What is a probe?
Oscilloscope is the most commonly used measuring instrument for electronic engineers, and the oscilloscope probe is undoubtedly the most commonly used accessory of oscilloscope. Oscilloscope probe is an electronic component that connects the tested circuit with the input end of oscilloscope. Without the probe, the oscilloscope becomes a decoration and can only be used as an ornament.
Before choosing an oscilloscope probe, we'd better read the instruction manual of the oscilloscope to find out what kind of probe the oscilloscope we use is suitable for. We believe that the following points should be more important when selecting probes:
Make sure that the interface of the probe matches that of our oscilloscope. The probe interface of most oscilloscopes is BNC. Some oscilloscopes may be SMA interfaces.
Observe whether the input impedance and capacitance of the selected probe match the oscilloscope. Because we all want the probe to have the least influence on the circuit under test. The matching degree of probe impedance and capacitance with oscilloscope will greatly affect the accuracy of measurement signal.
BNC interface
SMA interface
Some oscilloscopes will support 50Ω or1mΩ input impedance switching. But for most measurements, 1mω is the most common. The input impedance of 50 Ω is often used to measure high-speed signals such as microwaves. There are also signal transmission delay and circuit board impedance measurement in logic circuit.
The input impedance of oscilloscope can often be fixed at 1mω or 50 ω, but the input capacitance of oscilloscope is affected by design factors such as bandwidth. Generally speaking, the common input capacitance of an oscilloscope with an impedance of 1mω is 14pF. The value may also be between 5pF and 100pF. Therefore, in order to match the probe with the input capacitance of the oscilloscope, we should know 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 the first step we should pay attention to when using the probe.
So how many probes and which probes do we need?
According to our different measurement requirements, the number and types of probes are also different. Kind of like a SLR. Maybe he only has one camera, but he often has many lenses. For example, if the DC voltage is simply measured, a passive probe of 1mω is basically enough. However, if it is often necessary to measure the relative voltage difference between live wire and live wire or between live wire and neutral wire in three-phase power supply in power system testing, then we need to use differential probes.
Differential probe
passive probe
Passive probe is the most common probe. Generally, when buying an oscilloscope, the manufacturer will standard several. A common passive probe consists of a probe, a probe cable, a compensation device or other signal conditioning network and a probe connector. In these types of probes, no active components, such as transistors or amplifiers, are used, so there is no need to supply power to the probes. Generally speaking, passive probes are more common, easier to use and cheaper. Common adjustable attenuation ratios of passive probes are:
1×: no attenuation
10×: 10 times attenuation
100×: 100 times attenuation
1000×: 1000 times attenuation
Passive voltage probes provide different attenuation coefficients for different voltage ranges. Among these passive probes, 10× passive voltage probe is the most commonly used probe. For applications where the signal amplitude is 1V peak-to-peak or lower, the 1X probe may be suitable or even essential. In the application of low and medium amplitude signals (tens of millivolts to tens of volts), it is much more convenient to switch the1×10× probe. However, the switchable1×10× probe is essentially two different probes in one probe, which not only have different attenuation coefficients, but also have different characteristics in bandwidth, rise time and impedance (R and C). Therefore, these probes can't completely match the input of oscilloscope and can't provide the best performance achieved by standard 10× probes.
Probe attenuation extends the voltage measurement range of oscilloscope through internal resistance. When used with the input resistance of oscilloscope, it will produce a voltage divider. For example, a typical 10x probe is equipped with an internal 9mω resistor. When connected to an oscilloscope with an input impedance of 1mω, the attenuation ratio of 10: 1 will be generated on the input channel of the oscilloscope. This means that the signal displayed on the oscilloscope will be110 of the actually measured signal amplitude, so we often need to set the attenuation ratio to 10X in the channel setting of the oscilloscope.
This attenuation function allows us to measure signals that exceed the voltage limit of the oscilloscope. Moreover, the attenuation circuit will lead to higher resistance (usually a good thing) and lower capacitance, which is very important for high-frequency measurement.
