Test equipment cables are mainly used for signal transmission. We should pay attention to the following parameters in the transmission of signal cables
1. Length
The attenuation of the signal is proportional to the length of the cable. The longer the cable, the greater the attenuation. This is the physical law of the cable. The length of the cable is generally marked by feet or meters. Audio and video signals passing through long-distance cables will cause signal attenuation. The impact on the final effect is expressed in the reduction of signal-to-noise ratio, brightness reduction, image blur and poor synchronization. These obvious differences have also become the basis for comparing cable quality (different attenuation of cables per unit length for transmitting the same signal).
The capacitive reactance of the frequency cable and the wire material determine the range of the transmission signal frequency. Within the appropriate transmission distance, if the image is blurred, most of the cables do not reach the requirements of high-frequency transmission, resulting in high-frequency loss of the signal (details of the image).
The interference cable is also a huge antenna that absorbs electromagnetic waves in space. If the cable is not shielded or the shielding effect is poor, any type of electromagnetic interference will directly act on the useful signal, reducing the signal-to-noise ratio
Like all electronic circuits, the physical characteristics of the cable are also affected by the ambient temperature, and the physical parameters of the cable have different performances in different temperature ranges. In engineering applications, cables are typically overlaid on walls, ceilings, and instrument racks because the ventilation conditions in these places are not ideal and are prone to high temperatures. Therefore, the allowable temperature range of the selected cable should be suitable for these environments.
5. Slope slope is used to describe the time difference of signal transmission caused by different lengths of twisted pair, which depends on the twisting process and twisting type of twisted pair. When a large delay error occurs, slope compensation needs to be carried out on the cable. 6. Impedance Impedance is one of the important parameters to describe the technical specifications of the cable, which establishes a baseline for the correct flow of the signal. The flow of this signal maintains the power conversion of the entire system. Imagine water flowing through a large-diameter pipe. As long as the diameter of the pipe remains constant, the structure and flow of the water will not change. When this water flow was introduced into a small water pipe, the situation changed: due to the existence of the bottleneck, the structure of the water flow was disrupted, and all the water flow could not pass through the bottleneck at the same time, causing some water flow to flow in the opposite direction, and finally the flow of water was introduced into the mainstream again. The mismatch of cable transmission impedance will also cause a phenomenon similar to the above: the electronic signal is introduced again to the initial signal effect is called reflection, and the result of mutual interference between the transmitted wave and the reflected wave causes the voltage amplitude to form a standing wave, which is expressed by VSWR (voltage standing wave ratio). In video systems, impedance matching is a problem that needs to be taken seriously in system design.
In the early days, there were two specifications of 50_and 75_at the same time for BNC type connectors of coaxial cables. Now cables and connectors with uniform video impedance of 75_and 50_will only be applied to radio frequency signals at present. Short-distance impedance mismatch will affect the high-frequency details of the image, causing the phenomenon of "ghosting" in the picture;
7. Attenuation The attenuation of the signal by the cable is also called insertion loss, and the unit is decibels (dB). A regular cable will provide a loss table describing the attenuation value of the cable for different frequencies per unit length. For example, the attenuation value of a cable is expressed as -2.2dB/30m @ 100MHz, which means that when the cable is 30 meters long, it will generate-2.2dB insertion loss when transmitting 100MHz bandwidth signals. The insertion loss of the cable is cumulative, and the signal attenuation value is not the same for different frequencies.
For signals with the same bandwidth, the cable length is doubled, and the insertion loss is also doubled. For example, when the above-mentioned cable is 60 meters in length, the insertion loss of-4.4dB will be generated when transmitting 100MHz bandwidth signals.
