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.

POINT TO REMEMBER

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