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How to determine the performance of a cable assembly

Date:2021-11-20 16:18

Cable assemblies have two characteristic properties that are of interest: voltage standing wave ratio VSWR (or return loss) and insertion loss. Except for the shortest cable assembly (less than 6 inches) that uses extremely low loss cables, all insertion loss is mainly due to the attenuation of the cable itself, which can generally be determined from the manufacturer's information. How to confirm that the RF connector meets the standing wave requirements. On the other hand, VSWR is mainly due to the connector. Remember that VSWR is a vector quantity. When the frequency is scanned, the VSWR of each connector will jump up and down with the fluctuation of the phase shift. Where these maximum and minimum values occur will depend on the length of the cable and its dielectric constant. Generally speaking, the calculated maximum standing wave is determined by the reflection coefficient of each end connector. The worst case is the addition of two reflection coefficients. Although small, the return loss of the cable is also part of the VSWR. If the loss of the cable is ignored, VSWR will be reduced. For this example, we will ignore the attenuation of the cable and not as an influencing factor. For example, we say that the VSWR of one connector is 1.2 at a certain frequency, and the other connector is 1.25, the cable VSWR is 1.05, and the VSWR is converted into reflection coefficients of 0.091, 0.111 and 0.024, and the maximum reflection coefficient is 0.226. Converting back to VSWR is 1.584. One way to get the result quickly is to multiply the three VSWR values, in this case, it will be 1.2*1.25*1.05=1.575. This is very close to the previous calculation results. For return loss, VSWR can be converted to dB. If the return loss of each connector is different or if the return loss of the cable is not negligible, then each return loss will have to be converted into a reflection coefficient to be increased and then converted back to return loss. It is very important to realize that the VSWR of the connector and the cable are superimposed in a vector, and the VSWR of the cable assembly is higher than the VSWR of each individual component.

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