Common problems in the measurement process of oscilloscope

　　Oscilloscope is a measurement tool that people often use in design. Its main function is to convert invisible electrical signals into images that can be seen by the naked eye, so that people can observe the changes of electrical signals and make adjustments as needed during the design and research process. Today, the editor has sorted out some common problems that you may encounter when using an oscilloscope for measurement.

　　
　　How does the oscilloscope test the waveform without using a current probe?
　　
　　We need to find a precision Hall sensor with low current, such as 5A output 5V. Solder the collector leg or emitter leg from the pad, add a temporary short wire, pass through the window of the Hall sensor, and then solder it back to the original place, and then you can use the oscilloscope to test the working current. At that time, just convert the voltage waveform into current. Will the
　　
　　oscilloscope repair switching power supply burn the oscilloscope?
　　
　　The oscilloscope power supply wiring is as shown in the above part. The neutral and live wires are isolated by a transformer, the ground wire is connected to the casing, and the probe ground wire is also connected to the casing, so the probe ground wire and the power ground wire are connected. The
　　
　　switching power supply hot ground is separated from the live wire by only a rectifier diode. When the oscilloscope probe ground wire is connected to the switching power supply hot ground, the live wire is connected to the oscilloscope power ground wire through the rectifier diode. If the oscilloscope power ground wire is connected to the earth, it will short-circuit and burn the rectifier diode, as shown by the red line. 　　To prevent this from happening, either the oscilloscope power ground wire is not connected to the ground or the neutral wire, or the switching power supply is isolated by a transformer. It is also possible to isolate the probe ground wire, which requires a special probe. We only need to follow one principle, just disconnect the path indicated by the red line. 　　How does an oscilloscope measure AC? 　　We can connect a resistor to the output end of the transformer. If the voltage is very high, we can use several resistors to divide the voltage, and then measure the resistor with a small resistance value. Make sure that the voltage of the AC does not exceed the range of the oscilloscope, otherwise there will be a risk of fire. 　　How to choose the input impedance of the oscilloscope? 　　In the power supply noise test, there is still controversy over the selection of the input impedance of the oscilloscope channel. The oscilloscope channel has three options: DC50/DC1M/AC1M (for high-end oscilloscopes, there may be only one option: DC50). Some engineers believe that an input impedance of 1M ohm should be used, while others believe that an input impedance of 50 ohm is more appropriate. 　　During the test, we found that if a probe with 1x attenuation is used for testing, when the oscilloscope channel input is 1M ohm, the power supply noise measured is usually greater than the 50 ohm input impedance. The reason is that after the high-frequency power supply noise is transmitted from the coaxial cable to the oscilloscope channel, when the oscilloscope input impedance is 50 ohms, the characteristic impedance of the coaxial cable of 50 ohms is completely matched with the channel, and there is no reflection; while when the channel input impedance is 1M ohms, it is equivalent to high impedance. According to the transmission line theory, the power supply noise is reflected, so that the 1M ohm input impedance is higher than the 50 ohm input impedance of the tested power supply noise. Therefore, it is recommended to use a 50 ohm input impedance to measure small power supply noise. 　　How to distinguish CCM, DCM and BCM of power supply with an oscilloscope? 　　VDS will oscillate when it is turned off because of leakage inductance and COSS. When it is turned on again, in CCM mode, when the secondary is still releasing energy, the first refracted voltage (Vor) will still keep the coss in the previous charging state, so it will not oscillate when it is turned on again. However, in DCM, since the secondary energy is released, the first refracted voltage disappears. As can be seen from the figure, when the secondary current just drops to 0, the voltage loaded on coss and LM suddenly changes (only the input voltage), which will inevitably oscillate. For IDS, there will be oscillation when it is turned off, which is easy to understand. When it is turned on, energy is stored in LM. When it is suddenly turned off, an induced voltage will inevitably be generated. This voltage will oscillate in coss and LM. 　　When conducting, for CCM, since the secondary has been releasing energy in the continuous current before, the capacitance of the diode itself is in the energy storage state. When the primary is turned on, the diode capacitance current is reversely discharged, and a peak current is induced in the primary. For DCM, since the secondary releases energy completely, when the secondary continuous current drops to 0, the diode ci begins to discharge. Therefore, when it is turned on again, there will be no recovery current, so there is no induced spike. 　　What are the specifications of the oscilloscope probe? 　　The probe of the oscilloscope is generally divided into active and passive. Usually, the oscilloscope is only equipped with passive probes, and the input signal is not isolated from the oscilloscope. Ordinary passive probes have no attenuation function, and the higher-end ones have 1X10 or 1X100 attenuation functions. The upper limit of the use voltage of ordinary probes generally does not exceed 300V. If it is higher than 300V, a high-voltage probe needs to be selected. When using a non-original probe, it is necessary to pay attention to the distributed capacitance of the probe (usually 75pF) to match the oscilloscope. Too much error will affect the accuracy of the measurement. 　　Active probes have their own signal processing circuits and independent power supplies (some are provided by the oscilloscope). The input signal is completely isolated from the oscilloscope, and some have multiple attenuation ratios for selection. When the active probe is measuring, the voltage of the input signal will not have an adverse effect on the oscilloscope, and at most damage the probe. It can directly measure higher voltages (above 300V), but active probes are more expensive and easier to damage, especially afraid of vibration (especially Hall-type probes). In addition, passive probes cannot measure voltage signals that are not referenced to the ground when the oscilloscope is reliably grounded, otherwise it will damage the oscilloscope or the unit under test. 　　Oscilloscopes are widely used and can measure a variety of different electrical quantities. They are also the most commonly used measurement equipment for designers. Have you ever encountered any of the above problems? The editor will continue to collect questions and answers related to oscilloscopes to help you solve more problems.