Grounding concept
1. In video transmission, the most common fault phenomenon is that a black bar or white bar appears on the screen of the monitor, and it slowly scrolls up or down. Therefore, when analyzing this type of fault phenomenon, it is necessary to distinguish between two different causes of the fault.
To distinguish between the problem of power supply and the problem of ground loop, a simple method is to connect only the output signal of a camera with no power problem to the control host. If the above interference phenomenon does not appear on the monitor , It means there is no problem with the control host. Next, you can use a portable monitor to connect to the video output of the front camera, and check each camera one by one. If there is, deal with it. If not, the interference is caused by ground loops and other reasons.
2. Wood-like interference appears on the monitor. The appearance of such interference will not overwhelm the normal image when it is slight, and the image cannot be viewed when it is severe (even destroying the synchronization). The reasons for this failure phenomenon are many and complicated. There are roughly the following reasons:
⑴ The quality of the video transmission line is not good, especially the shielding performance is poor (the shielding network is not a good quality copper wire network, or the shielding network is too thin to play a shielding role). At the same time, the line resistance of this type of video cable is too large, so that the signal is greatly attenuated is also the cause of the increased failure. In addition, the characteristic impedance of this type of video cable is not 75Ω and the parameters are beyond the specifications is also one of the reasons for the failure. Because the above-mentioned interference phenomenon is not necessarily a failure caused by a bad video line, the cause of this failure must be accurate and cautious in judgment. Only after excluding other possibilities, can we consider from the perspective of poor video line. If it is really a cable quality problem, the best way is of course to replace all such cables and replace them with cables that meet the requirements. This is the best way to completely solve the problem.
⑵ Caused by the power supply system is not "clean". The power supply here is not "clean", it means that the interference signal is superimposed on the normal power supply (50-cycle sine wave). The interference signals on this kind of power supply mostly come from the equipment using SCR in this power grid. In particular, high-current, high-voltage thyristor equipment has serious pollution to the power grid, which results in the power supply in the same power grid not being "clean". For example, there are high-power thyristor frequency and speed control devices, thyristor rectifiers, and thyristor AC-DC converters in the power grid, which will pollute the power supply. The solution to this situation is relatively simple, as long as the entire system uses purified power or online UPS power supply can basically be solved.
(3) There are strong interference sources near the system. This can be judged by investigation and understanding. If this is the reason, the solution is to strengthen the shielding of the camera and ground the video cable.
3. The failure caused by short circuit or open circuit of the core wire of the video cable and the shielding network. The manifestation of this kind of failure is that a deep and chaotic large-area network interference is generated on the monitor, so that the image is completely destroyed, and the image and the synchronization signal cannot be formed. This situation often occurs on BNC connectors or other types of video connectors. That is to say, when this kind of fault phenomenon occurs, it is not always that the signals of all channels of the entire system are faulty, but only appears on the channels with bad connectors. As long as the joints are carefully checked one by one, it can be solved.
4. Failure phenomenon caused by mismatch of characteristic impedance of transmission line. The manifestation of this phenomenon is that there are several vertical bars with equal spacing on the screen of the monitor. The frequency of the interference signal is basically an integer multiple of the line frequency. This is caused by the impedance mismatch caused by the characteristic impedance of the video transmission line not being 75Ω. It can also be said that this interference phenomenon is caused by the combination of the characteristic impedance and distribution parameters of the video cable that do not meet the requirements. The solution is generally solved by the method of "resistance at the beginning of the connection" or "resistance at the end of the connection". In addition, it is worth noting that when the video transmission distance is very short (usually within 150 meters), the use of the above-mentioned impedance mismatch and distribution parameters of excessively large video cables will not necessarily cause the above-mentioned interference phenomenon. The fundamental solution to the above problem is to ensure quality when purchasing video cables. If necessary, the cable shall be sampled and tested.
5. Space radiated interference introduced by the transmission line. Most of this interference phenomenon occurs because there are strong, high-frequency spatial radiation sources near the transmission system, system front end, or near the central control room. One solution to this situation is to understand the surrounding environment when the system is established, and try to avoid or stay away from the radiation source; the other method is to strengthen the shielding of the front end and central equipment when the radiation source cannot be avoided. The pipeline of the transmission line is made of steel pipe and well grounded.
In fact, all grounds are the same, and grounding is only a matter of means. It depends on what your grounding is used for, so don't mess up the grounding connection problem without conditions, otherwise you will make many serious mistakes. ,
First of all, from the perspective of general circuit design, there is no potential for grounding, and the grounding is stable, but it is actually ---- impossible. In the actual production of the circuit, errors often occur due to dirty grounding. In general 1. The low-frequency circuit often adopts the single-point grounding method and the high-frequency circuit often adopts the multi-point grounding method. However, it must be noted that the high-frequency grounding is mostly large-area grounding. Why?
First of all, the theory of low-frequency circuit grounding is originally different from the theory of high-frequency grounding. Is it true that the audio circuit has a constant rule, that is, single-point grounding? If you do n’t see it, the low-frequency hum The so-called point is the area where the cross-sectional area is close to zero.The audio circuit, especially the rear stage, often hums due to the lack of single-point grounding.I have heard the loop current, the resistance of the wire, you can guarantee that you use the metal If zero impedance is impossible, then you must accept the fact that there is a potential difference in grounding, and there is current when there is a potential difference, which is the source of hum, and it is actually a current area with a very small flow range, but there is one here. The phenomenon is that your large-area grounding is on the chassis. If you use a single-point grounding output ground, the current will less affect the input grounding, and the current hum can be eliminated from the chassis because of the grounding current. But this is just a means,
There are many grounding methods, and the single point is just one of them. The low-frequency circuit uses wires to connect each unit circuit to the chassis. There will be no large loop, no large loop current, no large loop The current input and output are safe for each other,
But what about high-frequency circuits?
