Noise Floor
Understanding the Noise Floor
Every receiver has a noise floor that sets the absolute limit on sensitivity. The thermal noise floor of a 50-ohm system at room temperature (290K) is -174 dBm/Hz. This is a fundamental physical constant set by Boltzmann's constant and temperature. The total noise floor in a given bandwidth is -174 + 10 log10(BW) + NF.
Calculating the Noise Floor
The noise floor in dBm for a given bandwidth and noise figure is: N_floor = -174 + 10 log10(BW_Hz) + NF_dB. For a 1 MHz bandwidth with 5 dB noise figure: N_floor = -174 + 60 + 5 = -109 dBm.
Improving the Noise Floor
- Lower noise figure: Better LNA reduces the system NF, lowering the noise floor.
- Narrower bandwidth: Halving the bandwidth reduces noise power by 3 dB.
- Cryogenic cooling: Reducing physical temperature directly reduces thermal noise.
- Processing gain: Coherent integration, spread spectrum, and coding gain effectively reduce the noise floor relative to the signal.
N = kTB = -174 dBm/Hz at 290K
System noise floor:
N_floor = -174 + 10 log10(BW) + NF (dBm)
Example: BW = 10 MHz, NF = 3 dB:
N_floor = -174 + 70 + 3 = -101 dBm
Minimum detectable signal (MDS):
MDS = N_floor + SNR_min
Frequently Asked Questions
What is the noise floor?
The noise floor is the level of background noise in a receiver, below which signals cannot be detected. It is determined by thermal noise (dictated by physics), system noise figure, and measurement bandwidth. The noise floor sets the ultimate sensitivity limit of any receiver.
How do you calculate the noise floor?
N_floor (dBm) = -174 + 10 log10(bandwidth in Hz) + noise figure (dB). The -174 dBm/Hz is the thermal noise power spectral density at room temperature. For a 1 MHz bandwidth with 3 dB noise figure: -174 + 60 + 3 = -111 dBm.
Can the noise floor be below -174 dBm/Hz?
At room temperature (290K), -174 dBm/Hz is the absolute minimum. However, cryogenic cooling reduces thermal noise below this level. At 4K (liquid helium), the thermal noise floor drops to about -192 dBm/Hz. Radio telescopes use cryogenic receivers to achieve this.