Noise Figure Calculator

The cascaded noise figure is a single dB value that represents the total noise degradation introduced by a chain of RF amplifier or component stages. It is calculated using the Friis formula, which accounts for each stage's individual noise figure and the cumulative gain of all preceding stages. The first stage dominates because subsequent stages are divided by the gains before them.

Noise factor (F) is the linear ratio of input SNR to output SNR (dimensionless). Noise figure (NF) is the same quantity expressed in decibels: NF = 10 × log₁₀(F). A perfect, noiseless device has a noise factor of 1 and a noise figure of 0 dB. Both measure how much a device degrades the signal-to-noise ratio.

The Friis formula calculates total noise factor as: F_total = F₁ + (F₂−1)/G₁ + (F₃−1)/(G₁×G₂) + …, where F values are linear noise factors and G values are linear gains. The total noise figure is then NF_total = 10×log₁₀(F_total). This tool handles all the conversions and summations automatically.

In the Friis formula, the noise contribution of each subsequent stage is divided by the cumulative linear gain of all preceding stages. If the first stage has high gain, it suppresses the noise contributions from later stages significantly. This is why low-noise amplifiers (LNAs) are placed at the front of an RF receive chain.

For most RF receiver front-ends, a noise figure below 3 dB is considered good, and below 1 dB is excellent for LNAs in demanding applications like satellite communications or radio astronomy. Consumer Wi-Fi and cellular systems typically operate with NF values between 3–8 dB. The acceptable value depends heavily on the link budget and application.

In passive circuits with physical temperature below the reference temperature (290 K), the noise figure can theoretically be negative in dB. However, for active amplifiers operating at room temperature, noise figure is always ≥ 0 dB. A negative noise figure would imply the output SNR is better than the input SNR, which does not occur in standard amplifier stages.

Higher gain in early stages reduces the relative contribution of later-stage noise to the total. Conversely, if an early stage has low gain or even loss (e.g., a filter or cable), it amplifies the noise impact of the next stage. Total gain is simply the sum of all stage gains in dB, which equals the product of linear gains.

Noise figure = SNR_in − SNR_out = 40 dB − 35 dB = 5 dB. This is the direct definition: NF measures by how many decibels the device degrades the signal-to-noise ratio as the signal passes through it.