Devin Alex

While a spectrum analyzer passively measures signals generated by an external source, a Network Analyzer contains both a source and a receiver. Its primary purpose is to characterize the behavior of linear networks - components like filters, amplifiers, antennas, and cables - by stimulating them with a known signal and measuring their response across a range of frequencies.

Modern instruments are almost exclusively Vector Network Analyzers (VNAs), meaning they measure both the amplitude and the phase of the transmitted and reflected signals, allowing for complete characterization of the Device Under Test (DUT).

Scattering Parameters (S-Parameters)

At high frequencies (RF and microwave), traditional concepts of voltage and current become difficult to measure directly due to standing waves and the lack of a defined ground. Instead, networks are characterized using Scattering Parameters, or S-parameters, which relate traveling waves incident upon the ports of the network to the waves reflecting from or transmitting through those ports.

For a standard 2-port device (like a filter or amplifier):

$S_{11}$ (Input Reflection Coefficient): The ratio of the signal reflecting back from port 1 to the signal incident on port 1. Related to input return loss and VSWR.
$S_{21}$ (Forward Transmission Coefficient): The ratio of the signal exiting port 2 to the signal incident on port 1. Represents gain or insertion loss.
$S_{12}$ (Reverse Transmission Coefficient): The ratio of the signal exiting port 1 to the signal incident on port 2. Represents isolation or reverse isolation.
$S_{22}$ (Output Reflection Coefficient): The ratio of the signal reflecting back from port 2 to the signal incident on port 2. Related to output return loss.

VNA Calibration (Error Correction)

A VNA is inherently subject to significant systematic errors caused by imperfections in its internal hardware, test cables, and adapters. These errors include directivity, source match, load match, reflection tracking, and transmission tracking.

To make accurate measurements, a VNA must be mathematically "calibrated" immediately prior to use by measuring known standards (a calibration kit). The VNA calculates an error model (typically a 12-term error model for a 2-port calibration) and mathematically removes these systematic errors from subsequent measurements of the DUT. This process moves the measurement reference plane directly to the connection points of the DUT.

Common Calibration Techniques

SOLT (Short-Open-Load-Thru): The most common mechanical calibration method. It requires connecting four known standards to the test ports: a precise Short circuit, an Open circuit (with characterized fringing capacitance), a matched Load (usually 50 ohms), and a Thru connection (connecting the two test ports directly).
TRL (Thru-Reflect-Line): Often used for non-coaxial environments (like probing on a wafer or testing printed circuit board traces) where standard SOLT kits are unavailable. It relies on the electrical characteristics of a transmission line rather than discrete components.
ECal (Electronic Calibration Modules): Solid-state devices containing programmable impedance states that are thoroughly characterized at the factory. They automate the calibration process, significantly reducing operator error and connection wear compared to mechanical SOLT kits.

Metrological Traceability

While the user performs a "user calibration" with a cal kit prior to testing, the VNA itself and the calibration kit must be periodically calibrated by an accredited laboratory to maintain traceability to the SI. Calibrating a mechanical cal kit involves precise dimensional measurements and modeling, as well as electrical verification against higher-tier reference standard kits or airlines.