Specific electrical performance characteristics of coaxial cable assemblies

Previously, we studied some common specific electrical performance characteristics of coaxial cable assemblies. This article summarizes the most common parameters:Capacitance, group delay, propagation speed, RF shielding/leakage, maximum input power (power handling), maximum voltage/breakdown voltage, phase stability (when bending)

①　Capacitance

The capacitance per unit length of the coaxial cable (pF/L) is usually expressed in picofarad pF, which is a function of the distance between the inner and outer conductors and the relative permittivity of the dielectric. The capacitance of the coaxial cable is an important parameter that directly affects the cut-off frequency. Therefore, the cut-off frequency is lower when using a dielectric with a higher capacitance than a dielectric with a lower capacitance.

②　Group delay

The group delay of a coaxial cable assembly is obtained by calculating the phase change of the entire frequency, which refers to the estimated time it takes for the pulse to pass through the length of the coaxial cable as the frequency changes. In many applications, group delay flatness is an important factor, and inconsistent group delays may cause performance degradation, such as phase-sensitive radars. The group delay is measured by comparing the phase change of the VNA signal sent through the coaxial component and plotted against frequency.

③　Propagation speed (speed factor)

On the other hand, the propagation speed is the time it takes for the signal to travel through the transmission line (close to the speed of light). The propagation speed is easily determined based on the effective relative permittivity of the dielectric in the coaxial assembly. Therefore, coaxial cable assemblies of a dielectric with a higher relative permittivity will have a lower speed factor than a dielectric with a lower relative permittivity.

④　RF shielding or leakage

The shielding ability of a coaxial cable assembly refers to the ability of the assembly to prevent signal energy from escaping from it or to protect the assembly from external interference. Because coaxial cable assemblies are commonly used in precision applications, many critical applications usually have RF shielding efficiency as the minimum requirement. In many applications, a perfect radio frequency shielding layer cannot be used as the outer conductor of a coaxial cable assembly, and will affect flexibility, weight, size, and cost. Therefore, corrugated, braided wire, adhesive layer, foil, and other conductive materials are generally used. Material handling methods, etc. Radio frequency shielding or leakage can be measured by comparing the signal energy escaping from the coaxial cable with the signal energy contained in the coaxial component.

⑤　Maximum input power (maximum power capacity) and maximum voltage/breakdown voltage

The maximum input power that a coaxial cable assembly can handle before performance degradation depends on the geometry of the coaxial cable, the manufacturing method, and the material properties of the dielectric and conductors used in the cable. In essence, the maximum power capacity is the lowest power at which the coaxial cable assembly begins to deteriorate or fail due to thermal breakdown. The use of high-conductivity conductors and low-loss dielectrics can increase the power capacity of a given size coaxial cable assembly, but the power capacity will ultimately be limited by the size and heat dissipation capacity of the coaxial cable assembly and the temperature when the dielectric deteriorates.

Voltage breakdown or maximum voltage is usually determined based on the lowest voltage at which dielectric breakdown occurs and conduction through the dielectric. Both the maximum power capacity and voltage breakdown can be determined by destructive testing of coaxial component samples.

⑥　Phase stability (when bending)

The phase stability of the coaxial cable assembly refers to the degree to which the phase of the signal passing through the coaxial cable remains constant when the coaxial cable assembly is mechanically bent. Phase stability is very important for test and measurement applications, because the reproducibility of calibration and measurement depends on whether the test coaxial cable performs consistently over multiple life cycles. This can be measured with the VNA and the coaxial cable assembly connected to the fixture, which includes a mechanical structure that continuously bends the cable over time.