How real life ADSL filters at home work
The signal to telephone output is generally just low-pass filtered so that voice frequencies (frequencies up to 3.4 kHz) get nicely though, but higher frequencies gets filterted. This filtering generally consista of LC low-pass filter designed to some suitable operating frequency between 4 and 20 kHz (between voice and ADSL bands). This kind of filter causes that the high frequencies of the ADSL signal will be severely attenuated (usually by at least 30dB with a good filter) so the signal reaching your telephone equipment does not contain such amount of high frequency signals that could cause noise. The telephone LC filter is also designed in such way that the filter impedance towards the line that carries ADSL signals is high at the high frequencies, meaning that those telephone equipment and cables related to them look like they are look to high freuquency signals that they would ne "disconnected from the main line". line.
The ADSL POTS splitter is simply a series of coupled inductors and parallel capacitors forming a low pass filter that attenuates the higher frequency ADSL data and permits only the voice frequencies to reach the telephone. The series inductor shows high impedance to high freuqencies, so the ADSL signals on the line are not attenuated.
General design specifications for an ADLS filter should be somethign like this:
Return loss at voice frequencies (against 600 ohms) would be should be good enough.
Should not alter voice band freuqncy response too much
Should not have too high series resistance (commercial filters seems to have between 50 and 100 ohms for whole loop resistance)
Filter must pass the POTS tip-to-ring dc voltages (typically o-72V)
Filter must pass ring voltages well (40V to 80V rms at any frequency from15.3Hz to 68Hz with a dc component in the range from 0V to 72V)
Filter must
All requirements must be met in the presence of POTS loop currents (usually around 0-40 mA, can be up to 120 mA in some cases)
The ADSL output from filter (if it has such thing) is generally unfiltered line signal (normal home ADSL devices are not to be bothered with line voltage and voice signals.
for more details refer : http://www.epanorama.net/documents/telecom/adsl_filter.html