SNR Budgeting

After you've gained a little knowledge about cables, you will be sorely tempted to scratch out a rough signal attenuation budget on a napkin, add up the worst-case numbers , and declare that a certain system will or will not function. Be very careful with these calculations. Most communications product designers discover additional deleterious factors late in their design that weren't accounted for in the original SNR budget.

Always calculate the full amount of attenuation and crosstalk you expect from connectors, jumper cables, work area cables, equipment cables, chip packaging, board layout, receiver bandwidth, transmitter risetime, jitter, and anything else you can imagine. Then compute the reflections expected from all the connectors and cable junctures, including an allottment for structural return noise. After you've done all that, add another 2-dB margin for copper -based systems and a 3-dB power margin for fiber. The extra margin will cover you later when you discover other deleterious factors you forgot in the first-pass budget.


  • Don't underestimate the complexity of proper SNR budgeting.


Transmission Line Parameters

Performance Regions

Frequency-Domain Modeling

Pcb (printed-circuit board) Traces

Differential Signaling

Generic Building-Cabling Standards

100-Ohm Balanced Twisted-Pair Cabling

150-Ohm STP-A Cabling

Coaxial Cabling

Fiber-Optic Cabling

Clock Distribution

Time-Domain Simulation Tools and Methods

Points to Remember

Appendix A. Building a Signal Integrity Department

Appendix B. Calculation of Loss Slope

Appendix C. Two-Port Analysis

Appendix D. Accuracy of Pi Model

Appendix E. erf( )


High-Speed Signal Propagation[c] Advanced Black Magic
High-Speed Signal Propagation[c] Advanced Black Magic
ISBN: 013084408X
Year: 2005
Pages: 163 © 2008-2020.
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