Common grounding symbols: PE, PGND, FG- protective grounding or casing; BGND or DC- reflux-DC-48V (+24V) power supply (battery) reflux; GND-working place; DGND-Digital Terrestrial; AGND-analog grounding; LGND-lightning protection website.
There are many ways of grounding, including single point grounding, multi-point grounding and mixed grounding. And single point grounding is divided into series single point grounding and parallel single point grounding. Generally speaking, single-point grounding is used for simple circuits, grounding distinction between different functional modules, and low frequency (F
For an electronic signal, it needs to find a way to return the current with the lowest impedance to the ground, so how to deal with this signal return becomes very critical. First, according to the formula, we can know that the radiation intensity is directly proportional to the reflow area, that is to say, the longer the reflow bead is, the larger the reflow bead is, and the greater its interference to external radiation. Therefore, when laying PCB, we should minimize the area of power circuit and signal circuit. Secondly, for a high-speed signal, providing good signal reflux can ensure its signal quality, because the characteristic impedance of transmission lines on PCB is generally calculated with reference to the ground (or power layer). If there is a continuous ground plane near the high-speed line, the impedance of this line can be kept continuous. If there is no grounding reference near a section of line, the impedance will change, and the impedance discontinuity will affect the signal integrity. Therefore, when wiring, the high-speed line should be distributed to the floor near the ground level, or walk with one or two ground wires beside the high-speed line to shield and provide nearby backflow. Third, why try not to split across power sources when wiring? This is also because after the signal passes through different power sources, its return path will be very long and it will be easily disturbed. Of course, it is not strictly required not to cross the power score, but it is possible for low-speed signals, because the interference generated is negligible compared with the signal. For high-speed signals, check carefully and try not to cross. You can adjust the wiring of the power supply part. (This is for multilayer boards with multiple power supplies).
Some circuit boards have external input/output interfaces, such as serial connectors and RJ45 connectors. If their grounding design is not good, it will also affect their normal work, such as error code and packet loss in network interface interconnection, and it will also become an external electromagnetic interference source, sending the noise inside the board to the outside. Generally speaking, an independent interface ground will be divided separately, and the connection with the signal ground will be connected with a thin wire, which can be connected with a resistor of 0 ohm or a small resistor. Fine traces can be used to prevent the noise from the signal ground from being transmitted to the interface ground. Similarly, the filtering of interface ground and interface power supply should also be seriously considered.
Earth and electricity (signal), these are inseparable twins. Grounding usually refers to connecting with the earth with a conductor. But in electronic technology, the ground may have nothing to do with it, it is only the first level potential surface in the circuit. Such as the ground in radio and television, it is only a potential reference point in the receiver circuit. In power and electronic technology, grounding is simple and complex, and it is very important. According to the function of grounding, it can be divided into working grounding, protective grounding, overvoltage protection grounding, anti-static grounding, shielding grounding and signal grounding. In radio and television technology, the above grounding types will be encountered. This paper expounds some grounding technical problems in combination with practice.
I. Protective grounding
Protective grounding is a protective device to prevent equipment from being charged and endangering personal safety due to insulation damage. It has two ways: grounding and zero connection. According to the electric power regulations, the neutral wire of the three-phase four-wire power supply system should be grounded, so the metal shell of the equipment should be connected to the neutral wire through the conductor, and it is not allowed to directly ground the equipment shell. This is especially common in the switching equipment in the distribution room of radio and television system, power switching equipment such as central air conditioner and transmitter, and large power consumption equipment. When planning and designing, the grounding bus should be led out from the grounding grid to each device, and then the chassis should be connected to the grounding bus with wires. 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 equipment shell will be numb, which is caused by AC leakage and the equipment shell is not connected to zero. Generally, it can be solved by pulling out the power plug and plugging it in another position. In some frequently moving cataloging equipment, because the zero line is often ignored, some operators may encounter equipment with and without zero line at the same time, which happens from time to time.
2. Overvoltage protection grounding
This is a lightning protection and grounding device. Lightning rod and lightning arrester are the most widely used lightning protection devices. Lightning rods enter the ground through the steel bars of towers or buildings, and lightning arresters enter the ground through special grounding wires. Lightning arresters must be inspected before the thunderstorm season every year to prevent failure. If the hotline access equipment of our station is struck by lightning, it is caused by the failure of the lightning protection device of the telephone line. Do not connect the grounding wire of other equipment on the lightning protection downlead. Lightning protection downlead can only be directly grounded separately, otherwise lightning will damage other equipment through downlead. For example, a satellite TV receiver was struck by lightning many times. The original fault was that the feeder rubbed against the roof metal guardrail and the insulation was damaged, while the metal guardrail was welded with the lower part of the lightning rod, which caused lightning to rush in and damaged the receiver.
Three. Shielding land
In order to prevent electromagnetic induction, it is a protective measure to ground the shielding metal skin of audio and video wires, the metal shell of electronic equipment, the shielding cover and the metal shielding net of buildings (such as the shielding room for measuring sensitivity and selectivity). Among all grounding, shielding grounding is the most complicated, and there is a feeling of unclear and unclear. Because shielding itself can not only prevent external interference, but also interfere with the outside world through it, and electromagnetic interference must also be prevented between components in the equipment, such as the well-known middle shell and electron tube shield.
Poor shielding and improper grounding will cause interference, mainly including:
1. AC interference, mainly caused by AC power supply. The measures to prevent AC interference are usually to filter the power supply or to add a shielding layer between the primary of the power transformer and the ground. In addition to the large stray electromagnetic field, it is also necessary to shield grounding to prevent electromagnetic interference. For example, at the groundbreaking ceremony of Xinya New Mall in our city, there was a transformer near the recording and playback equipment, and its electromagnetic field interfered with the recording and playback on the spot. After that, the problem was solved by shielding the recording and expanding equipment and grounding it.
2. High frequency interference. This interference comes from the frequency conversion or overclocking signals of various wireless transmitting stations. After they broke into the electronic equipment, they got abnormal demodulation in the machine, resulting in audio interference.
The higher the signal frequency, the smaller the metal mesh hole of the building or equipment, and the denser the shielding layer of the signal line, otherwise the shielding effect will be lost. For signal wires that are often unplugged, the shielding layer should be prevented from loosening and falling off at the plug. Because sometimes the shielding of instruments and equipment is grounded through the shielding of signal lines (they are connected through plugs and sockets), if the shielding falls off, it is easy to cause interference. For example, when I was in an electronics factory in Shantou, testers reported that satellite TV receivers sometimes had noise interference, which affected the image quality. After tracking and observation, it is related to the passage of aircraft, which is obviously caused by the radar signal invasion and abnormal demodulation of Chenghai Airport. After analysis and search, it turned out that the shielding layer of the signal line outside the plug fell off, which led to the grounding of the satellite TV receiver.
Four. Signal grounding
All kinds of 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 and will not float and cause signal error.
The connection of signal ground is: the signal input ground and signal output ground of the same equipment cannot be connected together, but should be separated; The output ground of the previous stage (equipment) is only connected with the input ground of the latter stage (equipment). Otherwise, the signal may form feedback through the ground wire, causing the signal to float. When testing equipment, special attention should be paid to the connection of signal ground.