How do I convert between noise figure in dB and equivalent noise temperature in Kelvin?
Noise Figure and Noise Temperature Conversion
Noise figure and noise temperature are two representations of the same physical quantity: the noise added by a component or system. The conversion between them is exact, lossless, and always valid. The standard reference temperature T0 = 290 K (approximately 17°C or 62°F) serves as the bridge between the two scales.
The conversion is nonlinear. At low noise figures (below 1 dB), small changes in NF correspond to large changes in noise temperature. A change from 0.3 dB to 0.5 dB NF represents a noise temperature increase from 20.4 K to 35.4 K, a 73% increase. At higher noise figures, the relationship compresses: going from 6 dB to 7 dB NF changes the noise temperature from 865 K to 1163 K, a 34% increase.
This nonlinearity is precisely why satellite and radio astronomy engineers prefer noise temperature. When the system antenna temperature is 30 K, the difference between a 0.3 dB and 0.5 dB LNA is the difference between a 50 K and 65 K system noise temperature, a 30% degradation in sensitivity. Noise figure masks this impact behind seemingly small decimal differences.
NF = 10 × log₁₀(1 + Te/290) [dB]
Quick references:
0.5 dB → 35.4 K | 1.0 dB → 75.1 K | 3.0 dB → 288.6 K
Frequently Asked Questions
Is 290 K exactly room temperature?
No. 290 K (16.85°C) was chosen by the IEEE as the standard reference temperature for noise calculations. Actual room temperature is closer to 295-300 K, but 290 K is the universal standard used in all noise figure specifications and measurements.
Can noise temperature be negative?
No. A noise temperature of 0 K means the component adds no noise (perfect, noiseless device). Negative noise temperatures have no physical meaning in this context. The minimum noise figure is 0 dB, corresponding to Te = 0 K.
Which should I put on a datasheet?
Industry convention uses noise figure (NF in dB) for component specifications. Noise temperature is used in system-level specifications, particularly for satellite and radio astronomy receivers where G/T is the key figure of merit.