The grounding theory of high-frequency circuits is deeply affected by the skin effect. What is the skin effect? ​​The skin effect refers to the situation that the current distribution of the wire will be denser on the metal surface under a high-frequency operating environment. Unlike a wire, it will become a full reactance. The higher the frequency, the more severe the reactance of the wire, so you can use the wire to ground? Don't be stupid, when the antenna is almost the same, do you still want to use wires? If the circuit has many units, you divide it into many circuit blocks, and each unit is connected to its ground with a wire, then you simply take a resistor to connect faster, the resistance is relatively easy to grasp by us, the wire is complicated, so high frequency The grounding of the circuit is often avoided by using wires, the circuit units are each looking for the nearest large-area ground, directly connected by the shortest path, and multiple points are generated,
The high frequency is very particular about the grounding material, but in principle, the area must be large enough and the cabinet is large enough, but in fact, there must be something to declare here that your connection must be as short as possible. It is best to live in close proximity to multiple points. It is only effective to ensure that each point is connected to the same plane, but it is still an old saying. This is only one of the means. The methods of high-frequency grounding are very different. , ~~~~~
There are many methods for grounding, and the specific method depends on the structure and function of the system. The concept of "ground" was first applied in the design and development of telephones. From the beginning of 1881, a single cable was used as the signal channel, and the ground was a public circuit. This is the first grounding problem. However, using the earth as a signal loop will cause excessive noise and atmospheric interference in the ground loop. In order to solve this problem, a signal return line was added. Many existing grounding methods are derived from past successful experience, these methods include:
1) Single-point grounding: As shown in Figure 1, single-point grounding is a method of providing a common potential reference point for many circuits together, so that signals can be transmitted between different circuits. If there is no common reference point, an erroneous signal transmission will occur. Single-point grounding requires that each circuit be grounded only once and connected at the same point. This point is often referred to as an earth. Since there is only one reference point, it can be believed that there is no ground loop, so there is no interference problem.
2) Multi-point grounding: As shown in Figure 2, it can be seen from the figure that the internal circuits of the device all refer to the chassis as the reference point, and the chassis of each device uses the ground as the reference point. This grounding structure can provide a lower ground impedance, because when multiple points are grounded, each ground wire can be very short; and multiple wires in parallel can reduce the total inductance of the ground conductor. Multi-point grounding must be used in high-frequency circuits, and the length of each grounding wire must be less than 1/20 of the signal wavelength.
3) Hybrid grounding: Hybrid grounding includes both single-point grounding characteristics and multi-point grounding characteristics. For example, if the power supply in the system needs to be grounded at a single point, and the RF signal requires multiple points to be grounded, the mixed ground shown in Figure 3 can be used. For DC, the capacitor is open, the circuit is grounded at a single point, for RF, the capacitor is turned on, and the circuit is grounded at multiple points.
When many interconnected devices are bulky (the physical size of the device and the connecting cable are very large compared to the wavelength of any interfering signals), there is a possibility of interference through the action of the chassis and the cable. When this happens, the path of the disturbing current usually exists in the ground loop of the system.
When considering the grounding problem, there are two aspects to consider, one is the self-compatibility of the system, and the other is the external interference coupling into the ground loop, which leads to the wrong operation of the system. Because external interference is often random, it is often more difficult to resolve.
Grounding requirements
There are many reasons for requiring grounding. Here are a few:
1) Safety grounding: The equipment that uses AC power must be grounded through a yellow-green safety ground wire, otherwise, when the insulation resistance between the power supply in the equipment and the cabinet becomes small, it may cause electric shock injury.
2) Lightning grounding: The lightning protection system of the facility is an independent system consisting of a lightning rod, a lower conductor, and a connector connected to the grounding system. This grounding system is usually shared with the ground used as the power reference ground and the yellow-green safety ground wire. Lightning discharge grounding is only for facilities, equipment does not have this requirement.
3) Electromagnetic compatibility grounding: Grounding required for electromagnetic compatibility design, including:
* Shield grounding: In order to prevent mutual interference between circuits due to parasitic capacitance, circuit radiated electric field or sensitivity to external electric field, necessary isolation and shielding must be carried out, and these isolated and shielded metals must be grounded.
* Filter grounding: The filter generally contains bypass capacitors from the signal line or the power line to the ground. When the filter is not grounded, these capacitors are in a floating state and cannot function as a bypass.
* Noise and interference suppression: The control of internal noise and external interference requires many points on the device or system to be connected to ground, thereby providing the "lowest impedance" channel for the interference signal.
* Circuit reference: To transmit signals correctly between circuits, there must be a common potential reference point. This common potential reference point is ground. Therefore, all interconnected circuits must be grounded.
All the above reasons form a comprehensive requirement for grounding. However, generally only the requirements for safety and lightning protection grounding are specified in the design requirements, and others are implicit in the user's EMC requirements for the system or equipment.
3. Application of grounding technology The grounding technology and methods currently applied can be said to be a summary of past experience in solving problems. Typical grounding requirements are often limited to the so-called "single point grounding".
Usually, the specific requirements for grounding are not specifically proposed at the circuit level, because it is not appropriate to make specific requirements at this level. For digital circuits, most logic chip reads work with a single-ended circuit. In other words, if the potentials of all signals refer to the power circuit, the potential is 0V. In analog circuits, the situation is similar. When the distance between the components is very close, it is easy to complete the generation, processing and waveform shaping of the logic signal, but if the transmission line is too long or the reference point potential is incorrect, it will cause problems. We have to establish the concept that grounding is not required for every part or every system. For example, a single circuit board does not have to be grounded to work properly. When data is to be transferred between devices, grounding is very necessary
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