Noise and grounding issues
Time:2023-08-22
Views:649
Earth and electricity (signal) are inseparable twins. Grounding usually refers to connecting a conductor to the ground. In electronic technology, the ground may not be related to the ground at all, it is only an equipotential surface in the circuit. Like the ground in radios and televisions, it is only a potential reference point in the receiver circuit. Grounding, in power and electronic technology, is both simple, complex, and essential. According to the function of grounding, it can be divided into various types, such as working grounding, protective grounding, overvoltage protection grounding, anti-static grounding, shielding grounding, signal grounding, etc. In radio and television technology, the above types of grounding will be encountered. This article will elaborate on some grounding technology issues based on practical situations.
1、 Protective grounding
Protective grounding is a protective device set up to prevent insulation damage from causing equipment electrification and endangering personal safety. It has two methods: grounding and grounding. According to electrical regulations, for systems that use three-phase four wire power supply, due to the grounding of the neutral wire, the neutral wire should be connected to the neutral wire. The metal shell of the equipment should be connected to the neutral wire through a conductor, and direct grounding of the equipment shell is not allowed. This is particularly common in the switchgear in the distribution room of the broadcasting and television system, as well as in power switchgear such as central air conditioners and transmitters, as well as in high power consuming equipment. When planning and designing, the grounding busbar should be introduced from the grounding grid to each equipment, and then the machine casing should be connected to the grounding busbar with conductors. It is worth pointing out that the grounding wire should be connected to the dedicated grounding terminal of the equipment, and the other end should preferably be welded.
Sometimes the equipment casing may feel numb, which is caused by AC leakage and the equipment casing not being connected to zero. Generally, the power plug can be unplugged and replaced before being inserted. In some frequently moving cataloging devices, due to the neglect of the neutral line, some operators may come into contact with both the neutral and non neutral devices, which may lead to the above phenomenon.
II Overvoltage protection grounding
This is a grounding protection device set up to prevent lightning. The most widely used lightning protection devices are lightning rods and arresters. The lightning rod enters the ground through iron towers or building steel bars, while the lightning arrester enters the ground through a dedicated ground wire. Lightning arresters must be inspected before the thunderstorm season every year to prevent failure. If the hotline telephone access device on our station is struck by lightning, it is due to the failure of the lightning protection device on the telephone line. On the lightning protection down lead, never connect the ground wire of other equipment. The lightning protection down lead can only be directly grounded separately, otherwise lightning will damage other equipment through the down lead. For example, a certain satellite TV receiver has been struck by lightning several times, and the original problem was that the insulation was damaged due to friction between the feeder and the metal guardrail on the roof, and the metal guardrail was welded together with the lightning rod downlead, causing lightning to enter and damage the receiver.
III Shielded ground
A protective measure taken to prevent electromagnetic induction by grounding the shielding metal skin of visual and audio cables, the metal casing of electronic equipment, the shielding cover, and the metal shielding mesh of buildings (such as the shielding room for measuring sensitivity, selectivity, and other indicators). Among all grounding, shielded grounding is the most complex, with a feeling of confusion and ambiguity. Because shielding itself can prevent external interference and may cause interference to the outside world through it, and electromagnetic interference must also be prevented between various components in the equipment, such as the well-known mid range casing and electronic tube shielding cover.
Poor shielding and improper grounding can cause interference, mainly including:
1. AC interference, mainly caused by AC power supply. The protection against AC interference usually involves filtering the power supply or adding a shielding layer between the primary stages of the power transformer and grounding it. It is necessary to shield and ground to prevent electromagnetic interference outside large stray electromagnetic fields. For example, during the opening ceremony of Xinya New Mall in our city, there was a transformer near the recording and amplifying equipment, and its electromagnetic field interfered with the recording and amplifying sound on site. Later, 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 ultra frequency conversion signals of various wireless transmission stations, which enter electronic devices and undergo abnormal demodulation inside the machine, forming audio interference.
The higher the signal frequency, the smaller the metal mesh aperture of the building or equipment, and the denser the weaving of the signal wire shielding layer, otherwise the shielding effect will be lost. For frequently unplugged signal wires, it is necessary to prevent the shielding layer from loosening and falling off at the plug. Sometimes the shielding of instruments and equipment is grounded through the shielding of signal wires (they are connected through plugs and sockets), and if the shielding falls off, it is easy to cause interference. When I was at an electronics factory in Shantou, the testers reported that there may be a noisy interference in satellite TV receivers that affects image quality. After tracking and observation, it is evident that it is related to the passing of the aircraft and is caused by the intrusion of radar signals from Chenghai Airport and abnormal demodulation. After analysis and investigation, it was found that the shielding layer of the signal line fell off outside the plug, causing the shielding of the satellite TV receiver to not be grounded.
IV Signal ground
Various electronic circuits have a reference potential point, which is the signal ground. Its function is to ensure that the circuit has a unified reference potential, which will not cause signal errors due to floating.
The connection of signal ground is that the signal input terminal and signal output terminal of the same equipment cannot be connected together, but should be separated; The output ground of the previous stage (device) is only connected to the input ground of the subsequent stage (device). Otherwise, the signal may form feedback through the ground wire, causing the signal to float. In the testing of equipment, the connection of signal ground should be particularly noted. For example, when I worked at an electronics company, the quality inspection department reported that the quality test results of satellite receivers were inconsistent. Originally, some of the testing instruments in the quality inspection department had their shells grounded, while others had their shells ungrounded (the test signals were transmitted from the signal center to various departments), resulting in inconsistent measurement results due to signal feedback through the ground. After setting all grounded testing instruments as ungrounded, this phenomenon will disappear.
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