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Why do network analyzers measure S-parameters (scattering parameters) instead of H, Y, and Z parameters?

If the signal frequency is in the microwave range, the widespread H, Y, and Z parameters (used in basic network theory) cannot be measured for the following reasons:

  • It is hard to measure total voltages and currents at the ports of the device under test
  • Active devices may be unstable or self-destruct with short and open circuits

S-parameters measure traveling waves rather than total voltages and currents. The following are some advantages of using S-parameters:

  • The basic measurements to determine the S-parameters are familiar and well established microwave measurements: reflection coefficient, attenuation (gain), phase
  • S-parameters are analytically convenient; they allow for calculations of system performance by cascading the individual components.
  • Flow-graph analysis can be used, which simplifies the analysis of a microwave system.

For further information on S-parameters and S-parameter design, refer to HP Application Note 154 (literature number 5952-1087).