The main factors affecting the characteristic impedance are as follows:
1. The fluctuation of conductor diameter d0 will affect the size of inductance and capacitance, and then become a factor affecting the characteristic impedance ZC, which cannot be ignored. It is known from the electrical characteristics of the cable that the conductor diameter should not be less than 1.5 mm. In order to control the conductor diameter, the precision of the mold is crucial. The diameter deviation of the wire drawing outlet mold should generally be controlled below 0.002 mm. Under the condition of good equipment operation, the conductor diameter deviation should not exceed 0.003 mm. Only this precision can meet the requirements of the cable for the characteristic impedance. Therefore, in the process of wire drawing, attention should be paid to the operation of the wire drawing machine and the control of the precision of the wire drawing die. In addition, the annealing state of the conductor and its elongation at break shall be strictly controlled. If the conductor is over annealed, resulting in excessive elongation, it will be difficult to control and easy to deform in the subsequent processing process. Therefore, the fluctuation range of the elongation of the conductor shall be strictly controlled to be less than ± 1.5%.
2. The deviation and concentricity of the outer diameter of the insulated core are difficult to control in the production process of insulated single wire. The fluctuation and eccentricity of the outer diameter of the insulated core will lead to the change of the distance between two wires. The result of this change will change the characteristic impedance ZC value. The greater the change of the distance between two wires, the greater the fluctuation of the characteristic impedance ZC value. In serious cases, it will be far from the nominal value. The deviation range of ± 5 Ω of the characteristic impedance of the cable at 1.8 kHz shall be controlled within ± 0.03 mm and the concentricity shall not be less than 95% considering the influence of other factors. In order to obtain a relatively uniform characteristic impedance, it is necessary to ensure that the insulation thickness and concentricity of the produced insulated wire core are very good. The selection of extrusion die is also very important in production (because the extrusion die will have a direct impact on the outer diameter and eccentricity of insulated wire core)
3. There is also a key variable factor, that is, the radius of the shielding layer. The radius of the shielding layer also has a great impact on the inductance. Therefore, in the shielding production process, the quality of the shielding layer is very important. The closer the wire is to the shielding layer, the greater the circuit capacitance. On the contrary, the smaller the circuit capacitance is, which will directly affect the characteristic impedance value. Therefore, the longitudinal shielding or wrapping of the shielding tape should be properly tightened, and the roundness and consistency of the shielding layer should be the focus of control in the production process. In addition, in the process of pair stranding, the uniformity of the tension of take-up and pay off wires and the consistency of the pitch of wire pairs will also have a certain impact on the distance between the centers of loop conductors and the size of the diameter of wire groups.
