AdvancedTopic 96 of 104Electronics & Embedded
Signal integrity
High-speed traces need impedance control and termination to avoid reflections.
In depth
At high speed, traces behave as transmission lines: control impedance (50 Ω single-ended, 100 Ω differential), terminate to avoid reflections, keep return paths continuous, match lengths and minimise crosstalk. Eye diagrams assess quality; simulation (IBIS, SPICE) predicts issues.
Key points
- Rise time versus trace length
- Via stubs
- Termination types
ExampleDDR memory lines are length-matched within a few mils.
At high speed, PCB traces behave like transmission lines, so impedance control and termination decide whether signals arrive intact.
What is Signal integrity?
High-speed traces need impedance control and termination to avoid reflections.
Why is it important to understand signal integrity?
At high speed, PCB traces behave like transmission lines, so impedance control and termination decide whether signals arrive intact.
When does a trace need signal integrity analysis?
When signal rise times are short compared with the propagation delay, which is a common rule of thumb for traces longer than about a sixth of the rise time distance.
What is termination?
Adding a resistor at the end of a line to match its characteristic impedance, which absorbs the wave and prevents reflections.
How is Signal integrity used in practice?
Typical applications include routing a 1 GHz differential pair with controlled impedance, adding a series resistor to damp reflections on a fast clock line and designing the stack-up of a multilayer board for high-speed memory.
What should I learn before Signal integrity?
Study "Design for testability" first (topic 95 of 104 in Electronics & Embedded). It covers the groundwork this topic builds on.
What are the key points of Signal integrity?
Rise time versus trace length. Via stubs. Termination types.
Can you give an example of Signal integrity?
DDR memory lines are length-matched within a few mils.
What comes after Signal integrity?
The next topic is "Electromagnetic interference mitigation", which builds directly on this one.
How difficult is Signal integrity?
It is classed as Advanced in Electronics & Embedded (topic 96 of 104). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Electronics & Embedded.