PCB Via Stub Resonance Calculator
Calculate PCB via stub length, stub resonant frequency that causes signal notch, and frequency improvement from back-drilling.
Formula
L_{stub} = T_{pcb}\left(1-\frac{N_{layer}}{N_{total}}\right),\quad f_{res} = \frac{v_p}{4 L_{stub}}
Reference: Eric Bogatin, "Signal and Power Integrity Simplified" 3rd ed.
How It Works
Via stub resonance is a critical signal integrity phenomenon in printed circuit board (PCB) design where electromagnetic waves reflect and interact within via structures. The stub length calculation considers board thickness, layer position, and total layer count, which influences the transmission line's electrical characteristics. Understanding stub resonance helps engineers predict and mitigate potential signal degradation and unwanted reflections.
Worked Example
Problem: Calculate via stub resonance for a 6-layer PCB with 1.6mm total thickness, located on layer 3 Solution: 1. Stub length = 1.6mm × (1 - 3/6) = 0.8mm 2. Assuming εr = 4.2 (typical FR-4), c = 3×10^8 m/s 3. Resonant frequency: fr = (3×10^8)/(4 × 0.0008 × √4.2) ≈ 5.3 GHz
Practical Tips
- ✓Use backdrilling to minimize stub-induced reflections
- ✓Choose via placement carefully to reduce resonance effects
- ✓Validate stub length calculations with simulation tools
Common Mistakes
- ✗Ignoring layer position when calculating stub length
- ✗Neglecting dielectric constant in resonant frequency calculation
- ✗Not considering backdrill techniques for stub removal
Frequently Asked Questions
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