Convert noise figure between dB and linear, cascade up to 6 stages with the Friis formula (per-stage contribution table + dominant-stage highlight), and convert to noise temperature (T0 = 290 K). Everything runs locally in your browser — no data leaves your device.
Type in either box — the other updates instantly. FdB = 10·log10(f), f = 10F/10.
Physical noise figure is ≥ 0 dB (f ≥ 1). Values below that are converted as pure math and flagged.
Enter each stage's gain and noise figure (both in dB). Gain may be negative — a passive attenuator with loss L dB has NF = L dB.
| Stage | Gain (dB) | NF (dB) |
|---|
| Stage | Term (linear) | Share of total |
|---|
The dominant stage is highlighted — it contributes the largest term to Ftotal. In a well-designed receiver this is almost always stage 1.
Te = T0·(f − 1), with T0 = 290 K (IEEE reference). Type in any box.
Validation is physical, not just syntactic: NF < 0 dB and Te < 0 K are rejected as unphysical (a passive two-port cannot improve SNR). Linear noise factor must be > 0. Gain may be negative (lossy stage).
Noise factor (f) is the linear ratio of input SNR to output SNR of a two-port network: f = SNR_in / SNR_out. Noise figure (F) is the same quantity expressed in decibels: F(dB) = 10·log10(f). A perfect noiseless device has f = 1, i.e. F = 0 dB.
Use f = 10^(F/10). For example, F = 3 dB converts to f = 10^(0.3) = 1.9953 linear. Type any dB value in the converter's left box and the linear factor appears instantly.
Use F(dB) = 10·log10(f). For example, f = 2 converts to F = 10·log10(2) = 3.0103 dB. The converter's right box accepts any f > 0.
For stages with linear noise factors F1, F2, … and linear gains G1, G2, …: F_total = F1 + (F2−1)/G1 + (F3−1)/(G1·G2) + …. Example: stage 1 with F1 = 3 dB and G1 = 10 dB followed by stage 2 with F2 = 6 dB gives F_total = 3.6047 dB. (Textbooks often round this to ≈3.62 dB using f1 ≈ 2.0.)
Each later stage's noise is divided by the gain product of all stages before it, so its contribution shrinks rapidly. In the example above (3 dB / 10 dB + 6 dB), stage 1 contributes 87.00% of F_total while stage 2 contributes only 13.00%. The calculator's contribution table highlights the dominant stage for any 1–6 stage chain.
A passive attenuator with loss L dB has NF = L dB — it degrades SNR exactly by its loss. Example: a 3 dB pad (gain −3 dB, NF 3 dB) ahead of an amplifier (gain 20 dB, NF 2 dB) gives a cascade NF of 5.0000 dB, dominated by the pad. Enter negative gain directly in the Friis tab.
Noise temperature Te expresses the same noise as an equivalent temperature: Te = 290·(f − 1) kelvin, where 290 K is the IEEE standard reference temperature T0. A 3 dB noise figure (f = 1.9953) corresponds to Te = 288.63 K — roughly room temperature, as expected.
Invert the relation: f = Te/290 + 1, then F(dB) = 10·log10(f). For example, Te = 290 K gives f = 2.0 and F = 3.0103 dB. Type a temperature in the third tab and all three forms update together.
No — a negative noise figure (f < 1) would mean the device improves SNR, which is impossible for any passive or active two-port at a single frequency. This calculator rejects NF < 0 dB and Te < 0 K as unphysical, while the pure dB↔linear converter still evaluates them as math with a warning.
The Resistor Noise Calculator covers Johnson–Nyquist thermal noise of a single resistor (nV/√Hz, dBu, kTB power, SNR) — component-level noise. This tool covers system-level noise figure: dB↔linear conversion, multi-stage Friis cascades, and noise temperature. Use both together when a resistor's thermal noise feeds a receiver chain.
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