I don't have access to the documentation I did where I used to work (darn it) but luckily, Jim Quarfoot at TI wrote a great article about ADSL problems in the 25 July 2002 _Comms Design_. I recommend it.
( http://www.commsdesign.com/main/9812/9812feat1.htm ) Per the article, maximum ADSL power at 1.1 MHz bandwidth runs 20 dBm. 100 milliwatts, if equally allotted, gives about 0.4 milliwatts to each of the 256 tones. At 100 ohms, this is almost 0.2 volts at the sending end. Adding in 115 dB attenuation at maximum reach, this gives .35 microvolts or so at the receiver for the 1.1 MHz tone. This is receivable. If that tone represents 16 bits, though, there is about .02 microvolts difference between bits. It takes little noise to confuse the detector in such a case! Even at 4 bits, we have less than -20 dBuV to work with. Increasing power a hundredfold would still give us less than a microvolt and a large RFI problem, too. Realistically, this tone would not be used at maximum reach, and the rate is constrained by attenuation in practice. But while maximum rate at maximum reach IS a straw man (I've seen it DONE, though, in a lab), it is indicative of the problems receiving broadband signals over VF copper. Some folks WANT it. (I got into this where I used to work due to my signal integrity and noise suppression hats. Bearer of bad news, etc.) Back to the conducted standard, the attenuation of telco copper at the low end of the ADSL spectrum is much lower. Receivers have a much easier time with 30 dB attenuation than over 100. So if a conducted limit must protect these users, it can allow more emissions at the low end, and we have a rising limit below 500 KHz. But even a millivolt of switching noise on the power an ADSL modem is plugged into can be a tough problem. Cortland ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
