Skip to main content

Clock jitter on a motherboard

Short answer

Jitter arises from crosstalk, ground bounce, reflections from unterminated traces, and poor decoupling on the clock distribution net. Remedies include routing clocks over a solid ground plane, keeping traces short and guarded, adding series termination, and using adequate decoupling capacitors and matched‑length clock buffers.

A faulty computer motherboard has severe clock jitter. The crystal producing the clock pulses is functioning properly, but clock signals arriving at various motherboard chips suffer from jitter. Discuss the source of the problem and provide some remedies.

Since the crystal is good, the jitter appears after the clock leaves the source, so it is caused by the distribution of the clock on the board. Jitter is the variation of the clock edge from its ideal time, caused by noise adding to the signal near the switching threshold.

Likely sources

  1. Crosstalk: clock traces running parallel to other fast signals pick up noise by capacitive and inductive coupling.
  2. Power supply/ground noise: simultaneous switching causes ground bounce and supply ripple, shifting the threshold crossing time. Poor decoupling worsens this.
  3. Impedance mismatch and reflections: unterminated or long clock traces reflect the wave, and the reflections add to the edge.
  4. Poor ground return / split planes: the clock return current detours, increasing loop area and noise.
  5. EMI from nearby switching regulators or other devices.
  6. Different trace lengths to different chips (skew) and excessive loading.

Remedies

  • Route clocks short, direct, and on a layer next to a solid ground plane.
  • Keep them away from noisy/parallel signals (3W rule) or surround with ground guard traces.
  • Terminate the line (series resistor near the source, or parallel/Thevenin at the end) to match impedance.
  • Add decoupling capacitors (0.1 uF) at each chip and bulk capacitors; use separate analog/digital supplies.
  • Use a clock buffer/PLL with equal-length traces to each chip.
  • Use a ferrite bead/LC filter on the clock supply, and shield if needed.

✓ Answer checked by Claude Sonnet 5.5

Related questions

All Instrumentation II chapter 6 questions →