviation of the expected response or ideal response of the input signal. In practice, distortion usually occurs immediately between fast waveform transitions, which manifests as so-called "damped oscillations." High-frequency probes with no specified limit distortion can provide completely misleading measurements. The presence of distortion can account for severely distorted bandwidth and roll-off characteristics.

4.4 Attenuation coefficient

When the terminal is correctly connected, the probe should have a constant attenuation coefficient. The attenuation coefficient is the ratio of the output signal to the input signal. Some probes may have selectable attenuation coefficients. Typical attenuation coefficients are 1×, 10×, and 100×.

4.5 Probe attenuation compensation

The so-called probe attenuation compensation refers to adjusting the variable capacitance in the probe when the oscilloscope and probe are used together to make the frequency relatively stable. Probe compensation means frequency compensation between the end of the probe and the input of the oscilloscope.

Although the input resistance of the oscilloscope is only 1MΩ, the input capacitance in parallel with it varies depending on the model. Even if it is 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.

4.6 Maximum rated voltage

The maximum rated 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 it exceeds This maximum rated voltage will damage the probe.

4.7 Decrease of voltage rating with frequency

The maximum input voltage of the voltage probe at low frequency has clear regulations, and the input voltage will decrease as the frequency increases. For high-frequency probes, attention must be paid 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.

Precautions

Connecting the signal under test to the oscilloscope correctly is the first step of the test work. Here we mainly introduce the precautions when connecting the probe to the circuit under test.

1. When connecting the probe to the circuit under test, the ground terminal of the probe must be connected to the ground wire of the circuit under test. Otherwise, in the floating 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 and high-frequency signals with a short set-up time, please try to place the grounding wire of the probe as close as possible to the location of the measured point. If the ground wire is too long, it may cause waveform distortion such as ringing or overshoot.

3. In order to avoid the influence of the ground wire on the high-frequency signal test, it is recommended to use the special grounding accessory of the probe.

4. In order to avoid measurement errors, please be sure to check and calibrate the probe before measurement. The principle and method of probe attenuation compensation calibration have been introduced before, so I won’t repeat them here.

5. For high-voltage testing, a special high-voltage probe should be used, and the positive and negative poles should be distinguished, and the connection can be confirmed before powering on to start the measurement.

6. When the two test points are not at the ground potential, a "floating" measurement, also called a differential measurement, should be performed, and a professional differential probe should be used.

7 Summary

Probes are very important for oscilloscope measurement, so the impact of the probe on the detection circuit must be minimized, and it is hoped that sufficient signal fidelity is maintained for the measured value. If the probe changes the signal or changes the circuit operation mode in any way, the oscilloscope will see the distortion result of the actual signal, which may lead to erroneous or misleading measurement results. From the above introduction, we can see that there are many points worth noting about the purchase and correct use of probes. Only the probes that are well matched with the oscilloscope and the circuit under test are the probes you should choose and use.

Measuring point

When using the digital oscilloscope probe, ensure that the ground clip is reliably grounded (the ground of the system under test, not the real ground), otherwise, when measuring, You will see a large 50Hz signal, which is caused by the inductive 50Hz power frequency mains in the space because the ground wire of the oscilloscope is not connected well. If you find that there is a strong 50Hz signal on the oscilloscope (the mains frequency in our country is 50Hz, and there is 60Hz in foreign countries), then you must pay attention to whether the ground wire of the probe is not connected properly. Due to the frequent use of the oscilloscope probe, the vibration meter may cause the ground wire to be disconnected. The detection method is: adjust the oscilloscope to the appropriate scanning frequency and Y-axis gain, and then touch the probe in the middle of the probe with your hand. At this time, you should be able to see the waveform, usually a 50Hz signal. If there is no waveform at this time, you can check whether the signal wire in the middle of the probe has been damaged. Then, clamp the ground wire clip of the oscilloscope probe to the probe (or hook) of the probe, and then touch the probe of the probe with your hand. At this time, you should not see the signal just now (or the amplitude is very weak). It means that the ground wire of the probe is good, otherwise the ground wire has been damaged. Usually the wire connecting the clip is disconnected, usually re-soldered, and can be replaced if necessary. Pay attention to the ground wire connecting the clip not to be too long, otherwise it is easy to introduce interference, especially in the high-frequency and small-signal environment.

The ground clip of the digital oscilloscope probe should be close to the measurement point, especially when measuring signals with higher frequencies and smaller amplitudes. Because the long ground wire will form a loop, it is like a coil that will induce electromagnetic fields in space. In addition, when the current in the ground wire in the system is large, a voltage drop will be generated on the ground wire, so the ground wire of the oscilloscope probe should be connected to the ground near the tested point. The X10 gear of the probe has a larger input impedance. For example, when measuring the oscillation waveform of a crystal oscillator, the X10 gear of the probe should be used. If the X1 gear is adopted, it may cause the crystal oscillator to stop oscillating, or it may not stop oscillating, or it may be impossible to see the true waveform due to excessive changes in the oscillation conditions.

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