A network analyzer measures the network parameters of electrical circuits, most often the S-parameters (scattering parameters) that describe the transmission and reflection characteristics of a device under test (DUT). It can also report Y, Z, and H parameters, along with gain, return loss, and impedance. The defining feature is that the instrument supplies its own stimulus, then measures the device's response relative to that stimulus, so it reports both amplitude and phase. For the underlying definitions, see the Keysight glossary entry on the vector network analyzer and what is a network analyzer.

S-parameters are the standard way to describe a linear network at RF. The scattering-matrix formulation dates to Kurokawa's 1965 paper, Power Waves and the Scattering Matrix, and remains the reference framework today. For a working treatment, Keysight's application note Understanding the Fundamental Principles of Vector Network Analysis walks through reflection, transmission, and the Smith chart.

Network analyzers come in handheld, benchtop, and modular forms, in 2-port, 4-port, and higher port counts. Time Domain Reflectometry (TDR) can be used to locate impedance discontinuities along a line.
 

The traditional split into three families still holds, but two types cover almost all real work:

1. Vector network analyzers (VNAs) measure magnitude and phase, and can apply vector error correction. They handle the four S-parameters of a two-port and are standard in modern RF labs.
2. Scalar network analyzers (SNAs) measure magnitude only (return loss, voltage standing wave ratio or VSWR) and are a legacy choice; large-signal network analyzers (LSNAs) remain a specialist case for nonlinear devices such as amplifiers driven into compression. For everyday characterization, the VNA is the current instrument.

The decision is simple: if you need phase, you need a vector instrument. Impedance matching, de-embedding, and group delay all depend on phase, which is why VNAs, not scalar analyzers, do the bulk of characterization.
 

Network analyzers characterize, match, and debug components and circuits.Common jobs include:

• Reflection measurements (S11, S22): return loss, VSWR, and input or output match for antennas, filters, and connectors.
• Transmission measurements (S21, S12): gain, insertion loss, isolation, and group delay through a device.
• Impedance and matching: reading complex impedance on a Smith chart and designing a matching network.
• Cables and antennas: distance-to-fault and return-loss measurements in the field, often with a handheld unit.

VNA accuracy comes from calibration, not just hardware. Vector error correction removes systematic errors (directivity, source and load match, tracking) measured during a calibration such as SOLT (short-open-load-thru) or TRL (thru-reflect-line). Keysight Application Note 1287-3, Applying Error Correction to Network Analyzer Measurements, explains how those systematic error terms are defined and corrected. The traceable form of TRL was formalized by NIST's Roger Marks, whose 1991 multiline method in the IEEE Transactions on Microwave Theory and Techniques is still the reference for broadband and on-wafer calibration. Reference standards are defined by documents such as IEEE Std 287.1-2021 for precision coaxial connectors, and the calibration chain traces back to national standards through metrological traceability. A well-calibrated VNA can reach a measurement accuracy a receiver-only instrument cannot, because it corrects errors rather than just displaying them.
 

A spectrum analyzer measures a signal's power across a range of frequencies. It plots amplitude on the vertical axis against frequency on the horizontal axis, so you see the spectral content of a signal (its power and frequency) rather than its shape over time. Unlike a network analyzer, it has no source. It is a tuned receiver that measures whatever input signal you connect. For more background, see the Keysight glossary entry on spectrum analyzers and the Keysight guide to spectrum analyzers versus oscilloscopes.

The classic architecture is the swept-tuned superheterodyne receiver, which mixes the input with a local oscillator to an intermediate frequency, then sweeps across the band. Keysight's Spectrum Analysis Basics (Application Note 150) is a useful reference for the block diagram, the receiver architecture, and how a spectrum analyzer works in practice.

One point is worth clarifying: a modern signal analyzer is a spectrum analyzer that also digitizes the signal for vector analysis (magnitude and phase of modulation). The frequency-domain display is the same; the signal analyzer adds demodulation and deeper measurements, which is why current Keysight instruments carry the "signal analyzer" name.
 

1. Swept superheterodyne analyzers sweep a tuned receiver across the band. They offer wide frequency coverage and good dynamic range, and remain the general-purpose choice.
2. FFT (fast Fourier transform) analyzers sample a block of the signal and compute its spectrum, which suits low frequencies and fine resolution.
3. Real-time analyzers process the input continuously so they can catch brief, intermittent, or hopping signals that a swept analyzer may miss.

The right type depends on the signal: real-time for transient interference, FFT for fine low-frequency detail, and swept for general RF work.
 

Spectrum and signal analyzers find, measure, and troubleshoot signals. Common jobs include:

• Signal and interference hunting: locating unwanted signals and interference in Wi-Fi, cellular, and other wireless systems.
• Frequency and power measurements: center frequency, occupied bandwidth, channel power, and spurious levels.
• Phase noise: the short-term frequency stability of an oscillator or source.
• Harmonics, spurious signals, and intermodulation: distortion and spurious outputs from amplifiers and mixers, including IMD and third-order intercept point (IP3).
• Modulation analysis: error vector magnitude and demodulation of Wi-Fi, LTE, and 5G signals.

For the measurement quality that governs these jobs, Keysight's Signal Analysis Measurement Fundamentals covers noise floor, sensitivity, and dynamic range. The floor those jobs work against is physical: Johnson-Nyquist thermal noise sets a floor of about -174 dBm/Hz at room temperature, the limit a signal analyzer’s displayed average noise level can approach but not beat.
 

Match the instrument to the job, frequency range, and RF application. The table below maps common needs to current Keysight models, with the exact frequency ranges from their product pages.