VNA (Vector Network Analyser)

Notes

Calibration and fixture de-embedding dominate measurement quality above about 40 GHz, more than the instrument itself.

Overview

A VNA (vector network analyser) is an instrument that measures how a component or interconnect reflects and transmits signals across a range of frequencies, recording both amplitude and phase. Results are expressed as S-parameters, from which engineers derive insertion loss, return loss, crosstalk and impedance. VNAs are used on RF parts such as filters, amplifiers and antennas, and on high-speed digital channels, packages and connectors, where the measured data feeds channel simulation and compliance checks.

How it works

A VNA contains a swept signal source, a test set with directional couplers that separate the incident and reflected waves at each port, and receivers that measure those waves against a phase reference. For a two-port device it reports four S-parameters: S11 and S22 describe reflection at each port, and S21 and S12 describe transmission through the device. Instruments with four or more ports can measure differential pairs and the coupling between neighbouring lanes.

Accuracy depends on calibration. Cables, adapters and fixtures add their own loss and reflections, so before measuring, the user connects known standards (through, open, short and match or load) at the reference plane, and the VNA calculates error terms to remove them. Through-open-short-match (TOSM, also called SOLT) is the most common full two-port method for coaxial setups, while through-reflect-line (TRL) suits wafer probing and test fixtures. Drift and random errors, such as connector repeatability, cannot be calibrated out and are controlled with stable temperature and careful handling.

In signal integrity work, measured S-parameters feed tools such as the channel operating margin, which derives a pulse response from them. Instruments span from about 1 Hz to beyond 1 THz, and NIST provides traceable S-parameter calibration of RF components up to 110 GHz, with on-wafer methods up to 1 THz under development.

Related terms

• TDR
• Insertion loss
• Return loss
• Compliance test

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