Tags: video cable low frequency interference
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Visual appearance identification:
1. PVC sheath: The surface can be seen to have a regular "unevenness" in the inner mesh, indicating that the processing technology is good and there is no relative sliding, which is a good cable. The appearance is smooth, and the "unevenness" of the compacted net is not seen. The hand-pinched jacket has a loose feeling, which is a poor cable;
2. Check the shielding layer network: Is the number of the number enough? The copper mesh is used to check the solderability. The tinned copper wire is scraped to see if the copper wire is inside. The hardness of the aluminum-magnesium alloy wire is obviously larger than that of the copper wire; the mesh is sparse. The distribution is not uniform, and the insulation layer is not tightly wrapped, etc.
3. Check the core wire: diameter - SYV cable is 0.78-0.8mm, SYWV cable is 1.0mm; recently there is a SYV75-5 core wire diameter of 1.0mm cable, the characteristic impedance of this cable is definitely not 75 Ohm, not applied to 75 ohm transmission systems;
4. Check the adhesion between the core wire and the insulation layer: cut the insulation layer obliquely, and pull the core wire in the peeling direction to see if the core wire and the insulation layer are in contact with the process material; the cable has a large adhesion force and is poor. The cable is not glued;
5. Longitudinal tensile test: take one meter of cable, peel off the core wire, insulation layer, shielding layer, outer casing, each 10 cm long. The method is: two hands respectively hold the adjacent two layers of the cable and pull in the opposite direction; the good cable generally pulls the power, and the difference cable can be easily pulled out without much effort - the elevator cable is very important, many so-called "Elevator-specific cables" have problems in this regard;
Transmission performance test:
A video line, as the name suggests, is a transmission line used to transmit video signals. Since it is to transmit video signals, at least it is necessary to understand the transmission characteristics of the transmission line in the 0-6M frequency band, or transmission performance. Here mainly talk about "oscilloscope measurement method", a total reference, because the oscilloscope is one of the must-have equipment for engineering experts, the qualification inspection equipment; the following description is based on the already skilled use of the oscilloscope.
1. Color camera video signal can be used as â€œstandard video sourceâ€: camera video output for test engineering, the amplitude on the 75 ohm load should be 1Vp-p, that is, the bottom of the line sync head to the highest white level of the video signal. â€œPeak-peakâ€; note that the line sync head amplitude is â€œ-0.3Vâ€, and the color sync head (4.43M sine wave pulse) has an amplitude of 0.3Vp-p; select the oscilloscope sensitivity and hit the amplitude calibration state. Select a camera with better indicators as the "video source";
2. Test the cable as long as possible to reduce measurement error, such as 1000 meters, the cable intermediate connector must use "F-type connector" and coaxial double-pass (cable TV equipment), do not use welding method, because the welding method destroys the cable Coaxiality and continuity of characteristic impedance.
3. Measure the DC resistance data of the cable: If the SYV75-5 cable is 1000 meters, the DC resistance core wire is 35-40 ohms, and the outer shield layer resistance is 1000-36 ohms (the shielding layer is different, the resistance is very different) SYWV75-5 cable 1000 meters, DC resistance core wire is 18-22 ohms, outer shield layer resistance 1000 meters is 24-36 ohms; accumulation of this information is very useful, not only can judge the quality of cable materials, but also used for engineering Wiring and quality of the pipe are checked. If the person wears the pipe, the wire is broken, the resistance value becomes larger, the video signal becomes weaker, and the interference that should not occur also occurs. The probability of such an "accident" is very high, but Often overlooked;
4. Measure the high and low frequency attenuation characteristics of the cable: measure the head head and the color sync head amplitude at the end, calculate the attenuation amount with 0.3V as the 0db reference, the same head represents the low frequency attenuation, and the color sync head represents the 4.43M high frequency attenuation. , for example, the measured sync head of the 1000 meter line is 0.15V, and the db number of the attenuation multiple is calculated as â€œ-6db/1000mâ€ according to 20log, and the amplitude of the attenuation of the 4.43M color sync head after 1000 meters is 30mv, that is, 1 /10 times, the attenuation is -20db/1000m; with this method, the transmission quality of different cables can be accurately grasped, and the "frequency distortion (high and low frequency attenuation difference)" has an intuitive concept, you can accurately measure The difference between the SYV and SYWV cables of the same model and structure is good or bad. Comparing the differences and performances of different manufacturers, it is also possible to compare the changes of different batches of products from the same manufacturer;
5. The above method can also detect the performance of video transmission systems and equipment: such as the transmission characteristics of each coaxial video cable in the project, the transmission characteristics of the optical transceiver (can be measured as good or bad, do not think so ideal), RF transmission, microwave Transmission characteristics, transmission characteristics of the twisted pair, distribution characteristics of the video distributor, switching characteristics of the matrix host, special attention should be paid to when the multi-output simultaneously switches the same input signal, if the number of switching paths is found to be greater, the attenuation is greater. Itâ€™s not right, it should be the same. After testing, you will master a lot of unqualified products;
6. Observe the field signal and see that the field sync position distortion is not big (flat) - it should be very flat;
7. At the same time, you can also use the oscilloscope to check the low-frequency interference: if the field signal has slow fluctuations, it is 50/100 weeks of interference, there are many â€œthatchedâ€ beatings, most of which are frequency conversion harmonic interference, disconnecting the far-end camera, cable The inner and outer conductors of the distal end are short-circuited, and the interference waveform and intensity can be directly observed by the oscilloscope at the end; this method can also check and test the real performance of the anti-jamming equipment.
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