Emissions

Written on 6:46 AM by ooe

Most DSL line signals up to and including ADSL use frequencies and signal levels that are so low that emissions are very unlikely to cause problems. Access network cables transmit DSL signals in a balanced mode (equal and opposite voltages on each wire) that tends to cancel out potential emissions. Any signal that does find its way into the unbalanced or common mode is likely to be poorly radiated because the wavelength is so long that the antenna efficiency of the cables is very low. This has been confirmed by measurements both by BT [5] and independent authorities.

At VDSL frequencies the picture changes somewhat. Although the signal levels are still very low, the frequencies are much higher to the extent that the degrading balance of the cables allows more of it to enter the (radiative) common mode. Once there, the shorter wavelength raises the antenna efficiency so that emission becomes more likely. By making some careful design choices, the problems can be reduced to manageable proportions, as has been confirmed by extensive practical and theoretical measurements [9–11].