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How to determine the electrical length stability of a cable assembly?

Author: Date:12/12/2019 12:01:37 AM
   What is a salt spray test?

   Most components on military products require suppliers to perform salt spray tests before they leave the factory, including RF cable assemblies and electrical connectors. The purpose is to ensure product reliability and service life.


   Corrosion is the destruction or deterioration of a material or its properties caused by the environment. Most corrosion occurs in the atmospheric environment, which contains corrosive components such as oxygen, humidity, temperature changes, and pollutants. Salt spray corrosion is one of the most common and most destructive atmospheric corrosion. The salt fog mentioned here refers to the atmosphere containing chlorides. Its main component is the chloride salt in the ocean-sodium chloride. It mainly comes from the ocean and inland saline-alkali regions. The corrosion of the surface of metal materials by salt spray is caused by the contained chloride ions penetrating the oxide layer and protective layer on the metal surface and the electrochemical reaction between the internal metal. At the same time, chloride ions contain a certain amount of hydration energy, which is easily absorbed by the pores and cracks on the metal surface and displaces the oxygen in the chlorinated layer. Wave surface. Causes extremely adverse reactions to the product.


2. Which indicators need to be considered when selecting a radio frequency (test) cable assembly?


     In fact, many of the previous articles have mentioned the indicators of RF cable assemblies. Today, I will give a detailed summary, and also tell engineers to recommend several aspects to consider when selecting high-performance and reliable RF test cable assemblies.


     In addition to the frequency range, standing wave ratio, insertion loss and other factors, the correct selection of RF cable components should also consider the cable's mechanical characteristics, the use environment and application requirements. In addition, cost is also a constant factor.


    RF coaxial cable is used to transmit RF and microwave signal energy. It is a distributed parameter circuit whose electrical length is a function of physical length and transmission speed, which is fundamentally different from low-frequency circuits. RF coaxial cables are divided into four types: semi-rigid, corrugated tube and semi-flexible flexible cable. Different applications should choose different types of cables. Semi-rigid and semi-flexible cables are generally used for internal interconnection of equipment; most of the early corrugated pipes were used in base station construction (there will be fewer and fewer applications after 5G). In the field of test and measurement, flexible cables should be used.


Semi-rigid cable


     As the name implies, this cable is not easy to bend and shape. Its outer conductor is made of aluminum or copper tube (with plating, and the commonly used coating is ternary alloy plating). Its RF leakage is very small (<-120dB). Signal crosstalk caused in the system is negligible. The passive intermodulation characteristics of this cable are also ideal. If you want to bend to a certain shape, you need a special molding machine or a manual abrasive tool to complete it. Such troublesome processing technology in return has very stable performance. The semi-rigid cable uses solid polytetrafluoroethylene as the filling medium. This material has very stable temperature characteristics, especially under high temperature conditions, and has very good phase stability. Sex. The cost of semi-rigid cables is higher than semi-flexible cables, and they are widely used in various RF and microwave systems. However, with the acceleration of product iterative updates, the use of semi-steel wires has also decreased accordingly.


Semi-flexible cable


    Semi-flexible cable is a substitute for semi-rigid cable. The performance index of this kind of cable is close to semi-rigid cable, and it can be formed by hand. However, its stability is slightly worse than semi-rigid cables. Because it can be easily molded, it is also easy to deform, especially in the case of long-term use.


Flexible (braided) cables--for test-grade flexible cables


   Flexible cable is a "test grade" cable. Compared to semi-rigid and semi-flexible cables, the cost of flexible cables is very expensive, because flexible cables have more factors to consider in their design. Flexible cables must be easy to bend multiple times and still maintain performance. This is the most basic requirement for testing cables. Softness and good electrical indicators are a contradiction, which is also the main reason for the high cost. The selection of flexible RF cable assemblies must consider various factors at the same time, and there are some contradictions between these factors. For example, a coaxial cable with a single inner conductor has a lower insertion loss and a more stable amplitude when bent than a multi-stranded coaxial cable. Performance, but phase stability performance is not as good as the latter. Therefore, in addition to the frequency range, standing wave ratio, insertion loss and other factors, the selection of a cable assembly should also consider the cable's mechanical characteristics, the use environment and application requirements. In addition, cost is also a constant factor.


   Indicators associated with RF cable assemblies


(1) Characteristic impedance


     RF coaxial cable consists of conductor, dielectric, outer conductor and sheath.


