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New technical paper: Exploiting Gating principle to circumvent the challenges of properly characterizing cables with improper terminations

After production, LAN cables have to be tested electrically.  However, on drums, the inner end of the cable is not always accessible.  Improper termination of the cable leads to reflections that strongly affect the measurement results and may cause the rejection of the cable length. The use of Fourier transforms allows mitigating this effect by gating in the time domain and can help in simplifying the testing procedure for parameters such as impedance and return loss.

This study describes the gating principle and demonstrates different application procedures.  It shows the impact of diverse parameters and the interpretation of their effects on the measured LAN cables curves.

Specifically, gating allows suppressing the effect of “Open end” reflections that can be seen at low frequency in the impedance/return loss spectrum when measuring a non-terminated cable, as illustrated in figure 1.

Along with the effect of gate shape and width, a procedure is presented that

• allows the determination of the cable electrical length for gate positioning
• uses a dedicated extrapolation to DC of the measured spectrum to overcome issues related to the use of Discrete Fourier Transform (DFT)
• defines the use of a “cutoff” frequency to improve the measurement and data processing speed

Gating is a powerful mathematical tool that allows mitigating the effect of spatially located defects (defaults, connectors …).  Nevertheless, given the discrete nature of the measured spectrum, the properties of DFT have to be taken into consideration not to misinterpret the results.