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Network characterization

S-parameter is basically a means of characterizing n-port networks. By reviewing some traditional network analysis methods we'll understand why an additional method of characterization is necessary at high frequency.

Figure 1: A two-port network

A two-port device in Figure 1 can be described by a number of parameter sets. All of these network parameters relate total voltages and total currents at each of the two ports.




To see how parameter sets of this type can be determined through measurement, let's focus on the H-parameter. text_wrap_inline2787 is determined by setting text_wrap_inline2789 equal to zero - applying a short circuit to the output port to the network. text_wrap_inline2787 is then the ratio of text_wrap_inline2793 to text_wrap_inline2795 - the input impedance of the network.
text_wrap_inline2797 is determined by measuring the ratio of text_wrap_inline2793 to text_wrap_inline2789 - the reverse voltage gain - with input port open circuited.

Moving to higher and higher frequencies. some problems arise:

  1. Equipment is not readily available to measure total voltage and the total current at the ports of the network.
  2. Short and open circuits are difficult to achieve over a broad band of frequencies.
  3. Active devices, such as transistors and tunnel diodes, very often will NOT be short or open circuit stable.
Some methods of characterization are necessary to overcome these problems. The logical variables to use at these frequencies are traveling waves rather than total voltages and total currents.