Table of Contents
What Is Noise Figure?
Noise figure (NF) is a measure of the degradation of signal-to-noise ratio (SNR) caused by a component in a signal chain. It is defined as the ratio of the input SNR to the output SNR, expressed in decibels. A perfect noiseless component has a noise figure of 0 dB. Real components always add some noise, resulting in NF > 0 dB.
Noise figure is critical in radio receiver design, satellite communications, radar systems, and any application where weak signals must be detected. The first stage in a receiver chain (typically a low-noise amplifier) dominates the overall system noise figure, which is why LNAs with very low noise figures are essential.
Friis Formula for Cascaded Stages
All values in the Friis formula use linear (not dB) quantities. This shows that the first stage noise figure dominates when G1 is large.
Design Considerations
| Component | Typical NF |
|---|---|
| Low-Noise Amplifier (LNA) | 0.3-2 dB |
| Cable/connector loss | Equal to loss in dB |
| Mixer | 5-8 dB |
| Filter (passive) | Equal to insertion loss |
Frequently Asked Questions
Why should the LNA be the first component?
The Friis formula shows that each subsequent stage's noise contribution is divided by the cumulative gain of all preceding stages. By placing a high-gain, low-noise amplifier first, the noise from later stages (mixer, IF amplifier) becomes negligible. Any loss before the LNA directly adds to the system noise figure.
What is noise temperature?
Noise temperature is an alternative way to express noise performance. A component with noise figure NF has equivalent noise temperature Te = 290(NF-1) Kelvin. This is the temperature of a resistor that would generate the same amount of noise. Lower noise temperatures mean better performance.