How to Achieve a Robust PCB Design Workflow for Signal Integrity?
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Signal Integrity (SI) signifies the signal’s ability to propagate along PCB traces without distortion. Signal integrity is about the quality of the signal passing through a transmission line. In this article, Tim Wang Lee acknowledges some of the designers’ pain points regarding robust PCB design workflow and signal integrity.
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About the presenter:
Tim Wang Lee, signal integrity application scientist, Keysight Technologies.
Q1: How to achieve a robust PCB design?
Tim Wang Lee: To achieve a robust PCB design workflow consider the following points:
Understand possible root causes of signal integrity issues.
Create a virtual prototype using Electronic Design Automation (EDA) software.
Rely on a dependable manufacturer for layout, fabrication, and assembly and that will be Sierra Circuits .
Q2: What is the difference between traditional design and the robust design approach?
Tim: In a traditional design, we fabricate the prototype once we create a design and layout based on our vendor guidance. In the case of a robust design approach, we create a virtual prototype, simulate and validate the prototype after creating the design.
Q3: What are the advantages of having a virtual prototype?
Tim: A virtual prototype helps us to troubleshoot and understand the design in an efficient manner. Investing a little more time in creating a virtual prototype gives you a lot in return. It also helps us to achieve better signal integrity. The advantages are summarized in the below image.
The effort is a little higher, but the ability of troubleshooting and performing what-if-analysis increases.
Q4: What is the robust design workflow for signal integrity?
Tim: The signal integrity workflow is as follows:
Rules of thumbs estimations – this addresses signal integrity issues
Create a virtual prototype
3D extraction
Perform what-if-analysis
The workflow diagram is as shown below:
Q5: What are the rules of thumbs to be followed?
Tim: The rules of thumbs are as follows:
8-7-3 rule:
8dB loss at Nyquist, Eye with 75% UI jitter, 30% height
0.1-0.2 dB/inch/GHz:
0.1-0.2 dB of loss per inch per GHz for transmission line
Single-ended 50 ohms (FR4)
Microstrip impedance: Width/Height = 2 – width of the trace is twice the substrate height.
Stripline impedance: Width/Height = 1 – width of the trace is the same as min of the substrate heights.
Differential 100 ohms (FR4):
Spacing = 3W uncoupled ; Differential impedance = 2 times single-ended impedance.
Move the traces closer, this decreases differential impedance.
Stub resonance (GHz) for FR4 is 1.5/stub length (inches)...