In the field of wireless communication, microwave RF test cable is a common high-precision system test consumables, which is connected with vector network analyzer, frequency sweep instrument and other test instruments.
Any DUT is located between the signal generator and the analyzer, and the bridge between the DUT and the instrument is the test accessory or test system. Do not ignore these test accessories. If possible, it is best to solidify these test accessories to make them a standardized measurement system. When providing the whole machine, the instrument supplier will provide the test cable that is consistent with the highest working frequency of the instrument at most. In the real test process, we will encounter different situations and need to use different accessories. All these accessories will affect the accuracy of the measurement results, which requires the tester to have an in-depth understanding of the relevant test accessories.
When selecting the specification of the cable in the test system, in addition to considering the insertion loss and VSWR, the stability of the cable must be better. In RF and microwave frequency bands, the commonly used cables are divided into semi-rigid cables, semi flexible cables and flexible braided cables.
Flexible cable is a kind of “test grade” cable. Compared with semi-rigid and semi flexible cables, the cost of flexible cables is relatively expensive, because more factors should be considered in the design of flexible cables. Flexible cables should be easy to bend many times and maintain performance, which is the most basic requirement for testing cables.
Soft and good electrical indicators are a pair of contradictions, which is also the main reason for the increase of cost. Flexible cables must be kept stable in amplitude and phase under bending conditions. Generally speaking, the cable with single inner conductor is conducive to the stability of amplitude; Multi strand inner conductor cable is conducive to phase stability. It can be seen that these two indicators alone are difficult to complete.
Pay attention to the process of the joint and cable connection, which will affect the service life of the cable. In this part, there is a hard contact point between the traditional cable and the joint, which is easy to cause the cable fracture, which is also the most troublesome problem for most test engineers in the process of using the traditional test cable, which can not be solved simply by using the heat shrinkable sleeve, because the fracture of this hard contact point is often that the tension of the test cable is transmitted to the hard contact point through the cable after frequent bending, Cause aging and fracture of hard contact points.
It goes without saying that the traditional flexible test cable without armor can not effectively prolong the service life of the test cable even if the reinforced heat shrinkable sleeve is used at the connection between the cable and the joint because of the lack of armor layer protection; However, for the traditional armored cable, due to the gap between the armored layers and between the armored layer and the signal transmission layer, the tension will still be transmitted to the hard contact point after the cable is bent, resulting in the jump of the index of the cable after being used for a period of time.
The material of the joint is also the main factor determining the service life of the test cable. Generally speaking, the service life of the joint with copper outer conductor is not as good as that of stainless steel. On the premise of meeting the torque, the service life of the former is 500 times and that of the latter is 5000 times. The definition of this index is that after reaching the service life, the factory index of the joint begins to decline, rather than that the joint is about to be scrapped.
Under normal circumstances, the service life of cable joints is much longer than the above indicators. For the production test environment that needs frequent plugging, the application of adapter is recommended. In short, for the relatively static interconnection scheme, it is recommended to choose the ordinary test cable without armor without frequent plugging and bending. For mass production test or heavy laboratory test, the armored cable is always the best choice for cost performance in the long term.
The design of flexible cable violates the design principle of low passive intermodulation to some extent, so there are few low passive intermodulation models of flexible cable.
Generally speaking, the index of flexible test cable must be good and stable. The selection of a flexible test cable should be comprehensively considered from the practical application, frequency, loss, VSWR, joint material, service life, RF leakage, passive intermodulation and cost, rather than purely from the price, because in the test process, the test efficiency and yield are often the most influential factors on the overall cost.
The following discusses various key indexes and performance of RF test cable. Understanding the performance of cable is very useful for selecting the best RF cable component:
characteristic impedance
“Characteristic impedance” is the most commonly mentioned index in RF cables, connectors and RF cable components. The maximum power transmission and minimum signal reflection depend on the matching of the characteristic impedance of the cable and other components in the system. If the impedance exactly matches, the loss of the cable is only the attenuation of the transmission line, but there is no reflection loss. The characteristic impedance (Z0) of the cable is related to the size ratio of its inner and outer conductors, as well as the dielectric constant of the filling medium. Due to the “skin effect” of RF energy transmission, the important dimensions related to impedance are the outer diameter (d) of the inner conductor and the inner diameter (d) of the outer conductor of the cable:
Z0(Ω) = (138 /√ ε) × (log D/d)
The characteristic impedance of most RF cables used in communication field is 50 Ω; 75 Ω cable will be used in radio and television.
Standing wave ratio (VSWR) / return loss
The impedance change in the cable assembly will cause the reflection of the signal, which will lead to the loss of incident wave energy. The connection between test cable components and the connection between cables / joints are the main causes of reflection loss. Due to manufacturing reasons, the cable will also produce some VSWR mutations at some specific frequency points The magnitude of reflection can be expressed by 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. The equivalent parameter of VSWR is reflection coefficient or return loss. The VSWR of a typical microwave cable assembly is between 1.1 and 1.5, which is converted into a return loss of 26.4 to 14dB, that is, the transmission efficiency of incident power is 99.8% to 96%. The meaning of matching efficiency is that if the input power is 100W and the VSWR is 1.33, the output power is 98W, that is, 2W is reflected back.
Attenuation (insertion loss)
The attenuation of cable is the ability of cable to effectively transmit RF signal. It is composed of dielectric loss, conductor (copper) loss and radiation loss. Most of the loss is converted into heat. The larger the size of 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 the increase of frequency, and the dielectric loss has a linear relationship with the increase of frequency, the proportion of dielectric loss in the total loss is greater. In addition, the increase of temperature will increase the conductor resistance and dielectric power factor, so it will also lead to the increase of loss. For the test cable assembly, the total insertion loss is the sum of connector loss, cable loss and mismatch loss. In the use of test cable components, incorrect operation will also produce additional losses. For example, for braided cables, bending also increases their loss. Each kind of cable has the requirement of minimum bending radius. When selecting the maximum loss of the cable according to the size of the system, then select the minimum acceptable loss of the cable.
propagation velocity
The propagation speed of the cable refers to the ratio of the speed of signal transmission in the cable to the speed of light, which is inversely proportional to the root sign of the dielectric constant of the medium:
Vp = (1/√ ε )× 100, the dielectric constant can be seen from the above formula（ ε） The smaller the, the closer the propagation speed is to the speed of light, so the insertion loss of cable with low-density medium is lower.
Phase stability in bending
Phase change is a measure of the stability of a cable when it is bent. Bending during use will affect the insertion phase. Reducing the bending radius or increasing the bending angle will increase the change of phase. Similarly, the increase of bending times will also lead to the increase of phase change. Increasing the ratio of cable diameter / bending diameter will reduce the change of phase. The phase change is basically linear with the frequency. The phase stability of low-density dielectric cable will be significantly better than that of solid dielectric cable.