    "Characteristic impedance" is the most commonly mentioned index in RF cables, connectors and RF cable assemblies. Maximum power transmission and minimum signal reflection depend on the characteristic impedance of the cable and the matching of other components in the system. If the impedance is perfectly matched, the loss of the cable is only the attenuation of the transmission line, and there is no reflection loss. The characteristic impedance (Zo) of a cable is related to the size ratio of its inner and outer conductors. Due to the "skin effect" of RF energy transmission, the important dimensions related to impedance are the outer diameter (d) of the inner conductor of the cable and the inner diameter (D) of the outer conductor: Zo (Ω) = (138 / √ε) x (log D / d)


     The characteristic impedance of most RF cables used in the communication field is 50Ω; 75Ω cables are used in broadcast television.


(2) Standing wave ratio (VSWR) / Return loss


      In RF and microwave systems, maximum power transmission and minimum signal reflection depend on the characteristic impedance of the RF cable and the matching of other components in the system. The impedance change of the RF cable will cause reflection of the signal, and this reflection will cause the loss of incident wave energy. The magnitude of the reflection can be expressed by the voltage standing wave ratio (VSWR), which is defined as the ratio of the incident and reflected voltages. The calculation formula of VSWR is as follows: VSWR = (1 + √Pr / Pi) / (1-√Pr / Pi)


Where Pr is the reflected power and Pi is the incident power.


     The smaller the VSWR, the better the consistency of the cable production. The equivalent parameter of VSWR is the reflection coefficient or return loss. The VSWR of a typical microwave cable assembly is between 1.1 ~ 1.5, which translates into a return loss of 26.4 ~ 14dB, that is, the transmission efficiency of incident power is 99.8% ~ 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.


(3) Attenuation (insertion loss)


      The attenuation of a cable is its ability to effectively transmit radio frequency signals. It consists of three parts: dielectric loss, conductor (copper) loss, and radiation loss. Most of the losses are converted into thermal energy. The larger the conductor, the smaller the loss; the higher the frequency, the greater the dielectric loss. Because the conductor loss has a square root relationship with increasing frequency, and the dielectric loss has a linear relationship with increasing frequency, the proportion of dielectric loss in the total loss is larger.


      In addition, the increase in temperature will increase the conductor resistance and the power factor of the dielectric, so it will also increase the loss. For test cable assemblies, the total insertion loss is the sum of the connector loss, cable loss, and mismatch loss. In the use of test cable assemblies, improper operation can also cause additional losses. For braided cables, for example, bending also increases their losses. Each cable has a minimum bend radius requirement. When selecting a cable assembly, determine the acceptable loss value at the highest frequency of the system, and then select the smallest cable size based on this loss value.


(4) Average power capacity


      The average power capacity refers to the ability of a cable to dissipate thermal energy generated by resistance and dielectric loss. In actual use, the effective power of the cable is related to VSWR, temperature and height:

Effective power = average power x standing wave coefficient x temperature coefficient x height coefficient

      The above factors should be taken into consideration when selecting a cable.


(5) Transmission speed


      The propagation speed of a cable refers to the ratio of the speed of the signal transmitted in the cable to the speed of light. It is inversely proportional to the root of the dielectric constant of the medium: Vp = (1 / √ε) x 100. The dielectric constant (ε The smaller), the closer the propagation speed is to the speed of light, so the cable with a lower density medium has a lower insertion loss.


(6) Phase stability during bending (explained in detail in the previous article)


      Bend-phase stability is a measure of the phase change of a cable as it bends. Bending during use will affect the insertion phase. Decreasing the bending radius or increasing the bending angle will increase the phase change. Similarly, an increase in the number of bends also results in an increase in phase change. Phase change and frequency are basically linear. The phase stability of low-density dielectric cables is significantly better than that of solid dielectric cables.


(7) Passive intermodulation distortion of power supply


      The passive intermodulation distortion of a cable is caused by non-linear factors within it. In an ideal linear system, the characteristics of the output signal are exactly the same as the input signal; while in a non-linear system, the output signal will produce amplitude distortion compared to the input signal. If two or more signals are input to a non-linear system at the same time, a new frequency component will be generated at its output due to the existence of intermodulation distortion. In modern communication systems, engineers are most concerned about third-order intermodulation products (2f1-f2 or 2f2-f1), because these unwanted frequency components often fall into the receiving frequency band and cause interference to the receiver.