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  • Buyer's Guide: How to identify video line performance good or bad

    日期:2012-11-20  阅读4065次

     Identification of the video line is good or bad, the real test is the need for specialized apparatus and equipment. Equipment and instruments which do not have the design and engineering units. Engineering practice, how to identify the good and bad of the video line:
    First, the visual appearance of identification:

    1.PVC jacket: the surface can be seen pressed inside netting regularly "irregularities", the processing does not produce the relative sliding, good cable. Smooth appearance, do not see compression netting "roughness" The Yong Shounie jacket loose movement, poor cable;

    Check shield netting: programmed number is enough? Copper netting, check weldability, the tinned copper scraping look inside is copper wire, aluminum-magnesium alloy wire hardness significantly greater than copper; netting sparse, uneven distribution, not tight wrapped with an insulating layer is Poor cable;

    3 Check core wire: diameter - SYV cables 0.78-0.8mm, SYWV cable 1.0mm; recently appeared a SYV75-5 core wire diameter is 1.0mm cable, the characteristic impedance of this cable, and certainly not 75 ohms, not applied to the 75 ohm transmission system;

    4. Check the core wire and the insulating layer stick together: obliquely cut insulating layer, pull the core wire in the peeling direction, the core wire and the insulating layer did not stick together to process material; the good cable larger stick together, Poor The cables do not stick together;

    The longitudinal tensile experiment: Take a one meter cable, layered peel core wire, the insulation layer shield outside the household sets, each leaving 10 cm long. : Hands grip the adjacent layers of the cable, pull in the opposite direction; generally force in the cable pull them, the poor cable charges vigorously can easily pull out - the elevator cable is a very important, a lot of so-called dedicated elevator cable, there is the problem in this regard;

    Second, the transmission performance tests: video line, as the name implies, is used to transmit video signal transmission line. Since it is the transmission of video signals, at least is necessary to understand the transmission characteristics of the transmission line in the frequency range of 0-6M, or that the transmission performance. Here to talk about the main point oscilloscope measurement method, a total of reference, because the oscilloscope Contractors must have "weapons", one of the qualification review will be seized equipment; described below been able to skillfully use the oscilloscope.

    Color camera video signal can be used as the standard video source: test project with the camera video output increase should be on a 75 ohm load 1Vp-p, line the bottom of the sync tip to the highest white level of the video signal sync amplitude peak - peak "; Note that the line"-0.3V ", burst head (4.43M sine pulse) amplitude 0.3Vp-p; the oscilloscope sensitivity had hit amplitude calibration status. The election measure a good indicator of the camera as a video source;

    2 try to take the test cable a little longer, to reduce measurement errors, such as 1000 meters, cable joints must use the "F-type connector and coaxial double-pass (cable TV equipment), do not use the welding method, welding way damaged cable coaxial and the continuity of the characteristic impedance.

    3. Measure the DC resistance of the cable data: SYV75-5 cable 1000 meters, DC 35-40 ohm resistor core wire, outer shield resistance 1000 m 24-36 ohms (shield compiled resistance difference between) 1000 m; SYWV75-5 cable, the DC resistance of 18-22 ohm core wire, the outer shield resistance 1000 m 24-36 ohms; accumulate this information useful, not only to judge the quality of the cable timber, and used to project wiring, wear tube quality checks, such as attendants to wear tube, pull off the line, resistance becomes larger, the video signal becomes weak, interference should not there have been a very high probability of occurrence of this type of "accident", but and often overlooked;

    4 measuring cables high and low frequency attenuation characteristics: sync amplitude at the end of the measuring head and the color of the line head the 0.3V to 0db benchmark, calculate the amount of attenuation line the same Butou on behalf of low-frequency attenuation, the burst head behalf 4.43M high frequency attenuation , - such as: measured 1000 Rice Merchants synchronized head 0.15V, calculated in accordance with 20log attenuation multiple db number of the "-6db/1000m". measured 4.43M color sync after the first 1000 m attenuation magnitude 30mv, namely 1 / 10 times, the attenuation -20db/1000m; using this method can accurately grasp the different cable transmission quality and frequency distortion (high and low frequency attenuation difference) "With intuitive concept, you can more accurately measure the same model and structure the the SYV and SYWV cable distinction and performance is good or bad, compare the difference between different manufacturers products and performance is good or bad, you can also compare different batches of the same manufacturer product changes;

    The above method can also detect the performance of video transmission systems and equipment: every way coaxial video cable transmission characteristics such as engineering, Optical transmission characteristics (which can be measured is good or bad, do not think that are less than ideal), radio frequency transmission, microwave transmission characteristics of the mass of twisted pair

    Transfer characteristics, the distribution of characteristics of the video distributor, the switching characteristics of the host matrix, to pay special attention to when the output of the multiplexer switched simultaneously to the same input signal, if it is found to switch the more the greater the attenuation, not the large ones, should be the same, measured after, you will master the many substandard products;

    Observation field signal, look at the field synchronization position distortion big (flat uneven) - should be flat; also look at the low-frequency interference with an oscilloscope: field signal slowly varying fluctuations, 50/100 weeks interference a lot of "thatched" beating, mostly frequency harmonic interference, disconnect the remote camera, cable remote and outer conductors short-circuited at the end with an oscilloscope directly observed interference waveform and intensity; This method can also check and test anti-jamming the true performance of the equipment.