Every amplifier adds noise. The first stage of a receiver dominates the total noise figure, which is why satellite dishes put a low-noise amplifier (LNA) right at the antenna.
How to use the Noise Figure Calculator
- Calculate
- Stage 1 noise figure (dB)
- Stage 1 gain (dB)
- Stage 2 noise figure (dB)
- Stage 2 gain (dB)
- Stage 3 noise figure (dB)
- Stage 3 gain (dB)
- Stage 4 noise figure (dB)
- Stage 4 gain (dB)
- Input SNR (dB)
- Output SNR (dB)
- Noise factor F (linear)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the mode and enter the values.
Inputs explained
| Field | What to enter |
|---|---|
| Calculate | Choose one: Cascaded stages (Friis), From input and output SNR (dB), Noise factor ↔ noise figure |
| Stage 1 noise figure (dB) | A number |
| Stage 1 gain (dB) | A number |
| Stage 2 noise figure (dB) | A number |
| Stage 2 gain (dB) | A number |
| Stage 3 noise figure (dB) | A number |
| Stage 3 gain (dB) | A number |
| Stage 4 noise figure (dB) | A number |
| Stage 4 gain (dB) | A number |
| Input SNR (dB) | A number |
| Output SNR (dB) | A number |
| Noise factor F (linear) | A number |
What each result means
- Noise factor (linear) = 10^([Noise figure]÷10)
- Equivalent noise temperature = 290×(10^([Noise figure]÷10) – 1)
- Total gain = [Stage 1 gain (dB)] + [Stage 2 gain (dB)] + [Stage 3 gain (dB)] + [Stage 4 gain (dB)]
Example values
How the main result changes when Stage 1 noise figure changes and everything else stays at the example values above:
| Stage 1 noise figure (dB) | Noise figure (dB) |
|---|---|
| 0.5 | 0.5645 |
| 0.75 | 0.811 |
| 1 | 1.0576 |
| 1.25 | 1.3044 |
| 1.5 | 1.5513 |
| 2 | 2.0458 |
Formula
F_total = F₁ + (F₂ − 1)/G₁ + (F₃ − 1)/(G₁G₂) + … (linear) · NF = 10·log₁₀F · NF = SNR_in − SNR_out (dB)
Worked example
Inputs — Calculate: Cascaded stages (Friis); Stage 1 noise figure (dB): 1; Stage 1 gain (dB): 20; Stage 2 noise figure (dB): 4; Stage 2 gain (dB): 15; Stage 3 noise figure (dB): 8; Stage 3 gain (dB): 10; Stage 4 noise figure (dB): 0; Stage 4 gain (dB): 0.
Results — Noise figure = 1.0576 dB; Noise factor (linear) = 1.27572; Equivalent noise temperature = 79.9598 K; Total gain = 45 dB.
Key terms
- Friis formula
- Total noise factor of cascaded stages.
Frequently asked questions
Why does the first stage matter most?
Later stages' noise is divided by the gain before them.
What is the difference between noise factor and figure?
Figure is the factor in decibels.
What is a good LNA noise figure?
Below 1 dB.
What is noise temperature?
An equivalent way to express noise: T = 290 K × (F − 1).