About this tool
Every stage of a receive chain – preamplifier, cable, filter, mixer, receiver – adds noise. The Friis formula F = F1 + (F2 − 1)/G1 + (F3 − 1)/(G1·G2) + … shows how much of it remains: the noise of a later stage is divided by the gain in front of it. That is why the first stage almost alone sets the noise figure.
The noise figure calculator takes the stages from the antenna to the receiver and shows the total noise figure, total gain and noise temperature, plus the noise figure up to each stage and its share of the noise. Enter passive losses such as cables as a loss; at room temperature their noise figure equals the loss. Get the cable loss from the Coax Cable Loss Calculator.
The advanced parameters add OIP3 values for large-signal handling, the bandwidth, the required SNR and the antenna temperature. From them the calculator derives the input IIP3 of the chain, the noise floor and the sensitivity (MDS) – the value to use as receiver sensitivity in the Link Budget Calculator. Everything is calculated locally in your browser.
How it works
- Enter the stages in order from the antenna: active stages with gain and noise figure, cables and filters as a loss.
- Read the total noise figure, gain and noise temperature; the table shows how much noise each stage contributes.
- Turn on the advanced parameters for OIP3, bandwidth, SNR and antenna temperature to get the IIP3 and the sensitivity.
Frequently asked questions
Why does the preamplifier belong at the antenna?
A loss in front of the preamplifier adds 1:1 to the noise figure; behind a preamplifier with enough gain it hardly matters. The default chain (LNA with 20 dB and 0.8 dB noise figure, 3 dB of cable, receiver with 6 dB) gives 1.04 dB. With the cable in front of the LNA it is 3.91 dB, without an LNA 9 dB.
What is the difference between noise figure and noise temperature?
Both describe the same thing. The noise temperature Te = 290 K · (F − 1) expresses the added noise as the temperature of a resistor at the input, with F the noise figure as a ratio. 3 dB is about 289 K, 1 dB about 75 K. For very low-noise stages, for example in satellite reception, the temperature is easier to read than the noise figure.
How do you calculate the sensitivity of a receiver?
Thermal noise at 290 K is −174 dBm per Hz. Add 10·log10 of the bandwidth in Hz, the noise figure and the required SNR. With the default chain, 12.5 kHz bandwidth and 10 dB SNR this gives about −122 dBm. The calculator works more precisely with the system noise temperature of antenna and receiver.
What role does the antenna temperature play?
The antenna itself delivers noise. 290 K is the reference temperature the noise figure is defined for. A satellite antenna pointing at the cold sky sees only 30 to 80 K – there every tenth of a dB of noise figure counts. On shortwave, atmospheric and man-made noise lies far above 290 K, so a low noise figure gains little.
How is the IIP3 of the chain calculated?
For each stage the input intercept is IIP3 = OIP3 − gain. For the chain, 1/IIP3 = 1/IIP3₁ + G₁/IIP3₂ + G₁·G₂/IIP3₃ + … in linear values. A lot of gain up front therefore lowers the IIP3 of the later stages; low noise and strong-signal handling are a trade-off. Passive losses count as linear in the calculator. If an active stage has no OIP3, the IIP3 stays empty.