Meaning
Signal distortion components appearing at integer multiples of a base system frequency emerge during high-speed switching operations on digital printed circuit boards. Unwanted clock harmonics degrade radio receiver sensitivity when trace layout routes high-speed clock lines near sensitive antenna feed points. Analysis of these spurious emissions forms a core part of electromagnetic compatibility testing, ending where clock frequencies drop below radio receiver operational bandwidths.
Spurious Generation
Square-wave clock signals generate rich frequency spectra containing odd multiples of the fundamental switching rate. Rising and falling edges on digital traces generate electromagnetic field spikes that radiate into nearby trace structures. Coupling mechanisms allow clock harmonics to induce noise onto power supply rails and integrated antennas, degrading receiver performance.
Board Layout
Routing clock traces on inner PCB layers between continuous ground planes provides effective field confinement. Differential routing reduces common-mode emissions while maintaining timing integrity across high-speed interfaces. Physical separation between clock drivers and wireless modules prevents clock harmonics from corrupting weak radio signals at the receiver input.
Harmonic Mitigation
Series termination resistors placed near driver outputs reduce high-frequency ringing on clock lines. Spread spectrum clocking techniques distribute spectral energy across a narrow band, reducing peak amplitude at specific harmonic frequencies. Applying spread spectrum modulation allows clock harmonics to pass strict regulatory radiation limits without requiring heavy metallic shielding cans.
Shielding enclosures offer additional suppression when trace level mitigations fall short of compliance goals.
