Measured surface transfer impedance of cable shields that use combinations of braid and foil and are used for 1 Gb/s data transfer

The transfer impedance of six cable samples that used combinations of braid and foil shields was measured using a 1.2 m long quadraxial test fixture and a computer controlled network analyzer. These measurements showed that the combination braid/foil cables do not have the classic R+jωM₁₂ frequency dependence. Above a few MHz, they exhibited a frequency dependence that was approximately proportional to the square root of frequency. This suggests that the coupling mechanism is due to contact resistance somewhere in the shield. The two cables with individually shielded pairs had lower transfer impedances than those that used an overall shield over a balanced quad core. The transfer resistance of the 6 cables ranged from 9 to 21 mΩ/m. This is in the range of single braid cable shields and is appropriate for 1 Gigabit/s interconnect cables, such as are used for the Fibre Channel protocol. At 500 MHz, the transfer impedance ranged from a little more than 10 mΩ/m to 135 mΩ/m. The cables with individually shielded pairs were best (12.3 and 21.S mΩ/m)