What is the difference between edge coupled and broadside coupled stripline?
Stripline Coupling Configurations
In edge-coupled stripline, two parallel traces on the same inner layer are separated by a lateral gap. The coupling depends on the gap-to-height ratio (S/b where b is the total ground-to-ground spacing). Typical 100 Ω differential pairs use S/b ≈ 0.5-1.0. The traces are symmetric with respect to the ground planes, providing excellent balance for differential signaling.
| Parameter | Semi-Rigid | Conformable | Flexible |
|---|---|---|---|
| Loss (dB/m at 10 GHz) | 0.8-2.5 | 1.0-3.0 | 1.5-5.0 |
| Phase Stability | Excellent | Good | Fair |
| Bend Radius | Fixed after forming | Hand-formable | Continuous flex OK |
| Shielding (dB) | >120 | >90 | >60-90 |
| Cost (relative) | 2-5x | 1.5-3x | 1x |
Frequently Asked Questions
Which is better for high-speed digital?
Edge-coupled is almost universally preferred for high-speed digital routing (PCIe, DDR, USB 3.0) because of its superior manufacturing tolerance sensitivity and proven design rules. Broadside-coupled is used only when routing density requires it.
Can I design couplers with stripline?
Yes. Stripline coupled-line couplers are widely used for 3-6 dB directional couplers. Edge-coupled stripline couplers achieve 3-10 dB coupling. Broadside-coupled stripline achieves tighter coupling (3-6 dB) due to the stronger electromagnetic coupling between vertically stacked traces.
What about common-mode rejection?
Both configurations provide good common-mode rejection if the pair is symmetric. Edge-coupled pairs have inherent symmetry (both traces see identical ground plane distances). Broadside-coupled pairs place one trace closer to one ground plane, which can create slight asymmetry and reduce common-mode rejection unless the stackup is carefully balanced.