Analysis of Ground Loop Interference in Twisted Pair System

Why the ground potential difference in the twisted pair system occurs frequently, we first analyze this issue as follows:

Symptom: The phenomenon of the loop interference is typically caused by the black and white bars rolling up and down in the image. In severe cases, the image can be severely distorted until no image is displayed.

Cause of failure: After the video signal from the camera is connected from the beginning, it forms a complete electrical connection loop with the monitoring room host (that is, the ground loop connection). Under normal conditions, the front and back ends are at equal potential (or the potential difference is extremely small). The potential difference is roughly within 20 millivolts. As the front camera and the back-end master control device are in the process of transmission, the work grounds of the two places are too long, the grounding method is incorrect, the grounding is poor, and the load is unbalanced, etc., and a large gap will be formed between the front and back devices. The ground current, which will cause a voltage drop between the two grounds when the current passes through the ground with ground resistance, if the cable is grounded at both ends, the current will be formed on the cable through the internal resistance of the signal source (a potential difference between the two ends is generated. ), superimposed on the video signal to create interference. The ground potential difference at the work site is superimposed on the video signal, which means that the potential difference at the work site generates AC interference to the video signal. As the potential difference increases, the black and white bars will move up and down in the image, similar to the phenomenon of insufficient power. When the equipotential difference is as large as about 1 volt, the image is severely distorted until no image is displayed on the DVR monitor.

Potential difference example

Analysis: So why is there less chance of potential difference in the video line? Because the cathode of the video line is used as the ground in the signal loop. At the same time as the connection, the camera and the monitoring room are inadvertently connected by equipotential bonding, or it can be understood that when the potential difference in the two ground systems is small, the high-potential interference flows through the ground line into the other low potential. In the ground (grounding body), the interference current remaining in the equipment ground is not enough to affect the image quality (visible to the naked eye). This is a phenomenon that some video images have some disturbing wobble patterns. Only when the potential difference between the two places exceeds 2 volts will a certain interference pattern be generated on the video line.

Everyone also knows that twisted-pair transmission is a "virtual" transmission system. Because there is no ground wire, can it be understood that the camera ground and the host ground are two grounds before it passes through the ground? Twisted pair connects these two places. Although this transmission method can reduce the influence of the ground loop on the transmission of the signal line, due to the lack of equipotential bonding between the two “grounds”, the potential difference can be transmitted through the twisted pair signal line, causing interference to the video. Increases the chance of influence of the ground potential difference.

There are two ways to solve:

1. Try to improve the unbalanced ground potential. Typically, the metal body is used to connect the two places equally. Or use a cable to connect the equipment to the host device, which is also a good method.

2. Completely separate the power ground from the video ground at the source of the signal. Do not artificially create excessive ground loop systems. It is usually better to use direct current, and at the same time provide sufficient power consumption for the camera. Or use a professional device to completely isolate the normal signal on the signal line from the ground loop. This is what we often refer to as isolators. Isolation transformers, optocouplers, and other methods are commonly used. Optocoupler efficiency is good, and the applicable environment is many. The disadvantage is that the power supply is needed and the occupied space is slightly larger. The isolation transformer does not require power supply, and is easy to use. The disadvantage is that the high frequency loss is obvious and the volume is too large.

Through the above analysis and summary, we should understand how the disturbance of the ground loop is going on, and how to troubleshoot and solve the problem after the ground loop occurs. Of course, the ground loop is not as terrible as we thought, but there must also be a problem that must be faced in engineering. We need to solve it in the right way.