The previous issue discussed the phenomena of image blurring and tailing caused by the amplitude frequency characteristics and group delay characteristics of transmission systems, and proposed corresponding solutions. This issue mainly discusses the following issues: 1. Low frequency interference caused by improper handling of the system power supply ground wire; 2. High frequency interference caused by environmental electromagnetic interference and equipment self-excitation; 3. Double reflection and unstable display caused by impedance mismatch of equipment, transmission system or connectors.

1. The low-frequency interference caused by improper handling of the system power ground wire can be divided into three categories:

One is due to the lack of grounding caused by switching power supplies; The second is poor contact of the equipment signal connection ground wire; The third reason is due to the confusion of zero, fire, and ground wires during wiring construction, resulting in a lack of common ground. The following will be explained separately.

Due to the lack of common ground caused by switching power supplies: Most of the equipment used in engineering now uses switching power supplies, which are isolated from each other during high-frequency voltage transformation. Even analog power supplies often use isolation transformers without a common terminal. As for the equipment itself, the signal ground wire is independent, or there is no relationship between the signal ground wires between the equipment, as shown in the following figure:

Therefore, there will be a potential difference, i.e. voltage, between the signal ground of the equipment. This voltage may range from tens of volts to over a hundred volts, and sometimes there may even be case discharge between two devices. When the device signal is connected, such as through a matrix switch, due to the voltage between the signal ground, a current is inevitably formed. If there is no discharge path, this current will flow back through the signal channel, which will bring fluctuations in the power or signal ground into the signal channel, forming interference. Especially when the number of signal channels increases, the voltage between them is different, and the impact will be greater. A common phenomenon is the appearance of low-frequency raceways, such as light and dark raceways from top to bottom, or from bottom to top. The rolling frequency is low, either at or below the power frequency (caused by differential shooting), or there is a change in image brightness at lower frequencies. The solution is to require that all related equipment's protective ground must be connected and the same protective ground, so that even if there is voltage between the signal ground of the equipment, it can be discharged through the protective ground without affecting the signal itself. Therefore, when selecting equipment, it is necessary to consider the connection method of its power line. If there is no protective ground (two wire system), problems may occur in applications, especially in larger systems. In addition, the power ground wire pins of the equipment must not be damaged. Another way is to find ways to isolate transmission during signal transmission, such as using balanced transmission or fiber optic transmission, to solve the common ground problem. These applications will be frequently used in the future and should be the direction of future development.

Poor contact of the ground wire in the signal connection of the second device: There are two aspects to the poor contact of the ground wire during the signal connection of the device. One is that the ground wire is poorly soldered or missed during joint welding, which can be avoided with some care. The other is that there are different understandings of the joint. According to the standard of VGA joints (15HD), the definitions of each pin are as follows:

    Among them, 1, 2, and 3 are simulated red, green, and blue signals, and 6, 7, and 8 are corresponding simulated ground; 13. 14 is a digital field signal, and 10 is a digital ground signal; ID Bit is the control or address code between the screen and the host. However, in practical engineering, errors often occur in the connection of the ground wire. If certain pins (such as 4, 5, 9, 15, etc.) are connected to the ground wire and displayed on a large screen, there may not be any problems. However, if the 10 pins are not grounded, it is possible to have a problem with the ground wire. Therefore, when using such connectors, it is recommended to measure them first to see if their definitions are the same. Of course, sometimes to avoid this situation, some devices will ground all the unused pins. Although it is not standard, it is quite practical, but there may be problems when using the corresponding control bits. This should be known, and it can be used in this way currently. Due to the confusion of zero, fire, and ground wires during wiring construction, there is no common ground. This type of problem is extremely common in on-site construction. Party A or the construction party will confidently claim that there is no problem with the power ground wire, but often find that the zero and live wire positions are chaotic, the ground (protective ground) is basically not connected, or the power and signal wires share a bridge, which can sometimes lead to direct burning of equipment. Therefore, it is recommended to thoroughly inspect all power sockets with a multimeter or shake table before entering the construction site to ensure that they comply with the rules of two phase three wire system for left zero, right fire, and center ground, in order to prevent equipment breakdown and other serious accidents. This type of problem is not a technical issue, and careful inspection during construction can avoid it. It will not be discussed here. 2. High frequency interference caused by environmental electromagnetic interference and device self-excitation: Due to the wide bandwidth of VGA signals, the chips and circuit designs used have a relatively wide bandwidth. In poor electromagnetic environments, electromagnetic interference in space can cause self-excitation of the circuit, resulting in thin patterns or thin edges on character edges. The solution is simple, just destroy its self-excited condition. In principle, unused ports (input and output ports) should be matched with a load of 75 ohms to prevent interference and self excitation damage. 3. Double reflection and unstable display caused by impedance mismatch of devices, transmission systems, or connectors: These types of problems are quite challenging. Its manifestation is the presence of slightly dark but clearly defined ghosting on the edges of characters, or irregular black screen on the large screen with no signal alarm. This is all caused by reflection, which affects R G. For B, reflection causes the formation of ghosting, which affects H V can cause instability. For any input and output circuit, we can simplify it into the following model:

