Metric · Analog, RF & Mixed-Signal
S-parameters (Scattering parameters)
A complex matrix describing how a network reflects and transmits incident waves at each port, measured against a reference impedance. Unlike impedance or admittance parameters they need no open or short terminations, so they work at RF.
- Also known as
- S-params, Touchstone data
- Type
- Metric
- Domain
- Analog, RF & Mixed-Signal, RF characterisation
Notes
S11 is input reflection, S21 forward gain, S12 reverse isolation and S22 output reflection, shared as .s2p Touchstone files.
Overview
S-parameters (scattering parameters) describe how a linear electrical network, such as an amplifier, filter, connector or package trace, reflects and transmits signals at each of its ports, as a function of frequency. Each one is a complex ratio, with magnitude and phase, between the wave leaving one port and the wave arriving at another, defined against a stated reference impedance. Because they are measured with matched terminations rather than open or short circuits, they work at RF and microwave frequencies where other network parameters are hard to measure.
How it works
An N-port network is described by an N by N matrix. For a two-port device, S11 is the reflection seen at port 1, S21 is the transmission from port 1 to port 2, S12 is the transmission in the reverse direction and S22 is the reflection at port 2; the first number names the port where the wave leaves and the second the port where it enters. Each parameter is measured with signals applied at one port while the other ports are terminated in the reference impedance, most often 50 ohms.
Familiar figures follow directly. Gain or insertion loss in decibels comes from the magnitude of S21, return loss from the magnitude of S11 or S22, and the voltage standing wave ratio from the same reflection terms. A passive network made of ordinary materials is reciprocal, so S21 equals S12, while an amplifier is not. For differential links, mixed-mode S-parameters separate differential behaviour, common-mode behaviour and conversion between the two.
S-parameters are normally measured with a vector network analyser and exchanged as Touchstone files, plain text files whose extension, such as .s2p or .s4p, gives the number of ports. Each file states its frequency unit, parameter type, number format and reference resistance, which defaults to 50 ohms. S-parameters describe only linear, steady-state behaviour, so non-linear effects need other kinds of measurement.
Examples
- A low-noise amplifier datasheet giving S21 as gain and S11 as input match across its operating band.
- A four-port Touchstone file describing a differential package channel for a high-speed serial interface, analysed with mixed-mode parameters.
- A four-port device measured on a two-port vector network analyser in stages, with the unused ports terminated in matched loads and the results assembled into the full matrix.
Why it matters
S-parameters are the common language between RF designers, signal and power integrity engineers, package and board designers and test engineers: a package model, a connector datasheet or a channel for a high-speed serial link is commonly supplied as a Touchstone file, the de facto exchange format. Simulators read them directly, so their accuracy limits the accuracy of every simulation that uses them.
Related terms
Part of the Analog, RF & Mixed-Signal glossary, with every term in the topic and the lifecycles, tools and standards that use them.
Where it appears on the Hub
Used on 20 pages of the Hub. In each group, the pages that mention it most come first.
Lifecycle stages and gates
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Last reviewed . Written by The Semiconductor Hub editorial team. The full glossary