Thermal kT Noise Primer
The thermal noise power in 50-ohm system with a 1 Hz bandwidth has a minimum value given by:
kT = 4.14 x 10-21 Watts (-174 dBm)
where the parameters have the following values:
k = Boltzmann's constant (1.38 x 10-23 J/K),
T = temperature (300 K),
The plot shows the typical noise floor level of a microwave spectrum analyzer with the thermal kT noise rising as a function of frequency. The frequency markers indicate that this spectrum analyzer has a Noise Figure of 44 dB at 10 GHz, 50 dB at 15 GHz and 52 dB at 20 GHz.