Thermal Noise
Understanding Thermal Noise
Thermal noise is the ultimate noise floor that no receiver can surpass (without cooling). It is present in every resistor, conductor, and semiconductor, generated by the random motion of electrons at any temperature above 0 K. All other noise contributions (shot noise, flicker noise) add to this irreducible floor.
Thermal Noise Properties
- Flat spectrum (white noise): Equal power per Hz across all frequencies (up to THz range).
- Gaussian distribution: Instantaneous voltage follows a Gaussian probability distribution.
- Temperature-dependent: Noise power proportional to temperature. Cryogenic cooling reduces thermal noise.
- Bandwidth-dependent: Total noise power proportional to measurement bandwidth.
N = kTB (watts)
k = 1.381 x 10^-23 J/K (Boltzmann)
T = 290K (standard reference)
Noise power density at 290K:
N0 = kT = -174 dBm/Hz
Total noise in bandwidth B:
N (dBm) = -174 + 10 log10(B_Hz)
1 MHz bandwidth: N = -174 + 60 = -114 dBm
1 GHz bandwidth: N = -174 + 90 = -84 dBm
Frequently Asked Questions
What is thermal noise?
Thermal noise is electromagnetic noise from random electron motion in any conductor above absolute zero. Its power density of -174 dBm/Hz (at 290K) is the fundamental limit of receiver sensitivity. It cannot be eliminated, only reduced by cooling.
Why is -174 dBm/Hz important?
It is the absolute noise floor at room temperature. Every receiver sensitivity calculation starts here: sensitivity = -174 + 10log(BW) + NF + required SNR. No room-temperature receiver can detect signals below this noise floor.
How does bandwidth affect thermal noise?
Total thermal noise power = kTB. Doubling the bandwidth doubles the noise power (+3 dB). Narrower bandwidth = lower total noise = better sensitivity. This is why narrowband receivers are more sensitive than wideband receivers.