Noise Meters and Grounding Issues
Earth and electricity (signal), this is a pair of twins who are inseparable. Grounding usually refers to connecting to the earth with a conductor. However, the ground in electronic technology may have nothing to do with the earth, it is just an equipotential surface in the circuit. Such as the ground in the radio and TV, it is only a potential reference point in the receiver circuit. Grounding, in electric power and electronic technology, is both simple and complex, but also essential. According to the function of grounding, it can be divided into working grounding, protective grounding, overvoltage protection grounding, anti-static grounding, shielding grounding, signal grounding, etc. In radio and television technology, the above several grounding types will be encountered. Now we will make an elaboration on some grounding technical issues in combination with reality.
one. Protective grounding The protective grounding is a protection device set up to prevent the equipment from being electrified and endangering personal safety due to insulation damage. It has two methods: grounding and zero connection. According to power regulations, for systems that use three-phase four-wire power supply, since the neutral wire is grounded, the zero connection method should be used, and the metal shell of the equipment should be connected to the neutral line through a conductor, and the equipment shell is not allowed to be directly grounded. This is especially common in switchgear in the power distribution room of the radio and television system, power switchgear such as central air conditioners and transmitters, and large power-consuming equipment. When planning and designing, the grounding bus bar should be drawn from the ground grid to each device, and then the machine shell should be connected to the grounding bus bar with a conductor. It is worth pointing out that the grounding wire should be connected to the special grounding terminal of the equipment, and the other end should be welded. Sometimes the device case will feel numb, which is caused by AC leakage and the device case is not connected to zero. Generally, it can be solved by pulling out the power plug and changing the position and plugging it in again. In some frequently moving recording equipment, because the zero line is often ignored, some operators may touch the zero-connected and non-zero-connected devices with both hands at the same time, and the above phenomenon may occur.
two. Overvoltage protection grounding This is a grounding protection device for lightning protection. The most widely used lightning protection devices are lightning rods and arresters. The lightning rod enters the ground through the iron tower or building reinforcement, and the lightning arrester enters the ground through a special ground wire. The lightning arrester must be inspected every year before the thunderstorm season to prevent failure. If the telephone access device of my station is struck by lightning, it is caused by the failure of the telephone line lightning protector. Never connect the ground wire of other equipment to the lightning protection down conductor. The lightning protection down conductor can only be connected directly to the ground alone, otherwise lightning will damage other equipment through the down conductor. For example, a satellite TV receiver has been struck by lightning several times. The reason is that the feeder wire rubbed against the metal guardrail of the roof and the insulation was damaged.
Discussion on Noise and Grounding (2)
three. shielding ground
In order to prevent electromagnetic induction, it is a protective measure to ground the shielded metal skin of video and audio cables, the metal casing of electronic equipment, the shielding cover, and the metal shielding net of buildings (such as the shielding room for measuring sensitivity, selectivity, etc.) . Among all the grounding, the shielding ground is the most complicated, and it feels unclear and unclear. Because the shield itself can not only prevent external interference, but also may cause interference to the outside world through it, and electromagnetic interference must also be prevented between components in the equipment, such as the well-known mid-week casing and electronic tube shielding cover. Improper shielding and grounding will cause interference, which mainly include:
1. AC interference, which is mainly caused by AC power. To protect against AC interference, the power supply is usually filtered or a shielding layer is added between the primary and secondary stages of the power transformer and grounded. Outside of large stray electromagnetic fields, shielding and grounding are necessary to prevent electromagnetic interference. For example, during the opening ceremony of Xinyaxin Mall in our city, there was a transformer near the recording and amplifying equipment, and its electromagnetic field interfered with the on-site recording and amplifying. Finally, this problem was solved by grounding the shielding of the recording and expansion equipment.
2. High-frequency interference. This type of interference comes from frequency-conversion or ultra-conversion signals of various wireless transmitters. After they enter the electronic equipment, they are demodulated abnormally in the machine to form audio interference. The higher the signal frequency, the smaller the metal mesh of the building or equipment should be, and the denser the weaving of the signal line shielding layer should be, otherwise the shielding effect will be lost. For signal lines that are frequently plugged and unplugged, the shielding layer should be prevented from loosening and falling off at the plug. Because sometimes the shielding of the instrument is grounded through the shielding of the signal line (they are connected through a plug and socket), if the shielding falls off, it is easy to cause interference. For example, when I was in an electronics factory in Shantou, the testers reported that sometimes there was a kind of buzzing interference in the satellite TV receiver and affected the image quality. After tracking and observation, it was related to the passage of the plane, and it was obviously caused by the intrusion of the radar signal of Chenghai Airport and the abnormal demodulation. After analysis, it turned out that the shielding layer of the signal line fell off outside the plug, so that the shielding of the satellite TV receiver was not grounded.
Four. Various electronic circuits of the signal ground have a reference potential point, and this reference potential point is the signal ground. Its function is to ensure that the circuit has a uniform reference potential, so as not to cause signal errors due to floating. The connection of the signal ground is: the signal input terminal and the signal output terminal ground of the same device cannot be connected together, but should be separated; the output ground of the previous stage (equipment) is only connected to the input ground of the subsequent stage (equipment). 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 be paid special attention. For example, when I was working in an electronics company, the quality inspection department reported that the quality test results of satellite receivers were inconsistent. It turned out that some of the test instruments in 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 and the measurement results were inconsistent. Finally, set all grounded test instruments to be ungrounded, and this phenomenon will disappear.
