|The technique for characteristic impedance measurement is the same for both twisted pair and coax on the DSP-4x00 Series. |
A voltage step is applied to the cable under test, and the response at 40 ns after the leading edge is digitized. This corresponds to a location on the cable of about 4 meters (12 ft) from the DSP. The amplitude of the step is a function of cable Z0, since a voltage divider is formed from the DSP's source impedance and the cable's Z0. Stored calibration data allows extraction of an estimate of cable Z0 at that location.
Note that the Z0 reported is NOT necessarily an indication of quality of the entire cable under test, but is rather an estimate of Z0 at the particular location 4 meters from the DSP.
Impedance testing is not required for UTP/FTP or STP cabling. Return Loss is a far more revealing measurement.