Thermal noise power (also known as Johnson-Nyquist Noise) is a function of
- Temperature T
- Bandwidth B
- Boltzmann’s Constant k = 1.380649 × 10-23 m2 kg s-2 K-1
kTB Noise Power Formula
Thermal noise power is calculated using the following:
Pn = 10 * log10 (kTB/1mW)
The units are dBm. If the bandwidth selected is 1 Hz, then the unit for noise power is dBm/Hz.
The temperature has to be higher than 0 Kelvin
Noise Power Example Calculation
At room temperature (27oC) the noise power in a 1 Hz bandwidth is -173.83 dBm/Hz.
Noise power increases with temperature and bandwidth.
🔎 Consider a Radio Receiver or Spectrum Analyzer that is set to 1 Hz resolution bandwidth (RBW). The noise power level is the threshold below which no signal can be detected. In practice, the threshold is higher by about 10 dB or more depending on the noise figure of the receiver. This calculator takes the noise figure into account to determine the Noise floor of a radio receiver.