Thermal Noise and Receiver Sensitivity Calculator (kTB)
The thermal noise floor of a receiver for its bandwidth and temperature, the weakest signal it can decode from its noise figure and the SNR it needs, and the noise voltage across a resistor.
Results
The dashed line marks the value you entered.
What this tool does
Every resistor and every receiver input has a noise power of k × T × B, which at room temperature (290 K) is −174 dBm in each hertz of bandwidth, so a channel 20 MHz wide has a floor near −101 dBm before the receiver adds anything. The noise figure says how much worse the receiver is than that ideal, and the SNR the modulation needs on top gives the sensitivity: floor + NF + SNR. Halving the bandwidth lowers the floor by 3 dB; that is why narrow, slow links reach so much further.
Formula
N = k T B (−174 dBm/Hz at 290 K) · noise floor = −174 + 10 log₁₀ B · sensitivity = noise floor + NF + required SNR · v_n = √(4 k T R B)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
bw | Receiver bandwidth | MHz |
nf | Receiver noise figure | dB |
sn | Signal-to-noise ratio needed to decode | dB |
t | Physical temperature | K |
r | Resistance for the noise voltage | Ω |
SE | Receiver sensitivity (minimum signal) | dBm |
NF | Thermal noise floor (kTB) | dBm |
ND | Noise power density | dBm/Hz |
VN | Noise voltage across that resistance (rms) | µV |
Worked example
- Receiver bandwidth20 MHz
- Receiver noise figure6 dB
- Signal-to-noise ratio needed to decode10 dB
- Physical temperature290 K
- Resistance for the noise voltage50 Ω
- Receiver sensitivity (minimum signal)-85.0 dBm
- Thermal noise floor (kTB)-100.96 dBm
- Noise power density-173.98 dBm/Hz
- Noise voltage across that resistance (rms)4.002 µV
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.