According to the circuit principle requirements, when the output impedance is equal to the input impedance, that is, when matched, the input point is a half power point, and the input circuit is non reflective absorption. When the output impedance is not equal to the input impedance, the input point is not a half power point, and reflection will form. To put it more vividly, the output circuit transfers energy to the input circuit, but the input circuit cannot fully absorb it, and some excess parts are sent back to the output circuit. It is obvious that the output circuit cannot absorb this energy and send it back to the input circuit, resulting in some energy loss and time delay. This is similar to the principle of television ghosting in the past. The signal emitted by the transmitting station directly reaches the antenna.

The main signal is formed, and at the same time, the signal from the transmitting station is transmitted to the building. After reflection, it reaches the antenna and is received, forming an auxiliary signal. In addition to different intensities, there is also a time difference between the main and auxiliary signals. The TV receives two identical signals with time difference, and the displayed content forms a double shadow, as shown in the figure:

   In the transmission system, R G. The reflection of signal B will form ghosting, which affects H As for the V signal, due to the high resistance of the TTL circuit, it is possible to range from 510 ohms to 5000 ohms. Generally, it is 1500 ohms, but the impedance of the connector cable is 75 ohms. Therefore, impedance mismatch in the transmission process is inevitable and depends on the transmission distance. This will cause distortion of the synchronous signal waveform, disrupting the clock phase locking in the receiving circuit that utilizes the rising or falling edges of its waveform. This distortion will cause instability in phase locking. Nowadays, most projectors and other devices use digital phase locking, which means that the number of clocks in each row has a clear number and the phase locking range is small (to improve phase locking accuracy). Slight distortion can cause uneven boundaries of the displayed content or vertical lines, while more severe distortion can cause out of step or phase lock failure, resulting in the projector reporting no signal input. In fact, in this situation, if a CRT display is used for inspection, signals will be found, but the phase locking may not be very good, or even not severe. Since the principle of reflection is caused by impedance mismatch, the solution should be to match or destroy the reflection as much as possible, in order to As for the V signal, it is feasible to break the reflection. We have many methods to break the reflection and have achieved good results in practical applications. But for R G. The B signal can only be matched as much as possible. Due to the difficulty or impossibility of adjusting the impedance of the transmission system, including joints, cables, and even the equipment itself, this problem is not easy to solve. For BNC interfaces, it is required to use 75 ohms, but a large number of BNC interfaces on the market are 50 ohms (convenient for production); The cable should require 75 ohms ± 2 ohms, but this indicator is sometimes difficult to guarantee; Some projection devices, in order to improve clarity, have a significant increase in high-frequency performance. Under normal circumstances, the phenomenon of edge curling can be observed. If there is a slight reflection, this phenomenon will be even more pronounced. There is currently no good solution to this type of problem, and the only solution is to ensure that R G. Impedance matching for B, using standard connectors and cables to reduce the number of transfers, etc. We will classify and summarize the reasons that affect the quality of signals during transmission, and form the following table for reference:

The issues mentioned above can basically cover 80-90% of common engineering problems. Sometimes several phenomena may occur simultaneously, and it is advisable to decompose them one by one as much as possible. Different phenomena have different causes and solutions

(reprinted)