The RF coaxial cable assembly connects the RF coaxial cable and the RF connector through crimping, welding, screwing and other methods. Since different cables correspond to different applications and environments, many factors should be considered when selecting cable components. Today, let's learn about the common indicators of RF cable components:
1) Characteristic impedance
Characteristic impedance refers to the ratio of incident wave voltage to incident wave current on the transmission line, or the negative value of the ratio of reflected wave voltage to reflected wave current. It is the most basic electrical characteristic of coaxial cable.
2) Voltage standing wave ratio (VSWR) and return loss (RL)
Voltage standing wave ratio VSWR and return loss RL are used to measure the size of the reflected signal. It is defined as the sum of reflected signals caused by impedance mismatch. The voltage standing wave ratio VSWR is obtained by the ratio of the peak to the trough of the reflected signal. The return loss RL is calculated by measuring the logarithm of the ratio of the power reflected back to the source to the input power.
The generation of standing wave or return loss is mainly due to the mismatch of impedance. The RF signal will be reflected when it encounters the change of impedance in the transmission process. The reflection coefficient can be calculated according to the change of impedance Γ， Thus, the values of standing wave ratio VSWR and return loss RL can be calculated.
3) Loss (also known as attenuation) and mechanical amplitude stabilization
Loss refers to the energy loss of signal in the transmission process of cable assembly. When the RF signal is transmitted in the cable assembly, part of the energy is converted into heat and consumed, and part of the energy leaks out through the outer conductor of the cable. The sum of these two parts of energy loss is called loss, or attenuation. It is usually expressed by the DB value of a fixed frequency point per unit length. The higher the frequency, the greater the loss.
Attenuation has a great loss of energy. 3dB attenuation is equivalent to 50% of energy loss. Therefore, for an RF system, there are strict requirements for loss. Reducing the loss of cables and cable components is very important for the RF system. The cost increased by selecting low loss cables is much less than the cost of power amplifier increased by selecting high loss cables.
Mechanical amplitude stabilization refers to the stability of the loss of cable components under bending and shaking conditions. The mechanical amplitude stabilization index is an important consideration index for dynamic application environmental conditions. The test of mechanical amplitude stabilization index is usually carried out by the method of 360 ° around a cylinder with a radius of 10 times the cable diameter.
4) Transmission rate and delay
Transmission rate refers to the ratio of the signal propagation speed in the cable to the speed of light C, and its dielectric constant with the insulating medium ε Is inversely proportional to the square root of. Dielectric constant ε The smaller, the faster the propagation speed.
The time delay refers to the time the signal passes through the cable, which also depends on the dielectric constant of the insulating medium ε， And the length of the cable L. Dielectric constant ε The lower the, the shorter the signal propagation time.
5) Mechanical phase stability
The phase change of cable assembly caused by bending and jitter is called mechanical phase stability, including bending phase stability (referred to as bending phase stability) and jitter phase stability (referred to as jitter phase stability). The bending phase stability index has a certain relationship with the bending method and bending radius. When paying attention to this index, we must understand the test method and bending radius. When testing the bending phase stability index of cable components, the industry usually adopts the method of 360 ° of a cylinder with a cable winding radius of 10 times the cable diameter.
6) Phase temperature stability
Temperature phase stability refers to the phase change of cable components caused by temperature change.
7) Phase tracking
Phase tracking refers to the ability of multiple cable components to approach each other in terms of temperature, bending or both. Generally speaking, it is the consistency of phase change. Generally, phase tracking refers to temperature phase tracking. Phase tracking is one of the most important indexes that phased array radar R & D engineers should consider when selecting RF cable components.
8) Shielding efficiency
Shielding efficiency refers to the ratio between the RF energy incident on one side of the shielding layer and the RF energy projected on the other side.