What Is Noise Figure?

Noise figure is a measure of the degradation in signal-to-noise ratio (SNR) introduced by an electronic component or system as a signal passes through it. It quantifies how much additional noise a device adds to the signal beyond what was present at its input, expressed in decibels. A theoretically perfect, noiseless amplifier would have a noise figure of 0 dB, meaning the SNR at its output equals the SNR at its input. Real components always add some noise, so practical noise figures are positive values, typically ranging from less than 1 dB for cryogenically cooled low-noise amplifiers to 10 dB or more for general-purpose circuits.

The concept was introduced by Harald Friis at Bell Labs in a 1944 paper and remains a central figure of merit in radio-frequency, microwave, and optical receiver design. Noise figure is defined in terms of the noise factor F, which is the ratio of the input SNR to the output SNR under standard conditions of a 290 K source temperature: NF = 10 log10(F). The IEEE standard definition specifies this 290 K reference temperature to allow consistent comparison across devices and measurement setups.

The Friis Cascade Formula

When multiple components are connected in series, the total system noise figure is not simply the sum of individual noise figures. The Friis formula for cascaded stages gives the total noise factor as F_total = F1 + (F2 - 1)/G1 + (F3 - 1)/(G1 * G2) + ..., where F1, F2, F3 are the noise factors of successive stages and G1, G2 are their available power gains. The key insight is that the noise contribution of each successive stage is divided by the cumulative gain of all preceding stages. This means the first stage in a receiver chain dominates the total system noise figure, and placing a high-gain, low-noise amplifier (LNA) as early in the chain as possible is the standard strategy for achieving low overall noise. Microwaves101's treatment of Friis noise figure theory covers both the Friis definition and the IEEE alternative formulation, which can differ for two-port networks under non-standard termination conditions. The same cascade principle applies to lossy passive elements: an attenuator or filter with a 3 dB insertion loss contributes a noise figure equal to that 3 dB loss.

Measurement Methods

Noise figure is typically measured using the Y-factor method or the signal-generator method. In the Y-factor method, a calibrated noise source is switched between a hot state and a cold state, and the ratio of the output noise power in the two states yields the device's noise figure after accounting for the excess noise ratio (ENR) of the source. Automated noise figure analyzers implement this process in hardware. The Analog Devices system noise figure analysis guide describes how these measurements are applied at the system level for modern radio receivers, including the effects of image rejection, mixer conversion loss, and gain compression on the effective noise performance. For radar applications, US Department of Energy technical documentation on noise figure for radar receivers provides measurement and analysis methodology suited to high-frequency systems.

Applications

Noise figure has applications in a wide range of fields, including:

• Radio-frequency and microwave receiver design, where low noise figure determines sensitivity and detection range
• Satellite and deep-space communication links, where the link budget depends critically on ground station receiver noise performance
• Radar systems, where noise figure sets the minimum detectable signal and therefore clutter rejection capability
• Cellular base-station front ends, where LNA noise figure affects coverage area
• Radio astronomy and scientific instrumentation, where sub-1 dB noise figure cryogenic amplifiers enable detection of extremely weak signals