Cascade Noise Figure Calculator
Calculate total system Noise Figure (NF) and Gain using Friis formula for cascaded stages.
Stage 1
Stage 2
Stage 3
Friis Formula Cheat Sheet
Engineering Principles: Friis Formula for Noise
The total noise factor of a cascaded system is dominated by the first stage. Friis' formula calculates the total noise factor by summing the noise contribution of each stage divided by the cumulative gain of preceding stages.
F and G are linear ratios (not dB). F = 10^(NF/10).
Or in dB: G_total(dB) = G1(dB) + G2(dB) + ...
The standard metric displayed in component datasheets.
Technical Notes
- •LNA (Low Noise Amplifier) must be the first stage to minimize system NF.
- •Lossy components (filters, cables) have a Noise Figure equal to their Insertion Loss (in dB).
- •A high gain first stage reduces the noise contribution of subsequent stages.
Frequently Asked Questions
Why is the first stage most important for Noise Figure?
The Friis formula shows that the noise contribution of any stage is divided by the gain of all preceding stages. Therefore, noise from the second, third, and subsequent stages is 'suppressed' by the gain of the first stage. This is why a high-gain, low-noise amplifier (LNA) is always placed first in a receiver chain.
Can a passive component (like a cable) have gain?
No, passive components have a gain of less than 1 (represented as negative dB, or attenuation). For passive lossy components at room temperature, the Noise Figure is numerically equal to the Insertion Loss in dB. For example, a cable with a 3dB loss contributes 3dB to the noise figure calculation.
What is a 'good' Noise Figure?
It depends entirely on the application. For deep-space communications (GPS, astronomy), a Noise Figure < 1dB is critical. For cellular systems, 2-3dB is common. For cheap consumer Wi-Fi, 4-6dB might be acceptable. Lower is always better.