Spectrum Analysis

Spectrum analysis examines a signal in the frequency domain, showing how its power is distributed across frequency rather than how its amplitude varies over time. While an oscilloscope plots voltage against time, a spectrum analyzer plots amplitude against frequency, so a pure sine wave appears as a single spike and a complex signal as a collection of lines and bands. This view exposes features that are hard to see in the time domain, such as harmonics, spurious tones, noise and the sidebands produced by modulation.

The traditional swept-tuned spectrum analyzer works much like a tunable radio receiver. It mixes the incoming signal with a local oscillator whose frequency is swept across the band of interest, so that each part of the input spectrum in turn is translated to a fixed intermediate frequency where it passes through a narrow filter and is measured. The result is traced across the screen as the sweep proceeds. The width of that filter, called the resolution bandwidth, sets how closely spaced two signals can be and still be seen as separate; a narrower setting resolves finer detail but takes longer to sweep.

Spectrum analyzers are essential in radio-frequency and communications work. They are used to check that a transmitter occupies only its assigned channel, to measure harmonic and intermodulation distortion, to characterise noise, and to verify modulation. Many modern analyzers compute the spectrum digitally using the fast Fourier transform on sampled data, which suits lower-frequency and transient signals, while swept designs remain common at high frequencies. Either way the frequency-domain view complements time-domain measurement.

Frequently asked questions

How does a spectrum analyzer differ from an oscilloscope?

An oscilloscope shows amplitude against time, while a spectrum analyzer shows amplitude against frequency, revealing harmonics, noise and modulation sidebands not obvious in the time domain.

What is resolution bandwidth?

It is the width of the analyzer's measurement filter, which determines how close two signals can be and still be distinguished. A narrower setting resolves finer detail but sweeps more slowly.

What are spectrum analyzers used for?

They check transmitter occupied bandwidth, measure harmonic and intermodulation distortion, characterise noise and verify modulation, making them central to radio-frequency testing.