Network Analyzer

A network analyzer is an instrument used to analyze the properties of electrical networks, especially those properties associated with reflection and transmission of electrical signals known as s-parameters. Network analyzers are used mostly at high frequencies; a typical unit can make measurements from a few hundred kilohertz up to several gigahertz.

The two main categories of Network Analyzers are
Scalar network analyzer (SNA) - amplitude properties only
Vector network analyzer (VNA) - both amplitude and phase properties; also called gain-phase meter

VNAs are the most common type of Network Analyzer. References to a 'network analyzer' most often mean a VNA since an SNA is functionally identical to a spectrum analyzer in combination with a tracking generator.

A new category of Network Analyzers has also been developed for the measurement of non-linear microwave signals:
Large signal network analyzer (LSNA) - both amplitude and phase of the fundamental and harmonics are acquired
The microwave transition analyzer (MTA) was historically commercialized before the LSNA but was lacking some of the user-friendly calibration features now available with the LSNA.

Spectrum Analyzer

A spectrum analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform.

Often, it measures the power spectrum.

There are analogue and digital spectrum analyzers:
An analogue spectrum analyzer uses either a variable bandpass filter whose mid-frequency is automatically tuned (shifted, swept) through the range of frequencies of which the spectrum is to be measured or a superheterodyne receiver where the local oscillator is swept through a range of frequencies.
A digital spectrum analyzer uses the Fast Fourier transform (FFT), a mathematical process that transforms a waveform into the components of its frequency spectrum. As a result, computer programs can compute such transforms, and makes audio processing easier. FFTs have applications in much wider fields.

Some spectrum analyzers (such as Tektronix's family of "real-time spectrum analyzers") use a hybrid technique where the incoming signal is first down-converted to a lower frequency using superheterodyne techniques and then analyzed using FFT techniques.