Network analyzer (electrical)

Lore & Background

Network analyzers are used mostly at high frequencies; operating frequencies can range from 1 Hz to 1.5 THz. Special types of network analyzers can also cover lower frequency ranges down to 1 Hz. These network analyzers can be used, for example, for the stability analysis of open loops or for the measurement of audio and ultrasonic components. The two basic types of network analyzers are scalar network analyzer (SNA)—measures amplitude properties only—and vector network analyzer (VNA)—measures both amplitude and phase properties. A VNA is a form of RF network analyzer widely used for RF design applications. A VNA may also be called a gain–phase meter or an automatic network analyzer. An SNA is functionally identical to a spectrum analyzer in combination with a tracking generator. VNAs are the most common type of network analyzers, and so references to an unqualified 'network analyzer' most often mean a VNA.

Reader's Guide

The basic architecture of a network analyzer involves a signal generator, a test set, one or more receivers and display. In some setups, these units are distinct instruments. Most VNAs have two test ports, permitting measurement of four S-parameters, but instruments with more than two ports are available commercially. The network analyzer needs a test signal, and a signal generator or signal source will provide one. Older network analyzers did not have their own signal generator, but had the ability to control a stand-alone signal generator using, for example, a GPIB connection. Nearly all modern network analyzers have a built-in signal generator. High-performance network analyzers have two built-in sources. The test set takes the signal generator output and routes it to the device under test, and it routes the signal to be measured to the receivers. It often splits off a reference channel for the incident wave. Directional couplers or two resistor power dividers are used for signal separation. The receivers make the measurements. For the VNA, the receiver measures both the magnitude and the phase of the signal. It needs a reference channel (R) to determine the phase, so a VNA needs at least two receivers. The usual method down converts the reference and test channels to make the measurements at a lower frequency. The phase may be measured with a quadrature detector. A VNA requires at least two receivers, but some will have three or four receivers to permit simultaneous measurement of different parameters. There are some VNA architectures (six-port reflectometer) that infer phase and magnitude from just power measurements. With the processed RF signal available from the receiver/detector section it is necessary to display the signal in a format that can be interpreted. Most RF network analyzers incorporate features including linear and logarithmic sweeps, linear and log formats, polar plots, Smith charts, etc. Trace markers, limit lines and pass/fail criteria are also added in many instances.

Did You Know?

• Entry-level VNAs and do-it-yourself projects are available for less than $100, mainly from the amateur radio sector.
• Another category of network analyzer is the microwave transition analyzer (MTA) or large-signal network analyzer (LSNA), which measure both amplitude and phase of the fundamental and harmonics.
• A network analyzer requires periodic calibration, typically performed once per year.