Spectrum Analysers: an Introduction

Quick Answer

A spectrum analyser plots amplitude against frequency, where an oscilloscope plots it against time. The same signal that shows as one trace on a scope resolves into a fundamental, its harmonics, any spurious tones and the noise floor beneath them. Vertical scales are logarithmic, usually in decibels.

Both instruments hold the same information, organised differently. A distorted sine looks slightly wrong on a scope and no amount of staring quantifies it; on an analyser the harmonics stand up as separate lines with a number against each. A tone sixty decibels down — a millionth of the carrier's power — is invisible in a time trace and plain on an analyser's screen, which is the whole reason the vertical scale is in decibels.

A bench wants one as soon as the questions turn spectral: what a transmitter emits outside its channel, where an unexplained tone on a supply rail comes from, how much a stage distorts, which switching harmonic is going to fail an emissions test.

Most oscilloscopes will compute an approximation from a captured record, which is genuinely useful for finding a spur and not good enough for measuring one, since neither the dynamic range nor the frequency resolution is comparable. This entry is an introduction and no more. Sweep rate, resolution bandwidth, reference level and the arithmetic of dBm all decide whether a reading can be trusted, and none of them is covered here.