Jul. 01, 2020

The S parameter, short for "scattering" parameter, is a measure of the ratio of the scattered voltage wave entering the device under test (DUT) to the incident voltage wave.The Sparameter is derived from the network analysis method in the frequency domain, where voltage wave refers to sine wave.

When we measure the response of high-speed interconnection to sinusoidal waves, we assume that the interconnection satisfies three important characteristics:

Passive - no gain, only loss;

The linear - interconnection does not change the frequency of the input sine wave; Time-invariant - static in that geometric shapes and other features do not change during measurement.

This means that when we send 1GHz sine wave to DUT, what we get is only sine wave, and the only changes are amplitude and phase. The description of sine wave in frequencydomain is a complex number, with real and imaginary parts or amplitude and phase.

A complex number is obtained by comparing two sine waves of the same frequency. The amplitude is the ratio of the amplitude of the output wave to that of the input wave, andthe phase is the phase difference between the output wave and the input wave.

We can plot complex values over the entire frequency range in any of three ways: real and imaginary, polar coordinates, and amplitude and phase. An example of these threeapproaches is shown in the following figure. The essence of the three approaches is exactly the same, but they are displayed differently.

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