The RF coaxial cable assembly is to connect the RF coaxial cable and the RF connector together by crimping, welding, screwing, etc. Because different cables correspond to different applications and environments, many factors must be considered when selecting cable assemblies. Today, let's take a look at the common indicators of RF cable assemblies:
1) Characteristic impedance
Characteristic impedance refers to the ratio of the incident wave voltage to the incident wave current on the transmission line, or the negative value of the ratio of the reflected wave voltage to the reflected wave current. It is the most basic electrical characteristic of the 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 magnitude of the reflected signal. It is defined as the sum of the reflected signals due to impedance mismatches. 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 waves or return loss is mainly due to impedance mismatch. When RF signals encounter changes in impedance during transmission, they will cause reflections. According to the change of impedance, the reflection coefficient Γ can be calculated, so that the values of the standing wave ratio VSWR and the return loss RL can be calculated.
3) Loss (also known as attenuation) and mechanical amplitude stabilization
Loss refers to the energy loss of a signal during transmission of a 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 the losses of these two parts of energy is called loss, or attenuation. Usually expressed by the dB value per unit length at a fixed frequency point, the higher the frequency, the greater the loss.
The loss of attenuation is very large, and 3dB attenuation is equivalent to 50% of the energy loss. Therefore, for a radio frequency system, there are strict requirements on the loss. It is very important for the radio frequency system to reduce the loss of cables and cable assemblies. The added cost of choosing a low-loss cable is far less than the cost of increasing the amplifier by choosing a high-loss cable.
The mechanical amplitude stabilization refers to the stability of the loss of the cable assembly under bending and jittering conditions. The mechanical amplitude stabilization index is an important consideration for the dynamic application environmental conditions. The testing industry of the mechanical amplitude stabilization index usually adopts the cable assembly winding radius of 10 times the cable diameter. The cylindrical 360° method is tested.
4) Transmission rate and delay
The transmission rate refers to the ratio of the propagation speed of the signal in the cable to the speed of light C, which is inversely proportional to the square root of the dielectric constant ε of the insulating medium. The smaller the dielectric constant ε, the faster the propagation speed.
The delay refers to the time for the signal to pass through the cable, which also depends on the dielectric constant ε of the insulating medium and the length L of the cable. The lower the dielectric constant ε, the shorter the signal propagation time.
5) Mechanical phase stability
The phase change of the 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). Among them, the bending phase stability index has a certain relationship with the bending method and bending radius. When paying attention to this index, be sure to understand the test method and the size of the bending radius. When testing the bending phase stability index of the cable assembly, the industry usually adopts the method of winding the cable 360° around a cylinder with a radius of 10 times the cable diameter.
6) Temperature phase stability
Temperature phase stability is the phase change of the cable assembly due to temperature changes.
7) Phase tracking
Phase tracking refers to the ability of multiple cable assemblies to approach each other in phase under the conditions of temperature, bending, or both. In layman's terms, the phase change consistency. Generally speaking, phase tracking refers to temperature phase tracking. Phase tracking is one of the most important indicators that phased array radar R&D engineers should consider when selecting RF cable assemblies.
8) Shielding efficiency
Shielding efficiency is the ratio between the RF energy incident on one side of the shield and the RF energy projected on the other side.