Noise and grounding issues
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One. Protective grounding
Protective grounding is a protective device set up to prevent equipment damage caused by insulation damage and personal safety. It has two methods of grounding and zero connection. According to power regulations, for systems that use three-phase four-wire power supply, the neutral line is grounded, so the zero connection method should be used, and the metal shell of the device is connected to the zero line through the conductor, and the device shell is not allowed to be directly grounded. This is especially common in switchgear in power distribution rooms of radio and television systems, power switchgears such as central air conditioners and transmitters, and large power-consuming equipment. When planning and designing, the ground bus should be led out from the ground network to each device, and then the machine shell should be connected to the ground bus with conductors. It is worth pointing out that the ground wire should be connected to the dedicated ground terminal of the device, and the other end is preferably welded.
Sometimes the casing of the device is numb, which is caused by AC leakage and the device casing is not connected to zero. Generally, it can be solved by pulling out the power plug, changing the position and inserting it again. In some frequently-moving cataloging equipment, because the zero-connecting line is often ignored, some personnel may touch both the zero-connecting and non-zero-connecting devices at the same time with both hands, and the above phenomenon may occur.
2. Overvoltage protection grounding
This is a ground protection device for lightning protection. The most widely used lightning protection devices are lightning rods and arresters. The lightning rod enters the ground through an iron tower or building reinforcement, and the lightning arrester enters the ground through a dedicated ground wire. Lightning arresters must be inspected before the thunderstorm season comes every year to prevent failure. If the hotline telephone accessor of our station is struck by lightning, it is caused by the failure of the lightning protector of the telephone line. Never connect the ground wire of other equipment on the lightning protection downline. The lightning protection downline can only be directly connected to the ground alone, otherwise lightning will damage other equipment through the downline. If a satellite TV receiver has been struck by lightning several times, the original trap was the friction between the feeder and the metal guardrail on the roof and the insulation was damaged, and the metal guardrail and the lightning rod were welded together, and even the lightning entered and damaged the receiver.
3. Shielding ground
In order to prevent electromagnetic induction, it is a protective measure to ground the shielding metal sheath of the video and audio cables, the metal shell of the electronic equipment, the shielding cover, and the metal shielding network of the building (such as the shielding room for measuring sensitivity and selectivity) . Among all the grounding, the shielding ground is the most complicated, and there is a feeling of being unclear and unclear. Because the shield itself can not only prevent external interference, but also may cause interference to the outside world, and the electromagnetic interference between the components in the device must also be prevented. For example, the well-known mid-peripheral shell and tube shield are examples.
Poor shielding and improper grounding can cause interference. These interferences mainly include:
1. AC interference, which is mainly caused by AC power. For the protection of AC interference, the power supply is usually filtered or a shielding layer is added between the primary and secondary of the power transformer and grounded. Outside the large stray electromagnetic field, shielding and grounding is necessary to prevent electromagnetic interference. For example, during the groundbreaking ceremony of Xinyaxin Mall in our city, there was a transformer near the recording and expansion equipment, and its electromagnetic field interfered with the on-site recording and amplification. Afterwards, this problem was solved by shielding and grounding the recording and expansion equipment.
2. High frequency interference. This type of interference comes from the frequency conversion or over-frequency conversion signals of various wireless transmission stations. After they enter the electronic equipment, they are abnormally demodulated in the machine and form audio interference.
The higher the signal frequency, the smaller the metal mesh of the building or equipment, and the denser the braid of the signal line shielding layer, otherwise the shielding effect will be lost. For signal cables that are frequently plugged and unplugged, the shielding layer should be prevented from loosening and falling off at the plug. Sometimes the shielding of the instrument and equipment is grounded through the shield of the signal line (they are connected through the plug and socket). If the shield falls off, it is easy to cause interference. For example, when I was in an electronics factory in Shantou, the tester responded that there was sometimes a nuisance in the satellite TV receiver that affected the image quality. After tracking and observation, it is related to the passing of the aircraft, which is obviously caused by the intrusion of the radar signal of Chenghai Airport and the abnormal demodulation. After analysis, it was found that the shielding layer of the signal line fell off the plug, which caused the shielding of the satellite TV receiver to not be grounded.
4. Signal ground
Various electronic circuits have a reference potential point, which is the signal ground. Its role is to ensure that the circuit has a uniform reference potential, so as not to float and cause signal errors.
The connection of the signal ground is: the signal input terminal ground and the signal output terminal ground of the same device can not be connected together, but should be separated; the output ground of the front stage (device) is only connected to the input ground of the rear stage (device). Otherwise, the signal may form feedback through the ground wire, causing the signal to float. In the test of the equipment, the connection of the signal ground should pay special attention. For example, when I was working in an electronics company, the quality inspection department reported that the satellite receiver quality test results were different. It turned out that some of the testing instruments of the Quality Inspection Department were grounded and some were not grounded (the test signal was transmitted from the signal center to each department), so that the signal was fed back through the ground to make the measurement results inconsistent. After setting all grounded test instruments to ungrounded, this phenomenon will disappear.