How to look at the quality of video cable

(1) Visual inspection of appearance:

1.PVC sheath: The cable is made of high-quality PVC with high temperature resistance of 70o and low temperature of minus 20o. The surface is smooth, smooth and fine, the color is black, and the tensile strength is very good. It should be able to see the pressure from the surface of the cable. Tightly inside the net has a regular "unevenness", which shows that the extrusion process of Zhenya cable is good, does not produce relative sliding, is a good cable. The ordinary cable, the surface is rough, the color is uneven, not only indicates that the plastic material, processing technology and plasticizing performance are poor, and at the same time, if the plastic surface has large pores, it is easy to get wet and affect the product performance and service life. If it is easily peeled off by hand, it may be a material of unqualified quality. 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: cable SYV75-5 outer shield is divided into 96 series, SYV75-5 encryption 128 series, Zhenya cable SYV75-3 outer shield is 96 series, the material is oxygen-free copper, shielding mesh weave evenly The welding performance is very good. The tinned copper wire is scratched to see if it is copper wire inside. For other manufacturers' cables, you must check the material first. Is it pure copper? Is it black copper? There are many manufacturers using copper-clad aluminum for the shielding net. The hardness of the aluminum-magnesium alloy wire is obviously larger than that of the copper wire. The mesh is sparse and the distribution is not Uniform, not tightly wrapped with insulation, etc. is a poor cable;

3. Check the core wire: SYV75-3 insulation outer diameter is 3.00+0.13mm, SYV75-5 insulation outer diameter is 4.80+0.20mm, recently appeared a SYV75-5 insulation outer diameter is 4.0mm cable The characteristic impedance of this cable is certainly not 75 ohms and is not applied to the 75 ohm transmission system;

4. Check the adhesion between the core wire and the insulation layer: obliquely cut the insulation layer of the video cable, pull the core wire in the peeling direction, and see if the core wire and the insulation layer are in contact with the process material; the cable has a large dip. Joint force, the difference cable is not glued;

5. Longitudinal tensile test: Take a one-meter screen cable, layer-peel the core wire, insulation layer, shielding layer, and outer casing, each leaving 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;

(2) 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 "standard video signal source": 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 “Peak-peak”; note that the line sync head amplitude is “-0.3V”, 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 the measurement error, such as 1000 meters, the cable intermediate joint must use "F-type joint" and coaxial double-pass (cable 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 0db reference, the same head represents low frequency attenuation, and the color sync head represents 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 according to 20log is “-6db/1000m”, and the amplitude of the attenuation of the 4.43M color sync head after 1000 meters is 30mv, that is, 1/10. Double, 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 same model. The difference between the SYV and SYWV cables and the performance of the structure, compare the difference between different manufacturers and the performance of the products, can also compare the changes of different batches of the same manufacturer;

5. The above method can also detect the performance of the video transmission system 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. It’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 "thatched" 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. At the end, the interference waveform and intensity can be directly observed with an oscilloscope; this method can also check and test the true performance of the anti-interference device.