Schematic diagram of 10X passive probe
Active detector
An active probe is called an active probe because it contains active components similar to transistors and amplifiers and needs power support. In the most common case, the active device is a field effect transistor (PET), which provides a very low input capacitance, which will lead to a high input impedance in a wider frequency band. The rated bandwidth of an active FET probe is usually between 500 MHz and 4 GHz. In addition to higher bandwidth, the high input impedance of the active FET probe allows measurement at the test point with unknown impedance, and the risk of load effect is much lower. In addition, because low capacitance reduces the influence of 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 usually between 0.6V and 10V.
Active detector
Differential probe
The differential probe measures the differential signal. Differential signals refer to each other, not to signals grounded. The differential probe can measure the signal of the floating device. Essentially, it is composed of two symmetrical voltage probes with good insulation and high impedance respectively. The differential probe can provide high * * * mode rejection ratio (CMRR) in a wider frequency range. Compared with ordinary single-ended signal wiring, the most obvious advantages of differential signal are as follows:
Strong anti-interference ability, because the coupling between the two differential lines is very good, when there is noise interference from the outside, it is coupled to the two lines almost at the same time, and the receiving end only cares about the difference between the two signals, so it can offset the external * * * mode noise to the greatest extent.
EMI can be effectively suppressed. Similarly, because the polarities of the two signals are opposite, the electromagnetic fields radiated by them can cancel each other out. The closer the coupling is, the less electromagnetic energy is released to the outside.
Timing positioning is accurate, because the switching change of differential signal is located at the intersection of two signals, unlike ordinary single-ended signals, which are judged by the level of threshold voltage, it is less affected by process and temperature, which can reduce timing error and is more suitable for circuits with low amplitude signals. At present, the popular LVDS refers to this small-amplitude differential signal technology.
The principle of differential amplification is to input a pair of signals into the amplification circuit at the same time, and then subtract them to get the original signal. Differential amplifier is an amplifier composed of two transistors with the same parameter characteristics through direct coupling. If signals with the same magnitude and phase are input to two input terminals respectively, the output is zero, thus overcoming the zero drift.
Schematic diagram of differential probe
Current probe
Maybe you will think that it is easy to get the current value by dividing the voltage value measured by the voltage probe by the impedance value measured. Why do you need a special current probe to measure? Because in fact, the error introduced by this measurement is very large, we generally do not adopt the method of converting voltage into current. The current probe can accurately measure the current waveform by using the input of the current transformer, and the signal current flux is converted into voltage by the mutual inductance 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 without external power supply, while AC /DC current probes are usually active probes. Traditional current probes can only measure AC signals, because stable DC current can't induce current in transformers. Alternating current in transformer, with the change of current direction, produces the change of electric field and induces voltage. 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 needs an external power supply.
Ac /DC current probe
Finally, let's look at some suggestions related to detectors:
Correct compensation of probe: the input capacitance of different oscilloscopes may be different, even different channels of the same oscilloscope may be slightly different. In order to solve this problem, learning to compensate and adjust the probe is the most basic skill that engineers should master.
When the probe is connected 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 suspended state, the potential difference between the oscilloscope and other equipment or the earth may cause electric shock or damage to the oscilloscope, probe or other equipment.
Try to make the grounding conductor of the probe close to the position of the measured point. Too long grounding conductor may cause waveform distortion, such as ringing or overshoot.
When the two test points are not at ground potential, "floating" measurement, also called differential measurement, should be carried out, and a professional differential probe should be used.
Probe is very important for oscilloscope measurement. First of all, it is required that the influence of the probe on the detection circuit must be minimized, and sufficient signal fidelity should be maintained for the measured value. If the probe changes the signal or changes the working mode of the circuit in any way, the oscilloscope will see that the actual signal is seriously distorted, which may lead to errors or misleading measurement results. From the above introduction, it can be concluded that there are many points worthy of our attention in the selection and correct use of probes.