S-parameters are mathematical representations of energy propagation within a multiport electrical network. It is written as a complex matrix file known as scattering parameters. The S-parameter file represents full combination of the reflection and transmission parameters in a multiport network with a frequency component. It gives an idea of how the DUT behaves with a signal in both forward and reverse directions.
Let’s consider a two-port network. In this case, a two-port passive band pass filter as DUT, and the measurement is two port s-matrix. The industrial standard of s-parameters is the touchstone file format. It is also possible to record S-parameters as CSV and Matlab formats in most of the Vector Network Analyzers (VNA). VNAs are used to measure S-parameters.
Parameters
S11 – Reflection coefficient at port 1. A signal sent from port one of VNA and received at port one
S12 – Reverse transmission (gain/loss). The signal was sent from port 2 of VNA and received at port 1
S21 – Forward transmission (gain/loss). The signal was sent from port one of VNA and received at port 2
S22 – Reflection coefficient at port 2. The signal was sent from port 2 of VNA and received at port 2.
Formats
S-parameters can be presented in many formats as below:
Reflection S11, S22: Return loss, Impedance, Admittance, VSWR.
Transmission S12, S21: Gain/Loss (IL – Insertion Loss), Phase and Group Delay.
N port S-matrix
If the number of ports in a network increases, the s-parameter will become complex according to the number of ports. Two port network has four parameters; three ports will have nine parameters and so on. The parameters of any given s-matrix are represented as Snn, where S is the parameter and n is the number or port.
Mix mode s-parameters
Mix mode s-parameters are representations of balanced circuits in a network. Two input or output ports can be combined into one balanced port for combined measurement.
Touchstone format:
Touchstone file was originally introduced by EEsof, and it was acquired by HP (now Keysight EEsof EDA). It became an industry standard and was used worldwide. Standard touchstone file contains information about the type S, Y or Z parameters, formats such as magnitude angle (DB), Real Imaginary and frequency component, etc. It has file extension as .Snp where n is the number of ports.
Vector Network Analyzer
VNA is RF test equipment electrical and RF engineers use to measure a complex network’s electrical characteristics. It has multiple transmitter and receiver combinations to calculate a DUT’s gain, loss, reflection, VSWR, and impedance. Modern network analyzers are capable of performing more complicated tasks.
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