Mode of Excitation/Termination Scheme

Scope
The following are descriptions of the various termination schemes used in specifying or measuring twisted pair cable performance. Care should be taken in choosing a method, as each affects the data obtained in a unique manner.

Procedure

Single End: True single end measurements are performed with one conductor driven and all other conductors tied to any existing shields and designated as ground. The measurements are taken in an unbalanced mode.

SCSI Method: This method is an adaptation of the single end method and is, in fact, described as single end in all SCSI papers. One conductor of each pair is connected to the overall shield and designated as common ground. The remaining conductors are left floating and tested one at a time to common ground. The measurements are taken in an unbalanced mode.

Conductor to Conductor (Common Mode): A measurement taken between two conductors of a pair with single ended instrument with one conductor driven, the other conductor grounded, and all other conductors and shields allowed to float. The measurements are taken in an unbalanced mode.

Differential: A differential measurement is obtained by each conductor in a pair being excited by a signal equal in magnitude but opposite in polarity in reference to a common ground (shield). These signals can be obtained by either using a single ended source with a balancing transformer (balun) or by using two synchronized sources (differential TDR) with equal buy opposite signals. All other pairs are allowed to float and the shield is grounded.

Mutual: A three measurement technique used for capacitance measurement of balanced transmission line (i.e. twisted pair) that prevents stray capacitance effects from causing significant error.

\[{C_m} = \frac{{2({C_a} + {C_b}) - {C_c}}}{{4L}}\]

Where:

  • Cm is the Mutual Capacitance in pF/Ft.
  • Ca is 1st conductor to all other conductors and shield(s)
  • Cb is 2nd conductor to all other conductors and shield(s)
  • Cc is 1st and 2nd conductors tied together measured against all other conductors and shield(s)
  • L is the length of the sample in feet

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