Noise margin

In electrical engineering, noise margin is the amount by which a signal exceeds the minimum amount for proper operation. It is commonly used in at least two contexts:

• In communications system engineering, noise margin is the ratio by which the signal exceeds the minimum acceptable amount. It is normally measured in decibels.
• In a digital circuit, the noise margin is the amount by which the signal exceeds the threshold for a proper '0' or '1'. For example, a digital circuit might be designed to swing between 0.0 and 1.2 volts, with anything below 0.2 volts considered a '0', and anything above 1.0 volts considered a '1'. Then the noise margin for a '0' would be the amount that a signal is below 0.2 volts, and the noise margin for a '1' would be the amount by which a signal exceeds 1.0 volt. In this case noise margins are measured as an absolute voltage, not a ratio. Noise margins for CMOS chips is usually much greater than TTL because the V_{OH min} is closer to the power supply voltage and V_{OL max} is closer to zero.

In simple words, Noise margin (in circuits) is the amount of noise that a circuit can withstand. Noise margins are generally defined so that positive values ensure proper operation, and negative margins result in compromised operation, or perhaps outright failure.

See also

• Digital circuit
• Signal integrity
• Substrate coupling
• ITU G.992.1
• signal-to-noise ratio

External links

• DMT, a DSL monitoring and downstream noise margin tweaking program.