KreativeKommit LogoKreativeKommit
Signal Integrity
Learn2026-01-14

Signal Integrity

#electronics#signal-integrity#si

Overview

Signal integrity is critical when signal rise-times are comparable to trace propagation delays. This chapter covers transmission lines, reflections, proper termination, and crosstalk mitigation for both digital and analog signals.

Prerequisites

  • Basic PCB layout knowledge and oscilloscope use

Learning objectives

  • Recognise when traces behave as transmission lines and calculate characteristic impedance
  • Apply termination strategies to avoid reflections
  • Reduce crosstalk and measure SI problems with an oscilloscope or TDR

Tools & materials

  • Oscilloscope, TDR or simulation tool, high-speed probe, PCB stackup information

Hands-On Mini Task

  1. On a test board or simulation, create a long trace with a fast edge source and observe reflections with and without proper termination.
  2. Measure eye diagrams or signal overshoot and compare results after adding series termination or improving route geometry.

Expected result: observable reduction in ringing and cleaner signal edges after proper termination and routing changes.

Transmission lines and when they matter

  • A trace behaves as a transmission line when the signal rise time is comparable to the propagation delay across the trace (rule of thumb: trace length > tr/ (2 × propagation velocity)).
  • Characteristic impedance Z0 depends on geometry and stackup — calculate or use PCB toolset to extract.

Termination strategies

  • Series termination: simple resistor near source to match source impedance to line.
  • Parallel termination: resistor at load to match line impedance to avoid reflections.

Crosstalk and mitigation

  • Keep aggressive spacing between noisy and sensitive traces; use ground fills and stitched vias to reduce coupling.
  • Differential routing reduces common-mode coupling but requires matched lengths and spacing.

Measurement techniques

  • Use a high-bandwidth scope and proper probes; probe grounding can introduce artefacts — use short ground spring tips when possible.
  • TDR can reveal impedance discontinuities; use it to validate routing and connector behaviour.

Worked example — series termination

  1. Create a 50 Ω trace and drive it from a 25 Ω source with a 50 Ω load. Add a series resistor at the source to reduce reflections and observe waveform cleanliness.

Troubleshooting

  • If you see persistent ringing after termination, check for multiple impedance discontinuities (via stubs, connector transitions).
  • Large ground pours without stitching can create resonances; stitch with vias near critical nets.

Navigation