What is a network analyzer?
Measuring with a network analyzer
Figure 1: R&S®ZNA vector network analyzer with touch screen operation for four-port networks
(Courtesy of Rohde & Schwarz)
Figure 1:
R&S®ZNA
vector network analyzer with touch screen operation for four-port networks
(Courtesy of Rohde & Schwarz)
Measuring with a network analyzer
A network analyzer is a measuring device that tests the electrical behavior of high-frequency components. It is a stimulus-response test system consisting of an RF source and several test receivers. It is specifically designed to measure the forward and reflection and transmission responses (also called scattering parameters or S-parameters) of RF components as a function of frequency. These S-parameters have both magnitude and phase components and characterize the linear power of the assembly under test. If the network analyzer is capable of analyzing not only magnitude but also phase, it is called a vector network analyzer. Only since the advent of vector network analyzers, those devices that can only measure the magnitudes have been called scalar network analyzers for distinction.
Network analyzers are typical workshop equipment. The measurement process requires that the component to be measured has been removed from its application environment and tested under laboratory conditions. In addition to (linear) scattering parameters, some nonlinear properties such as amplifier compression, intermodulation distortion, or noise parameters can also be measured.
Figure 2: Scattering parameters on a two-port network (here: a band pass filter)
Scattering parameters
The scattering parameters describe the behavior of high-frequency components, which are called networks here. The number of scattering parameters depends on the number of connections (gates) of a network. The number of S-parameters required depends on the number of gates of the network and is calculated by squaring its number of gates. For example, a ferrite circulator is a three-port network and is measured by means of 9 scattering parameters. A band filter is a two-port network with one input (port 1) and one output (port 2). Its four scattering parameters are:
- S11: is the input port voltage reflection coefficient;
- S21: is the forward voltage gain;
- S12: is the reverse voltage gain;
- S22: is the output port voltage reflection coefficient.
In a passive network such as the bandpass filter shown in Figure 2, S21 and S12 should be equal. However, if the network contains an amplifier, S12 should be as small as possible and its inverse value is then called isolation.
Figure 3: S-parameters on a three-port ferrite circulator.
S-parameters of a multi-port network can be summarized in an S-matrix:
(1)