RF Component & Device Test Series: Coaxial Cable Assembly Performance Testing Part 1
By: Peter McNeil
In the previous post we explored quality testing of coaxial cable assemblies. In this post, we are exploring the electrical performance testing of coaxial cable assemblies. The following is a list of the most common electrical performance parameters given for coaxial cable assemblies.
Key Coaxial Assembly Performance Tests
- Frequency Range (Cutoff Frequency)
- Insertion Loss (S21/S12) & Attenuation
- Group Delay
- Velocity of Propagation
- RF Shielding
- Maximum Input Power (Power Handling)
- Dielectric Withstand Voltage (maximum voltage or voltage breakdown)
- Phase Stability *during flexure
Testing coaxial cable assemblies typically requires only a precision 2-port Vector Network Analyzer (VNA). A VNA is able to send and receive precision signals across a wide frequency range from it’s ports and measure the time it takes for the signals to travel from one port to another and back to the originating port. This capability provides highly accurate measurements of the Scattering Parameters, or other network parameters, which can be used to calculate the electrical performance characteristics of a coaxial cable assembly.
A VNA with a frequency range beyond that of the cutoff frequency of a coaxial cable assembly is able to perform a frequency sweep that shows the degradation of the transmission line properties of a coaxial cable. In this way the maximum frequency, or cutoff frequency, of a coaxial cable assembly can be determined. The cutoff frequency of a coaxial cable is a direct result of the geometries of the cable and connectors used, so could also be relatively accurately determined from physical measurements.
Insertion Loss & Attenuation
The insertion loss is a measure of the amount of signal energy lost from a signal sent from the first port of the VNA and received by the second port of the VNA. As coaxial cable assemblies should be symmetrical passive networks, the S12 and S21 responses should be identical. Given the frequency dependent losses associated with conductors and dielectrics, the insertion loss of a coaxial cable assembly typically degrades toward higher frequencies somewhat consistently. It is generally desirable to have the lowest insertion loss possible for a coaxial cable assembly, and is often specified for a given application.
Attenuation is merely the loss per unit length of a coaxial cable, as is largely dependent on the dielectric losses of the coaxial cable. The resistive losses of the inner and outer conductor also contribute. As these losses are also a function of frequency, attenuation is often provided at a given frequency, worst case, or as a range over frequency.
VSWR is a measure of the maximum to minimum voltage of the standing wave developed by the impedance imperfections within a transmission path. VSWR can be derived by the reflection S-parameters, which are a measure of the power sent via port one and what is reflected back to port one after the calibration plane. VSWR should be somewhat symmetrical on each side of the coaxial cable assembly, though physical variations may cause somewhat different port parameters.
The characteristic impedance and capacitance can both be determined from the reflection coefficient derived from the S11/S22 parameter. It is generally desirable for the specified characteristic impedance of the coaxial cable assembly to be the same as the port impedance of the coaxial connectors and the characteristic impedance of the coaxial cable assembly. This requires a precise match and precision construction to achieve a consistent impedance throughout the assembly structure. Higher precision cables will likely exhibit much lower impedance variations.
Stay tuned for Part 2 of the 2-part section of, “RF Components & Device Test Series: Coaxial Cable Assembly Performance Testing.”
Learn more about Pasternack’s extensive line of Coaxial Cable Assemblies here https://www.pasternack.com/nsearch.aspx?Category=Cable+Assemblies&sort=y&view_type=grid
Courtesy of Pasternack