Metric · Analog, RF & Mixed-Signal
Noise figure
The decibel measure of how much a component degrades the signal-to-noise ratio of a signal passing through it, referenced to a source at 290 K. Cascaded stages combine by Friis's formula, giving the first stage outsized importance.
- Also known as
- NF, Noise factor (linear form)
- Type
- Metric
- Domain
- Analog, RF & Mixed-Signal, RF characterisation
Notes
A passive lossy element's noise figure equals its loss in decibels, so every filter ahead of the LNA costs sensitivity directly.
Overview
Noise figure is a measure, in decibels, of how much a circuit such as an amplifier, mixer or receiver worsens the signal-to-noise ratio of a signal passing through it. A perfect, noiseless circuit would have a noise figure of 0 dB; every real one adds some noise of its own. It is one of the headline specifications of any radio receiver chain and of the low-noise amplifier at its front.
How it works
The underlying quantity is the noise factor, F: the signal-to-noise ratio at the input divided by the signal-to-noise ratio at the output, as a plain power ratio. Noise figure is ten times the base-ten logarithm of F. Because the result depends on how much noise arrives at the input, the definition assumes a source at a standard temperature, T0, of 290 K. Noise factor can also be written through an effective noise temperature, Te, as F = 1 + Te/T0, which is often more convenient for very low-noise systems such as satellite receivers that look at cold sky.
In a chain of stages, the total noise factor follows the Friis formula: the first stage contributes its full noise factor, and each later stage's excess noise is divided by the combined gain of the stages before it. A low-noise first stage with good gain therefore sets most of the system's noise figure, which is why receivers start with a low-noise amplifier, and why loss placed before that amplifier, such as a filter or cable, adds directly to the noise figure.
Noise figure is measured with dedicated instruments against calibrated noise references. Because the quantities involved are tiny, careful calibration and correction for the instrument's own noise are part of every measurement.
Examples
- A low-noise amplifier datasheet quoting its noise figure in decibels at each frequency band it supports.
- A receiver budget showing how a lossy filter placed before the LNA raises the total noise figure by roughly its own loss.
- A characterisation engineer measuring an LNA across frequency and temperature with a noise figure analyser.
- A satellite ground receiver specified by noise temperature in kelvin rather than noise figure, because its antenna sees sky much colder than 290 K.
Why it matters
Noise figure largely decides how weak a signal a receiver can pick up, which sets range, data rate and battery life in phones, Wi-Fi, radar and satellite links. LNA and RF front-end designers work to minimise it, RF systems architects budget it across the chain, and characterisation and test engineers measure it on every new device. NIST notes that industry's two most requested noise parameters are the noise temperature of a source and the noise figure of an amplifier.
Related terms
Part of the Analog, RF & Mixed-Signal glossary, with every term in the topic and the lifecycles, tools and standards that use them.
Where it appears on the Hub
Used on 11 pages of the Hub. In each group, the pages that mention it most come first.
Lifecycle stages and gates
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Last reviewed . Written by The Semiconductor Hub editorial team. The full glossary