Spurious vs. Harmonics: Understanding RF Frequencies

This article explains the difference between harmonics and spurious frequencies in the context of RF systems. Both are unwanted frequencies generated in addition to the desired output frequency in an RF transceiver or any frequency translation system involving mixers.

Spurious Frequencies

As shown in the figure below, in the frequency domain view, spurious frequencies are present at non-integer multiples of the input frequency.

Harmonics

As shown in a frequency domain view, harmonic frequencies are present at integer multiples of the input frequency. Harmonics are essentially a subset of spurious frequencies, specifically those occurring at integer multiples of the input frequency.

Both spurious and harmonic frequencies should be measured in dBc (decibels relative to the carrier signal). Their levels should ideally be down by at least 40-50 dBc for proper functioning of the RF system.