Cascaded Noise Figure Calculator

Receiver sensitivity depends on the noise figure of the whole receive chain. Enter the NF and gain of each stage (LNA, filter, amplifier, mixer …) and the Friis formula instantly gives the total noise figure, total gain and noise temperature – and shows which stage hurts most.

Friis formulaUp to 5 stagesPassive losses includedPer-stage contribution chart

① Input Parameters

Stage	Noise figure NF	Gain
Stage 1 e.g. LNA	dB	dB
Stage 2 e.g. SAW / BPF	dB	dB
Stage 3 e.g. AMP	dB	dB
Stage 4 e.g. Mixer	dB	dB
Stage 5 e.g. IF AMP	dB	dB

For a 2- or 3-stage chain simply leave the remaining stages blank (they are set to 0 and do not change the result). For a filter or cable with L dB loss, enter NF = L and gain = −L.

② Results

Item	Value
Total noise figure NF	dB
Total gain	dB
Noise temperature Te	K

Te = 290 × (F − 1), referenced to 290 K.

Receiver Chain Block Diagram

Each stage’s NF / gain is shown under its block. Passive parts (filters, cables, lossy mixers) are stages too: enter NF = loss L and gain = −L.

Noise Contribution per Stage

Computed as (F_k − 1) ÷ total gain ahead of stage k, shown as a share of total (F − 1). Orange is stage 1.

How to Use

Starting at the antenna, enter the noise figure NF (dB) and gain (dB) of each stage in signal-path order.
For passive parts (filters, cables, switches, passive mixers) the NF equals the loss and the gain is the negative loss.
Click Calculate to get the total noise figure, total gain and equivalent noise temperature.
Check the contribution chart on the right: the stage with the largest share is where improvement pays off most.

Theory & Formulas

Convert dB to linear ratios, then cascade the stages with the Friis formula:

F_k = 10^NF_k/10　G_k = 10^Gain_k/10
F = F₁ + (F₂ − 1)/G₁ + (F₃ − 1)/(G₁G₂) + (F₄ − 1)/(G₁G₂G₃) + (F₅ − 1)/(G₁G₂G₃G₄)
NF = 10·log₁₀F　　Gain = Gain₁ + … + Gain₅ (dB)
T_e = 290·(F − 1)

Why does the first stage matter most?

The noise of later stages is divided by the gain of all stages in front of them. With enough first-stage gain the total NF is almost equal to the first stage’s NF; conversely, any lossy filter or cable placed before the LNA adds its loss directly to the total NF.

Design Tips

Place the LNA as close to the antenna as possible – every 1 dB of loss in front of it adds about 1 dB to the total NF.
Make the first-stage gain comfortably larger than the following stage’s NF (typically 10–20 dB or more) so later stages become negligible.
Receiver sensitivity ≈ −174 dBm/Hz + 10·log(bandwidth) + NF + required SNR – handy for estimating link range.
If the front end needs selectivity, design it with the band-pass filter calculator and enter its insertion loss as a stage.

FAQ

How do I enter a passive component?: At room temperature (about 290 K) a passive part with L dB loss has NF = L dB and gain = −L dB.
Why is the total gain a simple sum?: Cascaded gains multiply, which in dB means they add; the total gain is the sum of the stage gains in dB.
What if I only have two stages?: Leave stages 3–5 blank or 0; they are treated as NF 0 dB and gain 0 dB and do not affect the result.

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