Noise Power
Understanding Noise Power
Thermal noise (also called Johnson-Nyquist noise) is generated by the random thermal motion of electrons in any conductor at a temperature above absolute zero. It is present in every resistor, cable, and component, and cannot be eliminated except by cooling. The noise power available from a matched source at temperature T in bandwidth B is precisely kTB watts.
The kTB Formula
k = 1.38 x 10^-23 J/K (Boltzmann's constant), T = temperature in Kelvin, B = bandwidth in Hz. At T = 290K (standard reference temperature), kT = 4.00 x 10^-21 W/Hz = -174 dBm/Hz.
Noise Power in Practice
- The -174 dBm/Hz figure is the starting point for every link budget and sensitivity calculation.
- In a 1 MHz bandwidth: N = -174 + 60 = -114 dBm
- In a 100 MHz bandwidth: N = -174 + 80 = -94 dBm
- System noise figure adds directly to this: N_system = kTB x F = -174 + 10log(BW) + NF
N = k x T x B (Watts)
N (dBm) = -174 + 10 log10(B_Hz) (at 290K)
Boltzmann constant: k = 1.38 x 10^-23 J/K
Reference temperature: T0 = 290K (16.85 C)
Examples at 290K:
1 Hz BW: -174 dBm
1 kHz BW: -144 dBm
1 MHz BW: -114 dBm
1 GHz BW: -84 dBm
Frequently Asked Questions
What is kTB noise?
kTB is the thermal noise power formula: k (Boltzmann's constant) x T (temperature in Kelvin) x B (bandwidth in Hz). At room temperature (290K), this equals -174 dBm per Hz of bandwidth. It represents the fundamental noise floor of any electronic system.
Can noise be lower than -174 dBm/Hz?
Yes, by cooling below 290K. At 4K (liquid helium), the noise floor drops to ~-192 dBm/Hz. Radio telescopes and quantum computers use cryogenic cooling to achieve noise levels far below the room-temperature floor. However, -174 dBm/Hz is the standard reference.
How does bandwidth affect noise?
Noise power is proportional to bandwidth: doubling bandwidth doubles noise power (adds 3 dB). A 1 MHz receiver has 30 dB more noise power than a 1 kHz receiver. This is why narrowing bandwidth is an effective way to improve sensitivity.