Spurious vs. Harmonics: Understanding RF Frequencies

This article explains the difference between harmonics and spurious frequencies in the context of RF systems. Both are undesired 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.

Cause : It arises from various sources like mixer products, oscillator leakage or power supply noise.

Mitigation : Requires careful design, shielding and filtering to minimize.

Regulatory concern : Strictly regulated; excessive spurious emissions can lead to penalties.

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.

Cause : It is generated due to non-linearities in amplifiers or other components.

Mitigation : Use of filters such as low pass filters to suppress higher order harmonics.

Regulatory concern : Subject to regulatory limits to prevent interference.

• 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.

Summary

In summary, while both harmonics and spurious signals are unwanted frequencies in RF systems, harmonics are specific, predictable multiples of the fundamental frequency, whereas spurious signals are broader and can arise from various non linear interactions within the system.