How to solve the monitoring problem of image interference signals

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How to solve the monitoring problem of image interference signals [Copy link]

Sometimes, on the monitor screen, there will be several equally spaced vertical lines. The frequency of this interference signal is basically an integer multiple of the line frequency. If you use an oscilloscope to observe the waveform of the interfered image, you will see that the high-amplitude harmonic oscillation waveform is superimposed on the back shoulder of the line synchronization head, causing vertical bar interference. : 1. Causes of this interference This is due to the impedance mismatch caused by the characteristic impedance of the video transmission line is not 75 ohms. Observing the waveform and quantitative measurement of the video cable, we can see that its impedance does not meet the requirements of the video cable, and its distribution parameters do not meet the requirements. This is also one of the reasons for the impedance mismatch. Therefore, the interference is caused by the combination of characteristic impedance and distribution parameters of the video cable. If the "start" or "end" impedance of the poor quality coaxial cable or the signal transmission over 250m is seriously mismatched, or even has an open circuit, the above interference may be caused. 2. Solution Rely on "resistance in series at the beginning" or "resistance in parallel at the end". That is, a 75Ω resistor is connected in series at the "beginning" and a 75Ω resistor is connected in parallel at the "end" to avoid impedance mismatch and excessive distribution parameters; Use high-quality coaxial cable. That is, the outer shielding net is copper, not less than 96 braids, and the core wire is also multi-strand copper; It is worth noting that when the video transmission distance is within 150m, the use of the above-mentioned video cable with impedance mismatch and excessive distribution parameters may not necessarily cause the above-mentioned interference phenomenon. Therefore, in a system with a long transmission distance, when analyzing this fault phenomenon, you should not be confused by the short distance and interference. The fundamental way to solve these problems is to ensure the quality when selecting video cables. Cable sampling should be carried out when necessary. (This article is from Wuyang Monitoring Company. Please indicate the source when reprinting.)

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Rely on "starting end series resistor" or "terminal parallel resistor". That is, "starting end" series 75Ω resistor, "terminal" parallel 75Ω resistor, to avoid impedance mismatch and excessive distributed parameters;