In the process of selling weak current cables, whether online or offline, many customers will ask how to identify the quality problems of weak current cables:

1. Jacket: The regular "unevenness" of the pressed inner mesh can be seen on the surface, which indicates that the processing technology is good and will not cause relative sliding, so it is a good cable. The appearance is smooth, and the "unevenness" of the pressed mesh cannot be seen. The sheath is loose when you pinch it, which is a poor cable;

2. Check the screen of shielding layer: whether the number of screens is enough for copper screen, check the solderability, scrape the tinned copper wire to see if it is copper wire, and the hardness of aluminum magnesium alloy wire is obviously greater than that of copper wire; Cables with sparse netting, uneven distribution and poor insulation

3. Check the core wire: diameter - 0.78-0.8mm for SYV cable and 1.0mm for SYWV cable; Recently, a cable with SYV75-5 core wire diameter of 1.0mm has appeared. The characteristic impedance of this cable is definitely not 75 ohms, and it is not used in 75 ohm transmission systems;

4. Check the bonding force between the core wire and the insulating layer: cut the insulating layer diagonally, pull the core wire in the stripping direction, and check whether the core wire and the insulating layer are bonded with process materials; Good cables have a large adhesion force, while poor cables have no adhesion;

5. Longitudinal tensile test: take one meter of cable, peel off the core wire, insulating layer, shielding layer and outer sheath layer by layer, and leave 10 cm long each. The method is: two hands hold two adjacent layers of the cable respectively, and pull in the opposite direction; The good cable can't be pulled with general force, while the poor cable can be pulled out easily without great effort - the elevator cable is very important, and many so-called "elevator special cables" have this problem;

Fire test:

Nowadays, there are too many kinds of fake copper wires and their simulation is also very high. It is difficult for ordinary people to distinguish them. The worst thing is that they cheat merchants everywhere and say that they are all copper. Those ordinary merchants will only be deceived. I will now share my only experience with you, hoping it will be useful to all friends; There are several types of fake copper wires on the market:

1) General false copper wire: copper clad aluminum, copper clad aluminum magnesium alloy, copper clad steel, copper clad iron

2) High imitation copper wire: copper clad aluminum magnesium alloy is added to the multi strand copper wire in a certain proportion, and other materials are processed by electroplating copper. Generally, the price is much higher than that of the previous one. After all, there is real copper in it.

3) Unqualified copper wire: the material is not drawn copper wire made of oxygen free copper, the transmission performance is poor, or the number of cores is insufficient;

As the saying goes, true gold is not afraid of fire. True and false SYV, RVV and other copper wires can also be identified by fire. First, open a section of wire to expose a section of copper wire and divide them into thin parts (that is better). Generally, it will take shape when it is burned with a wind proof lighter. The real copper wire is very resistant to high temperature. It will definitely change color at most if it is copper clad aluminum Copper clad aluminum magnesium alloy will bend quickly. If it is copper clad steel or copper clad steel, it will be ready after burning with a knife;

Transmission performance test:

Video cable, as its name implies, is a transmission cable used to transmit video signals. Since the video signal is transmitted, at least we should understand the transmission characteristics of the transmission line in the 0-6M band, or the transmission performance. Here we mainly talk about "oscilloscope measurement method" for common reference, because oscilloscope is a necessary "weapon" for engineers and one of the necessary inspection equipment for qualification review; The following description is based on the ability to skillfully use the oscilloscope.

1. The video signal of the color camera can be used as a "standard video signal source": the amplitude of the video output of the camera used in the test project on a 75 ohm load should be 1Vp-p, that is, from the bottom of the line synchronization head to the highest white level "peak peak" of the video signal; Note that the amplitude of the line synchronization head is "-0.3V", and the amplitude of the color synchronization head (4.43M sine wave pulse) is 0.3Vp-p; Select the sensitivity of the oscilloscope and set it to the amplitude calibration state. Select a camera with good indicators as the "video source";

2. The test cable shall be as long as possible to reduce the measurement error. For example, if the cable is 1000m long, the middle joint of the cable must be "F type joint" and coaxial double pass (cable television equipment), and welding method shall not be used, because the welding method destroys the coaxiality of the cable and the continuity of the characteristic impedance.

3. Measure the DC resistance data of the cable: for example, SYV75-5 cable is 1000m long, the DC resistance core wire is 35-40 ohms, and the resistance of the outer shielding layer is 24-36 ohms (the resistance varies greatly with the number of shielding layers); SYWV75-5 cable is 1000m long, DC resistance core wire is 18-22 ohm, and external shield resistance is 24-36 ohm when it is 1000m long; It is very useful to accumulate this information, which can not only judge the quality of cable materials, but also be used to check the quality of engineering wiring and pipe threading. For example, when someone is threading, the line is pulled off, the resistance value becomes larger, the video signal becomes weaker, and unwanted interference also occurs. This kind of "accident" has a very high probability of occurrence, but it is often ignored;

4. Measuring the high and low frequency attenuation characteristics of the cable: measure the amplitude of the line head and color synchronization head at the end, and calculate the attenuation amount with 0.3V as the 0db benchmark. The line head represents the low frequency attenuation, and the color synchronization head represents the 4.43M high frequency attenuation. For example, the measured 1000m line synchronization head is 0.15V, and the db number of the attenuation multiple calculated according to 20log is "- 6db/1000m". The measured 4.43M color synchronization head attenuation amplitude after 1000m is 30mv, that is, 1/10 times, and the attenuation is - 20db/1000m; With this method, you can accurately master the transmission quality of different cables, and have an intuitive concept of "frequency distortion (high-low frequency attenuation difference)". You can accurately measure the difference and performance of SYV and SYWV cables of the same model and structure, compare the difference and performance of products from different manufacturers, and also compare the changes of products of different batches from the same manufacturer;

5. The above methods 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 (good or bad can be measured, don't think it is so ideal), the radio frequency transmission and microwave transmission characteristics, the transmission characteristics of twisted pairs, the distribution characteristics of the video distributor, and the switching characteristics of the matrix host. Special attention should be paid to when multiple outputs switch the same input signal at the same time, If you find that the more switching paths are, the greater the attenuation is. It should not change. After testing, you will master many unqualified products;

6. Observe the field signal to see if the field synchronous position distortion is large (flat) - it should be very flat;

7. At the same time, you can also use an oscilloscope to observe low-frequency interference: if the field signal has slow fluctuation, it is 50/100 cycles of interference, and there are many "thatch" jumps, most of which are frequency conversion harmonic interference, disconnect the remote camera, short-circuit the inner and outer conductors at the far end of the cable, and directly observe the interference waveform and intensity at the end with the oscilloscope; This method can also check and test the resistance to dryness.