Formula and Method for Noise Figure
Noise figure (NF) measures how much a component or system degrades the signal-to-noise ratio (SNR) of a signal as it passes through, expressed in decibels. It starts from the noise factor F, the linear ratio of the SNR at the input to the SNR at the output: F = SNRin / SNRout. Because every real device adds some of its own noise, the output SNR is always lower than the input SNR, so F ≥ 1 and NF ≥ 0 dB. An ideal, perfectly noiseless device would have F = 1 and NF = 0 dB.
How the calculation works
Enter the input and output SNR in decibels, as measured on a bench or specified on a datasheet. Because decibels are already logarithmic, the noise figure is just the difference of the two: NF(dB) = SNRin(dB) − SNRout(dB). The calculator then converts that figure to the linear noise factor with F = 10^(NF/10), and derives the equivalent noise temperature Te = T0 × (F − 1), where T0 = 290 K is the IEEE standard reference temperature used throughout the RF and microwave industry.
Common mistakes
- Confusing noise factor with noise figure: F is a plain linear ratio (always ≥ 1); NF is F expressed in decibels (always ≥ 0 dB). Do not plug a dB value into a formula that expects the linear F, or vice versa.
- Averaging noise figures directly: NF in dB cannot be summed or averaged across cascaded stages. Convert to linear F (or to noise temperature Te, which is additive) before combining stages with the Friis cascade formula.
- Ignoring the reference temperature: equivalent noise temperature depends on the assumed T0. The IEEE standard is 290 K; using a different reference (such as 300 K or actual room temperature) will shift Te and should always be stated.
Real-world applications
- Low-noise amplifier (LNA) datasheets specify NF, often 0.3-1 dB, to show how little extra noise the first amplifier stage adds to a weak received signal.
- Satellite ground stations and radio telescopes track noise temperature rather than dB because noise temperature adds directly across cascaded stages, simplifying system noise budgets.
- Receiver sensitivity and link-budget calculations combine noise figure with bandwidth and antenna temperature to predict the minimum detectable signal.
- RF engineers apply the Friis cascade formula, Ftotal = F1 + (F2−1)/G1 + (F3−1)/(G1G2) + …, to show that the first stage's noise figure and gain dominate an entire receiver chain's noise performance.
Frequently Asked Questions
What is noise figure?
Noise figure (NF) is a decibel measure of how much a device degrades the signal-to-noise ratio of a signal passing through it. It is defined as NF = 10 x log10(F), where F (the noise factor) is the linear ratio of input SNR to output SNR. A perfectly noiseless device has NF = 0 dB.
What is the difference between noise factor and noise figure?
Noise factor (F) is the plain linear ratio SNRin / SNRout and is always 1 or greater for a real device. Noise figure (NF) is simply F expressed in decibels: NF = 10 x log10(F). For example, F = 2 corresponds to NF ≈ 3.01 dB.
How is noise figure related to noise temperature?
Equivalent noise temperature is Te = T0 x (F − 1), where T0 = 290 K is the IEEE standard reference temperature. A low-noise amplifier with NF = 0.5 dB has F ≈ 1.122, giving Te ≈ 290 x 0.122 ≈ 35 K.
What noise figure is considered good for an amplifier?
For RF and microwave amplifiers, roughly 0.3-1 dB is excellent (typical of specialized low-noise amplifiers), 1-3 dB is very good, 3-6 dB is a typical general-purpose stage, and above 10 dB is considered poor for a front-end device.
Common Tip Percentages
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| Restaurant (sit-down) | 15–20% | 20%+ for exceptional service |
| Food delivery | 10–20% | Consider extra for long distances |
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Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |