Glossary term
Noise Figure
A receiver or component metric describing how much internal noise degrades signal-to-noise ratio.
Definition
metricNoise figure measures how much a component, receiver, or network degrades signal-to-noise ratio compared with an ideal noiseless device.
Noise figure expresses the degradation of signal-to-noise ratio caused by a component, receiver chain, or network, referenced to a standard input noise temperature. It is a key metric for low-noise amplifiers, radio receivers, radar front ends, optical receivers, and any system where weak signals must be detected above thermal and electronic noise.
Noise figure measures how much a device or receiver chain worsens the signal-to-noise ratio. The linear quantity is noise factor:
Noise figure is the same degradation expressed in decibels:
An ideal noiseless component has F=1 and NF=0 dB. Real amplifiers, mixers, filters, cables, and detectors add noise or attenuate signal, so their noise figure is positive.
Receiver-chain significance
The first active stage of a receiver is especially important. In a cascade, the noise contribution of later stages is divided by the gain of earlier stages, so a low-noise amplifier placed close to the antenna or sensor can dominate overall sensitivity. Loss before the first amplifier, such as cable loss, switch loss, filter insertion loss, or impedance mismatch, directly worsens system noise figure.
Noise figure is tied to bandwidth and reference temperature. Standard RF definitions commonly use a source temperature of 290 K. Equivalent noise temperature is often used in radio astronomy, satellite links, and radar:
where T_0 is the reference temperature. This form is useful when combining antenna temperature, sky noise, receiver noise, and propagation effects in a link or radar budget.
Measurement and interpretation
Noise figure can be measured with calibrated noise sources, spectrum analyzers, noise figure analyzers, or Y-factor methods. The result depends on source impedance, frequency, gain compression, measurement bandwidth, temperature, and whether the device is operated in its intended bias condition. For mixers and frequency converters, image noise and conversion loss must also be handled correctly.
Common mistakes
A common mistake is to compare noise figure values without frequency, bandwidth, gain, impedance, and operating temperature. Another is to ignore losses before the first low-noise stage. A strong receiver review checks the full cascade, source match, filter losses, gain distribution, dynamic range, calibration uncertainty, and whether the quoted noise figure is typical, maximum, or measured on the actual hardware.