Noise Figure Calculator
Quantify the noise degradation introduced by a receiver or amplifier above the thermal noise floor.
What is Noise Figure?
Noise Figure (NF) is a measure, in dB, of how much a device (receiver, amplifier, mixer, or attenuator) degrades the signal-to-noise ratio (SNR) of a signal passing through it. It represents the additional noise power added by the device above the theoretical minimum set by thermal physics (Johnson-Nyquist noise).
At room temperature (290 K), the thermal noise power density is −174 dBm per Hz of bandwidth. A perfect, noiseless receiver would preserve this as the noise floor. Real receivers add extra noise; a receiver with NF = 6 dB has a noise floor 6 dB higher than the theoretical minimum, meaning it is 6 dB less sensitive than a perfect receiver of the same bandwidth.
NF = 0 dB is an ideal noiseless device (impossible in practice). Practical LNAs achieve 0.5–2 dB. General-purpose receivers typically have NF of 5–12 dB.
Why Does It Matter?
- Every 1 dB improvement in noise figure directly improves receiver sensitivity by 1 dB — equivalent to doubling transmit power
- NF is the dominant parameter for low-signal applications: GPS, satellite, and long-range IoT links
- Cascaded noise figure (Friis formula) shows that the first amplifier in a receive chain dominates overall system NF
- Cable loss before an LNA adds directly to the system noise figure — minimizing RX cable length is critical
- In SATCOM and radio astronomy, even 0.1 dB NF improvements are significant
Quick Noise Floor Calculator
290 K = standard room temp
Formula Used by LinkBudgetPro
\(-174\) dBm/Hz = \(kT_0\) at 290 K · \(B_{\text{Hz}}\) = channel bandwidth · NF = noise figure in dB
The \(-174\) dBm/Hz constant is the thermal noise power spectral density (kTB) at 290 K. The bandwidth term accounts for the total noise power integrated over the channel. Noise figure is added directly because both are in dB.
For a cascaded system (antenna → LNA → cable → radio), use the Friis noise figure formula: \( \text{NF}_\text{total} = \text{NF}_1 + (\text{NF}_2 - 1)/G_1 + \cdots \) (linear values), where \(G_1\) is the gain of the first stage.
Parameter Explanation
| Parameter | Symbol | Unit | Description |
|---|---|---|---|
| Noise Figure | NF | dB | SNR degradation introduced by the receiver (0 dB = ideal, 5–10 dB = typical) |
| Noise Factor | F | linear | Linear version: F = 10^(NF/10). NF = 10·log₁₀(F) |
| Thermal noise density | kT | dBm/Hz | −174 dBm/Hz at 290 K (room temperature) |
| Bandwidth | BW | Hz | Channel bandwidth — noise floor = kT + 10·log₁₀(BW) + NF |
| Noise floor | N | dBm | Total in-band noise power at receiver input |
| Noise temperature | T_e | K | Equivalent noise temperature: T_e = 290 × (F − 1). NF=3 dB → T_e ≈ 290 K |
Worked Example
Compare the noise floor for two receivers with different NF values, both in a 20 MHz channel:
When Should You Use It?
- Comparing receiver hardware — NF is a key spec for choosing low-noise front ends and SDRs
- Designing receive chains — Friis formula guides component ordering for minimum system NF
- Sensitivity budget — compute noise floor as the starting point for receiver sensitivity calculation
- LNA placement — quantify the benefit of mounting an LNA directly at the antenna vs. at the radio
- Long-range IoT/LoRa links — NF is a primary limiter for links attempting maximum range
Related Calculations
- Receiver Sensitivity — sensitivity = noise floor + required SNR
- Shannon Capacity — channel capacity given SNR
- Fade Margin Calculator — overall link budget including noise floor
- dBm ↔ Watt Conversion — convert noise power between units
- RF Documentation Index — all RF engineering reference pages
Enter your bandwidth and noise figure in the full calculator to compute sensitivity and fade margin.
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