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Hi, 1 A step test waveform is generally unbalanced, one side is earthed, while telephone lines are balanced. Unbalancing a telephone line causes so much noise that test results are buried in the noise. 2 A pulse waveform contains the same information as a step waveform, except for the low frequencies, which is unimportant anyway in TDR. 3 Telephone lines generally passes through filters and transformers, which precludes low frequencies and DC anyway. 4 Telephone lines have finite bandwidth. The attenuation of a long open wire telephone line (eg. 100km), for instance, practically limits the bandwidth to between 600Khz and 1Mhz. Shorter cables of course have higher bandwidths, but more than 2Mhz is not practical. This means that very short pulse risetime is irrelevant, and therefore very small resolution testing is impossible. In any case, a resolution of 0.5km on a 100km line is very good. Who can physically measure the distance to better resolution anyway? Consider that when using TDR on loaded cables, the bandwidth is limited to less than 4 Khz, yet TDR is a very useful tool on those cables. The resolution required and obtained is then to a loading section, which traditionally was 2000 yards. The pulse need not even be shaped to sine square, the bandwidth of the cable will shape it. The pulse length must be adjusted to the bandwidth of the transmission medium. Regards Johan Smit smit.js@pixie.co.za P.O.Box 9551 George 6530 Republic of South Africa