Noise Figure vs Noise Temperature
Noise figure (NF) and noise temperature (Te) are two equivalent ways to describe how much noise an RF component adds to a signal. They are mathematically related by a simple formula, but each is preferred in different engineering contexts — noise figure in commercial RF design, noise temperature in satellite and radio astronomy work.
Noise Figure (NF)
Noise figure is the degradation of signal-to-noise ratio caused by an RF component, expressed in dB. NF = 10·log₁₀(F) where F is the noise factor (linear). A perfect (noiseless) component has NF = 0 dB.
Advantages
- Intuitive — directly tells you how many dB of SNR are lost
- Standard in commercial datasheets for LNAs, mixers, and amplifiers
- Easy to use in Friis cascade formula for cascaded noise figure
- Directly relates to receiver sensitivity degradation
Disadvantages
- Referenced to 290 K (17°C) — misleading at very low temperatures
- Becomes inaccurate for very low-noise systems (NF < 1 dB)
- Not directly addable in cascaded systems (must convert to linear first)
When to use
Use noise figure for all commercial RF design: LNA selection, receiver chain analysis, link budget calculations, and component datasheets.
Noise Temperature (Te)
Equivalent noise temperature is the temperature of a resistor that would produce the same noise power as the component. A noiseless component has Te = 0 K. Te = T₀(F − 1) where T₀ = 290 K.
Advantages
- More accurate for very low-noise systems (cryogenic LNAs, space receivers)
- Directly addable in cascade: total Te = Te1 + Te2/G1 + ...
- Standard in satellite, radio astronomy, and deep-space communication
- Useful when the reference temperature differs from 290 K
Disadvantages
- Less intuitive — requires understanding of Johnson-Nyquist noise
- Not on commercial datasheets — must convert from NF
- The "temperature" is not the physical temperature of the device
When to use
Use noise temperature for satellite ground stations, radio telescopes, cryogenic receivers, and any system where physical temperature matters or NF < 1 dB.
Key Differences
- ▸NF = 10·log₁₀(1 + Te/290) — both describe exactly the same noise, just scaled differently
- ▸NF is referenced to 290 K; Te is absolute (0 K for noiseless device)
- ▸NF 1 dB = Te ≈ 75 K; NF 3 dB = Te ≈ 290 K; NF 0.1 dB = Te ≈ 7 K
- ▸Noise figure is used in commercial RF; noise temperature in satellite/astronomy
- ▸Noise temperatures cascade additively (after dividing by gain); noise figures do not
Summary
Both metrics describe the same physical phenomenon. Use noise figure for everyday RF design and component selection. Use noise temperature for satellite and deep-space applications, or whenever you need to account for physical temperature effects on system noise. Carry a conversion calculator for quick switching between the two.
Frequently Asked Questions
How do I convert noise figure to noise temperature?
Te = 290 × (10^(NF/10) − 1). Example: NF = 1 dB → Te = 290 × (1.259 − 1) = 75.1 K. Conversely: NF = 10·log₁₀(1 + Te/290).
What noise figure is "good" for an LNA?
For 2.4 GHz WiFi/cellular, 1–3 dB NF is typical. High-performance LNAs achieve < 1 dB. Cryogenic LNAs for radio astronomy achieve NF < 0.1 dB (Te < 7 K). The first stage dominates — Friis formula shows that subsequent stages add relatively little noise when G1 is large.
Why is 290 K used as the reference temperature?
290 K (≈ 17°C) was chosen by IEEE as a standard reference temperature for noise figure measurements — close to room temperature. This makes NF measurements reproducible across labs. The actual noise contributed depends on both the NF and the physical temperature of the source.
Does physical temperature of an LNA affect its noise figure?
Yes, but differently than you might expect. The shot noise and thermal noise of the active device depend on physical temperature, and NF can decrease when an LNA is cooled. Cryogenic LNAs cooled to 15–20 K achieve noise temperatures of 5–15 K. The 290 K reference is just for defining NF, not the operating temperature.