Meaning
Unwanted electromagnetic emissions appearing at integer multiples of the fundamental operating frequency that fall outside the intended channel. These spurious harmonic spikes result from non-linearities in the power amplifier or the switching behavior of the digital circuitry. They can interfere with other radio services and must be suppressed to meet international emission standards.
Filtering Strategy
Low pass filters are placed at the output of the radio to attenuate high frequency noise. If the spurious harmonic spikes are particularly strong, a notch filter might be needed for a specific frequency. Shielding the radio section of the board also prevents these signals from radiating directly from the circuit traces.
Clean power delivery networks help reduce the generation of these harmonics at the source.
Measurement Difficulty
Identifying these signals requires a spectrum analyzer with a wide dynamic range. During the certification of a wireless product, engineers scan the spectrum up to the tenth harmonic to find spurious harmonic spikes. Any peak that exceeds the regulatory mask requires a hardware redesign.
These measurements are performed in an anechoic chamber to eliminate background noise from the environment.
Interference Radius
Harmonics can land on the receiving frequency of a nearby cellular or global positioning system receiver. These spurious harmonic spikes degrade the sensitivity of those collocated radios.
