Detailed explanation of the grounding problem of 50HZ interference oscilloscope

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Detailed explanation of the grounding problem of 50HZ interference oscilloscope [Copy link]

Don't think that an oscilloscope probe is simple, in fact, it is very particular. 1. The first is the bandwidth, which is usually written on the probe, how many MHz. If the bandwidth of the probe is not enough, no matter how high the bandwidth of the oscilloscope is, it is useless, and it is a bottleneck effect. 2. Another thing is the impedance matching of the probe. The impedance matching part of the probe should be adjusted before use. Usually there is an adjustable capacitor at the end of the probe close to the oscilloscope, and some probes also have adjustable capacitors at the end close to the probe. They are used to adjust the impedance matching of the oscilloscope probe. If the impedance is not matched, the measured waveform will be deformed. The method to adjust the impedance matching of the oscilloscope probe is as follows: first, set the input selection of the oscilloscope to GND, and then adjust the Y-axis displacement knob to make the scan line appear in the middle of the oscilloscope. Check whether the scan line is horizontal (i.e., whether it coincides with the horizontal center line of the oscilloscope). If not, you need to adjust the horizontal balance knob (usually analog oscilloscopes have this adjustment terminal, in the small hole, you need to use a screwdriver to adjust it. Digital oscilloscopes do not need to adjust it). Then, set the oscilloscope input selection to DC coupling, and connect the oscilloscope probe to the test signal output terminal of the oscilloscope (generally oscilloscopes have this output terminal, usually a 1KHz square wave signal), and then adjust the scan time knob so that the waveform can display about 2 cycles. Adjust the Y-axis gain knob so that the peak-to-peak value of the waveform is about 1/2 screen width. Then observe the upper and lower sides of the square wave to see if it is horizontal. If overshoot, tilt, etc. occur, it means that the matching capacitor on the probe needs to be adjusted. Use a small screwdriver to adjust it until the upper and lower waveforms are horizontal and there is no overshoot. Of course, due to the quality of the oscilloscope probe, it may not be possible to adjust to a completely distortion-free effect, and at this time you can only adjust to the best effect. 3. In addition, there is a small switch on the oscilloscope probe to select the range: X10 and X1. When the X1 range is selected, the signal enters the oscilloscope without attenuation. When the X10 range is selected, the signal is attenuated to 1/10 before entering the oscilloscope. Therefore, when using the X10 range of the oscilloscope, the reading on the oscilloscope should be enlarged 10 times (some oscilloscopes can select the X10 range at the oscilloscope end to match the probe, so that after the oscilloscope end is also set to the X10 range, the reading can be directly read). When we want to measure higher voltages, we can use the X10 range function of the probe to attenuate the higher voltage before entering the oscilloscope. In addition, the input impedance of the X10 range is much higher than that of the X1 range, so when testing signal waveforms with weaker driving capabilities, setting the probe to the X10 range can provide better measurements. But please note that when the signal voltage is not clear, you should also use the X10 gear to measure it first. After confirming that the voltage is not too high, you can choose the correct range gear to measure. It is necessary to develop such a habit. Otherwise, if the oscilloscope is damaged one day, it will be too late to regret it. People often ask why the waveform on the crystal pin cannot be seen with an oscilloscope? One possible reason is that the probe's X1 gear is used, which is equivalent to a heavy load (an oscilloscope probe using the ×1 gear has a capacitance of hundreds of pF) connected in parallel to the crystal oscillator circuit, causing the circuit to stop oscillating. The correct method should be to use the probe's X10 gear. This should be noted during use. Even if the oscillation does not stop, the real waveform may not be seen due to excessive changes in the oscillation conditions. 4. When using the oscilloscope probe, make sure that the ground wire clip is reliably connected to the ground (the ground of the system under test, not the real earth). Otherwise, when measuring, you will see a large 50Hz signal. This is because the ground wire of the oscilloscope is not connected properly. If you find a 50Hz signal with a strong amplitude on the oscilloscope (the mains frequency in my country is 50Hz, and 60Hz abroad), you should pay attention to whether the ground wire of the probe is not connected properly. Since the oscilloscope probe is often used, the ground wire may be broken. 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 line in the middle of the probe is damaged. Then, clip 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), which means that the ground wire of the probe is good, otherwise the ground wire is damaged. Usually, the line connecting the clip is broken, usually re-weld, and can be replaced if necessary. Note that the ground wire connecting the clip should not be too long, otherwise it is easy to introduce interference, especially in high-frequency small signal environments. The ground wire clip of the oscilloscope probe should be close to the measurement point, especially when measuring signals with high frequency and small amplitude. Because the long ground wire will form a loop, it is like a coil, which will induce the electromagnetic field in space. In addition, when the current in the ground wire of the system is large, a voltage drop will also be generated on the ground wire, so the ground wire of the oscilloscope probe should be connected to the ground near the test point. 5. Sometimes the crosstalk of the pulse will interfere with the oscilloscope and cause false triggering. You can try to use the high-frequency suppression trigger mode of the oscilloscope, limit the bandwidth of the oscilloscope, and other methods. The above content is transferred from the Internet, but I don’t agree with the reason for the 50Hz interference on the Internet. I asked an engineer in the company and the explanation he gave me is quite reasonable. If the test ground of the oscilloscope is connected to the ground of the device under test, then the return path of the signal is the shortest, flowing directly from the probe of the oscilloscope to the test ground of the oscilloscope, passing through the ground of the device under test, and finally returning to the probe of the oscilloscope. However, if the test ground of the oscilloscope is disconnected, the signal can only flow back through a long route. The probe signal passes through the ground of the oscilloscope, flows to the ground of the device under test, and finally flows to the probe of the oscilloscope. It has made a big circle in the middle and introduced the interference of the 50Hz power frequency.

star_66666

Very good experience, give it a thumbs up

constant

Yes, when I use the oscilloscope, I always find 50Hz interference, and I don’t know where it comes from. . . .