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Various key indicators and performance of RF connectors
“Characteristic Impedance” is an indicator value that is often mentioned in RF connectors, cables, and RF cable assemblies. Maximum power transfer and minimum data signal reflection depend on the characteristic impedance of the cable and the pairing of the remaining components in the system. If the characteristic impedances are perfectly matched, the cable wear is only the attenuation of the transmission line, and there is no reflection loss. The impedance of the cable (Z0) is related to the ratio of the specifications of the inner and outer conductors, and also to the dielectric constant of the filled medium. Because of the "skin effect" of RF energy transfer, the important dimensions associated with the characteristic impedance are the outer diameter (d) of the inner conductor of the cable and the inner diameter (D) of the outer conductor:
Z0(Ω) = (138 /√ε) × (log D/d)
The vast majority of RF cables used in the communications industry have an impedance of 50 Ω; in broadcast television, 75 Ω cables are used.
Standing wave ratio (VSWR) / return loss, characteristic impedance changes in the cable assembly will cause reflection of the data signal, which will cause loss of incident wave energy. Detecting the connection between the cable assemblies and the connection of the cables/joints to each other is the primary cause of reflection losses. Due to the production, the cable will also form some VSWR mutations at a certain frequency. The level of reflection can be expressed by the voltage standing wave ratio (VSWR), which is defined as the ratio of incident and reflected voltage.
The smaller the VSWR, the better the consistency of cable production and processing. The equivalent parameter of VSWR is the reflection coefficient or return loss. A typical microwave cable assembly has a VSWR between 1.1 and 1.5, which translates into a return loss of 26.4 to 14 dB, ie, the transmission efficiency of the incident power is 99.8% to 96%. The meaning of matching efficiency is that if the input power is 100W, when the VSWR is 1.33, the output power is 98W, that is, 2W is reflected back.
Attenuation (insertion loss)
The attenuation of the cable is the ability to transmit RF data signals indicating the effectiveness of the cable. It consists of dielectric loss, conductor (copper) loss and radiation loss. Most of the losses are converted into heat. The larger the specification of the conductor, the smaller the loss; and the higher the frequency, the greater the dielectric loss. Since the conductor loss has a square root relationship with the increase of the frequency, and the dielectric loss has a linear relationship with the increase of the frequency, the ratio of the dielectric loss is larger in the total loss.
In addition, the increase in temperature will increase the conductor resistance and dielectric power factor, and thus also increase the loss. For test cable assemblies, the total insertion loss is the total number of joint losses, cable losses, and mismatch losses. In the use of test cable assemblies, the wrong operation also creates additional losses. For example, for braided cables, bending also increases its loss. Each cable has a minimum bend radius. When selecting a cable assembly, first determine the acceptable loss value at the highest frequency of the system, and then select the smallest cable based on this loss value.
The propagation speed of a cable is the ratio of the rate at which a signal transmits data in a cable to the speed of light, and is inversely related to the root of the dielectric constant of the medium:
Vp = (1/√ε) × 100. The smaller the dielectric constant (ε) is, the closer the propagation speed is to the speed of light, so the cable with low-density medium has lower insertion loss.
Shenzhen Twinlink Communication Technology Co., Ltd. is a professional manufacturer of RF connectors for 12 years. It has three professional laboratories of machinery, electrical and environment, and an integrated industrial chain of machining, assembly testing and electroplating. . More than 20 professional customer service teams, 24 hours online, quick and timely response, all products use ROHS-compliant raw materials, 90% of suppliers have cooperated for more than 10 years. The pursuit of high reliability of products is the guarantee of Twinlink people to customers.
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