yorkemc.co.uk » Consultancy & Research » EMC Research »

Investigation of impedance and mode conversion of telecommunications cables for xDSL systems Final Report (AY 3944)

A report for the Radiocommunications Agency by York EMC Services Ltd, University of York

Author: Dr. D W Welsh


At the time of writing, large-scale deployment of ADSL technology is occurring in the UK. The technology provides a broadband data link over existing telephone lines suitable for ‘video on demand’ or high speed internet applications. The existing telephone distribution cabling was not designed for the higher frequencies used by the ADSL modems; consequently, unintentional RF emissions will occur. A significant EMC threat from ADSL could therefore exist to radio communications in the medium and long wave bands. In the case of a complaint, the UK Radiocommunications Agency could test a suspect installation against the MPT 1570 1 standard. If the suspect system fails to meet MPT 1570, it may be turned off. However, the ADSL modems driving the cabling system may well have passed the existing EN 55022:1998 2 EMC standard at the compliance stage. Therefore, both the modem manufacturers/installers and the UK Radiocommunications Agency could be presented with a practical issue resulting from differences in the applicable standards. A solution would be to devise a better test method for compliance testing. This report has used state of the art transmission line modelling techniques, to understand the mechanisms that lead to the unintentional RF emissions, and hence suggest a possible test method that can give manufacturers a realistic appraisal of their products at the compliance stage. Its findings relevant to this problem are summarised below.

Unintentional radiated emissions are normally associated with common-mode currents on cables. The current EN 55022 standard limits the level of the common-mode voltage permitted on the output cables of the modem. However, the test ignores the fact that the cabling system itself provides a second source of common-mode voltage as well as an effective radiator. Imbalance in the cabling system causes desired differential voltages applied by the modems to be converted into undesirable common-mode voltages. This conversion is dependent on the cable balance, the length of the cable and the frequency of the applied differential voltage. For the cable type studied, it has been found that the worst-case common-mode voltage was consistently 44dB less than the applied differential voltage, independent of frequency. In a practical installation this amount of mode conversion would not always occur, as the cable length needs to be correct at a given frequency for maximum mode conversion.

The findings suggest that both common-mode and differential voltage being produced by the modem should be tested. The common-mode voltage due to mode conversion can then be calculated by subtracting a factor from the differential voltage. This factor would be dependent on the cabling system on which the modem was to be deployed.


You can download this report as a PDF file (286 kB).

Last Updated: 2006-Feb-01