Klokkejitter

Klokkejitter, or clock jitter, refers to the small, unwanted variations in the timing of a clock signal relative to its ideal periodic edges. It can manifest as deviations in edge position from cycle to cycle (period jitter) or as fluctuations in the phase of the clock edge (phase jitter). Jitter is typically quantified as RMS jitter, peak-to-peak jitter, or phase jitter expressed in degrees or as a frequency offset.

Jitter is measured with specialized equipment such as time interval analyzers, high-speed oscilloscopes, or dedicated jitter analyzers. Key metrics include RMS jitter, peak-to-peak jitter, and jitter transfer characteristics. In high-speed digital and communications systems, jitter specifications are often given in picoseconds or as a percentage of the clock period, sometimes with separate references for short-term and long-term stability.

Common causes of clock jitter include thermal and flicker noise in oscillators and power supplies, substrate and crosstalk noise, poor PCB layout, aging of components, and disturbances in the clock distribution network. PLLs and frequency synthesizers can also introduce or amplify jitter through their VCO noise, loop noise, and reference disturbances.

The impact of clock jitter depends on the application. Excessive jitter can degrade data integrity in SERDES interfaces, reduce eye opening in high-speed links, and cause timing violations in synchronous systems. In multimedia and measurement equipment, jitter can translate to audible or visible artifacts.

Mitigation strategies focus on reducing noise and improving timing integrity. These include using low-jitter clock sources (OCXO, TCXO, or MEMS oscillators), clean power delivery, robust clock distribution networks with proper impedance control and termination, careful PCB layout, shielding, and, when necessary, jitter-cleaning or jitter-attenuation devices. A jitter budget helps allocate acceptable jitter margins across system components.