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.

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. is determined by setting equal to zero - applying a short circuit to the
output port to the network. is then the ratio of to - the input impedance of the network.

is determined by measuring the ratio of to - the reverse voltage gain - with input
port open circuited.

Moving to higher and higher frequencies. some problems arise:

- Equipment is not readily available to measure total voltage and the total current at the ports of the network.
- Short and open circuits are difficult to achieve over a broad band of frequencies.
- Active devices, such as transistors and tunnel diodes, very often will NOT be short or open circuit stable.