AN 672: Transceiver Link Design Guidelines for High-Gbps Data Rate Transmission

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ID 683624
Date 1/29/2020
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Document Table of Contents
Document Table of Contents x
1. AN 672: Transceiver Link Design Guidelines for High-Gbps Data Rate Transmission
1. AN 672: Transceiver Link Design Guidelines for High-Gbps Data Rate Transmission x
1.1. PCB Material Selection 1.2. Stackup Design 1.3. Channel Design 1.4. Summary 1.5. Document Revision History for the AN 672: Transceiver Link Design Guidelines for High-Gbps Data Rate Transmission
1.1. PCB Material Selection x
1.1.1. Loss Tangent and Dissipation Factor 1.1.2. Dielectric Constant 1.1.3. Fiberglass Weave 1.1.4. Copper Surface Roughness
1.3. Channel Design x
1.3.1. BGA Channel Breakout 1.3.2. Channel Routing Design 1.3.3. Edge Coupling 1.3.4. Broadside Coupling 1.3.5. Transparent Via Design 1.3.6. Blocking Cap Optimization 1.3.7. Connectors Optimization
1.3.2. Channel Routing Design x
1.3.2.1. Trace Width Selection 1.3.2.2. Loose vs. Tight Coupled Traces 1.3.2.3. Crosstalk Control
1.4. Summary x
1.4.1. PCB Material Selection 1.4.2. Stackup Design 1.4.3. Channel Design 1.4.4. References
1. AN 672: Transceiver Link Design Guidelines for High-Gbps Data Rate Transmission

1.3.2.3. Crosstalk Control

Crosstalk is induced noise current resulting from mutual capacitive (Cm) and mutual inductive (Lm) coupling on a victim trace due to switching activity from nearby aggressor trace or traces. The current coupled from Cm travels along the victim trace in both the forward and reverse direction with the same polarity. Similarly, the current from Lm travels forward and backwards in opposite polarity. As a result, crosstalk can be separated into two distinct components referred to as near-end crosstalk (NEXT) and far-end crosstalk (FEXT). In NEXT, the coupled noise current is the sum of the induced currents from Cm and Lm as those currents are the same polarity. Conversely, for FEXT, the current is the difference of Cm and Lm due to the polarity difference. For signals entirely contained within a homogeneous dielectric material (such as stripline), the capacitive and inductive forward crosstalk are equal and cancel. For non-homogeneous dielectrics (such as microstrip), the inductive component tends to be larger and the resulting coupled noise is negative.

Figure 17. NEXT and FEXT Coupling Components


Crosstalk control usually involves reducing signal edge rates and maintaining enough trace-to-trace separation to reduce the mutual capacitive and mutual inductive coupling energy. In high-speed transceiver designs running at many gigabits per second, reducing the signal edge rate is usually not an option since the unit interval time (UI) is very small. Therefore, crosstalk control for high-speed transceiver designs is mainly determined by PCB layout spacing constraints to keep the transceiver traces far enough apart to minimize the coupling effect. For very high-speed traces, it is desirable to keep the coupling noise to less than 1% of the source signal if possible.

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