Demystifying S-Parameters in Network Analyzers

Modern network analyzers have been widely used in research and production. Network analyzers are widely used to analyze various components, materials, circuits, equipment and systems. Whether it is to optimize the design of analog circuits in the research stage or to debug and detect electronic components, vector network analyzers have become an indispensable measuring instrument.

Network is a term that is used frequently and has many modern definitions. In terms of network analysis, a network refers to a group of interconnected electronic components. One of the functions of a network analyzer is to quantify the impedance mismatch between two RF components to maximize power efficiency and signal integrity. Whenever an RF signal enters from one component to another, part of the signal is reflected while another part is transmitted. Let's learn and discuss what S parameters are in a network analyzer?

Today, the protagonist "S parameters" officially debuted. In simulation, we generally use S parameters to represent passive networks. S parameters are also called scattering parameters and are applicable to distributed parameter circuits.

The following figure shows the direction of the signal very vividly. A1 is the incident signal, B1 is the reflected signal, and B2 is the transmitted signal. S parameters are network parameters based on the energy relationship of these signals.

S parameters can be measured with a network analyzer and calculated using network analysis techniques.

The commonly used expression is Sij: it means the energy injected from port j and the energy measured at port i. For example, S11 is defined as the square root of the ratio of the energy reflected from port 1 to the input energy. It is also often simplified to the ratio of the equivalent reflected voltage to the equivalent incident voltage.

Now let us imagine the network as a "black box" and only care about the value of port energy.

Take a two-port example to analyze the meaning of S parameters:

S11: Reflection coefficient of port 1 when port 2 is matched

S22: Reflection coefficient of port 2 when port 1 is matched

S12: Reverse transmission coefficient from port 2 to port 1 when port 1 is matched

S21: Forward transmission coefficient from port 1 to port 2 when port 2 is matched

We often use S11 and S21, which respectively mean:

For a 2-port network, Port1 inputs the signal and Port2 outputs the signal, then:

S11 represents the return loss, that is, how much energy returns to Port1. Of course, the smaller the better, that is, very little energy is reflected back. Generally, S11<0.1 (-20dB).

S21 represents insertion loss, that is, how much energy is transmitted to Port2. Of course, the larger the better. The maximum is 1, that is, all energy is transmitted to Port2 without loss. Generally, S21>0.7 (-3dB).

Therefore, for our actual engineering applications, S11 and S21 are two relatively important parameters, which can characterize the performance of the network. Usually, we usually express the amplitude of S parameters in DB.

In practice, a single transmission line or a via that we often use can be equivalent to a two-port network, with one end connected to the input signal and the other end connected to the output signal. So if Port1 is used as the input port of the signal and Port2 is used as the output port of the signal, then S11 represents the return loss of this transmission line.