By Ian Darney

A method of characterising the electromagnetic coupling between a co-axial cable and the conducting structure of an electronic system is described. Frequency response tests are carried out on a rig and the data so collected is used to develop a circuit model which replicates that response. Assessment of the model provides a clear insight into the mechanisms involved in cross-coupling. This form of analysis can be integrated into the system design process.

Setup

Figure 1 shows the setup. The rig itself is a 15 mm copper pipe routed round three walls of a room. A wooden batten fixed along the top of the pipe acts as a spacer to separate the cable under test from the pipe. This setup allows different cable assemblies to be characterised. In this case it is a 50 ohm co-axial cable. One end is terminated at an interface module which allows a signal to be applied via a low impedance source and for the amplitude of that signal to be monitored by channel 1 of an oscilloscope. Both terminals at the far end are short-circuited to the copper pipe. A current transformer is used to measure the current delivered to the cable.