Introduction to various types and usage of oscilloscope probes

What is a probe?

The oscilloscope is the most commonly used measuring instrument for electronic engineers, and the oscilloscope probe is undoubtedly the most commonly used accessory for the oscilloscope. The oscilloscope probe is an electronic component that connects the circuit under test to the input terminal of the oscilloscope. Without the probe, the oscilloscope becomes a decoration.

Before choosing an oscilloscope probe, we'd better read the oscilloscope manual to understand what kind of probe is suitable for the oscilloscope we are using. The following points should be more important when choosing a probe:

• Make sure the probe interface matches the interface of our oscilloscope. Most oscilloscopes have a BNC probe interface. Some oscilloscopes may have an SMA interface.

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• Observe whether the input impedance and capacitance of the selected probe match the input impedance and capacitance of the oscilloscope. Because we all hope to minimize the impact of the probe on the circuit being measured. The matching degree of the probe impedance and capacitance with the oscilloscope will greatly affect the accuracy of the measured signal.

BNC connector

SMA interface

Some oscilloscopes support 50 Ω or 1 MΩ input impedance switching. But for most measurements, 1 MΩ is the most common. 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 circuit board impedance measurements.

The input impedance of an oscilloscope can often be fixed at 1 MΩ or 50 Ω, but the input capacitance of an oscilloscope is affected by bandwidth and other design factors. Generally speaking, the common input capacitance of an oscilloscope with 1 MΩ impedance is 14pF. This value may also be between 5pF and 100pF. Therefore, in order to match the probe to the input capacitance of the oscilloscope, it is necessary to understand the capacitance range of the probe before selecting the probe, and then adjust the capacitance of the probe through the calibration rod. This is the compensation of the probe, and it is also the first step we should pay attention to when using the probe.

So how many probes and which probes do we need?

Depending on our measurement needs, the number and type of probes required are also different. This is a bit like a person who plays with a SLR camera. Maybe he only has one camera, but often has many lenses. For example, if it is just a simple measurement of DC voltage, then a 1 MΩ passive probe is basically enough. However, if it is often required to measure the relative voltage difference between the live wire and the live wire in the three-phase power supply, or between the live wire and the neutral (neutral) wire in the power system test, then we need to use a differential probe.

Differential probes

Passive probes

Passive probes are the most common probes, and generally the manufacturer will provide several of them as standard when you buy an oscilloscope. Common passive probes consist of a probe head, a probe cable, a compensation device or other signal conditioning network, 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. In general, passive probes are more common, easier to use, and cheaper. Common passive probes have adjustable attenuation ratios:

1×: No attenuation

10×: 10 times attenuation

100×: 100 times attenuation

1000×: 1000 times attenuation

Passive voltage probes offer a variety of attenuation factors 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 less, 1× probes may be more suitable or even essential. 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 probe, 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 with standard 10× probes.

Probe attenuation is the process of expanding the voltage measurement range of an oscilloscope through an internal resistor that, when used with the input resistance of the oscilloscope, creates a voltage divider. For example, a typical 10x probe is equipped with an internal 9MΩ resistor, which, when used in conjunction with an oscilloscope with a 1MΩ input impedance, produces a 10:1 attenuation ratio on the oscilloscope's input channel. This means that the signal displayed on the oscilloscope will be 1/10 of the actual measured signal amplitude, so we often need to go to the oscilloscope's channel settings and adjust the attenuation ratio to 10X.

This attenuation feature allows us to measure signals that are beyond the voltage limits of the oscilloscope. Also, the attenuation circuit results in higher resistance (usually a good thing) and lower capacitance, which is important for high frequency measurements.

10X Passive Probe Schematic

Active probes

Active probes are called active probes because they contain active components such as transistors and amplifiers and require power support. 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. 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, because the low capacitance reduces the effect of the ground lead, longer ground leads 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.

Active probes

Differential probes

The differential probe measures differential signals. Differential signals are referenced to each other, not to the ground. The differential probe can measure the signal of a floating device. In essence, it is composed of two symmetrical voltage probes, each with good insulation and high impedance to the ground. The differential probe can provide a high common mode rejection ratio (CMRR) over a wider frequency range. Compared with ordinary single-ended signal traces, the most obvious advantages of differential signals are reflected in the following three aspects:

1. The anti-interference ability is strong because the coupling between the two differential lines is very good. When there is external noise interference, it is almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals, so the external common-mode noise can be offset to the greatest extent.

2. It can effectively suppress EMI. For the same reason, since the polarities of the two signals are opposite, the electromagnetic fields they radiate can cancel each other out. The tighter the coupling, the less electromagnetic energy is discharged to the outside world.

3. The timing positioning is accurate. Since the switching change of the differential signal is located at the intersection of the two signals, unlike the ordinary single-ended signal that relies on the high and low threshold voltages for judgment, it is less affected by the process and temperature, which can reduce the timing error and is more suitable for circuits with low amplitude signals. The currently popular LVDS refers to this small amplitude differential signal technology.

The principle of differential amplification is that a pair of signals are simultaneously input into the amplifier circuit, and then subtracted to obtain the original signal. The differential amplifier is an amplifier composed of two transistors with the same parameter characteristics using direct coupling. If the two input terminals are respectively input with signals of the same magnitude and phase, the output is zero, thus overcoming the zero drift.

Differential Probe Schematic

Current probes

You may think that it is easy to get the current value by measuring the voltage value with a voltage probe and dividing it by the measured impedance value. Why do we need a current probe to measure it? Because in fact, the error introduced by this measurement is very large, and we generally do not use the method of converting voltage to current. The current probe can accurately measure the current waveform by using the current transformer input, and the signal current flux is converted into voltage by the transformer, and then amplified by the amplifier in the probe and 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 and do not require external power supply, while AC/DC current probes are usually active probes. Traditional current probes can only measure AC signals because stable DC current cannot induce current in the transformer. In the transformer, the AC current generates a change in the electric field as the current direction changes, and induces a voltage. However, using 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 an external power supply.

AC and DC current probes

Finally, let's look at a few suggestions related to probes:

1. Correctly compensate the probe: Different oscilloscopes may have different input capacitance, and even different channels on the same oscilloscope may have slight differences. To solve this problem, learning to compensate the probe is the most basic skill that engineers should master.