Physics:Noise figure

Short description: Figure used to evaluate the performance an amplifier

Noise figure (NF) and noise factor (F) are figures of merit that indicate degradation of the signal-to-noise ratio (SNR) that is caused by components in a signal chain. These figures of merit are used to evaluate the performance of an amplifier or a radio receiver, with lower values indicating better performance.

The noise factor is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard noise temperature T₀ (usually 290 K). The noise factor is thus the ratio of actual output noise to that which would remain if the device itself did not introduce noise, which is equivalent to the ratio of input SNR to output SNR.

The noise factor and noise figure are related, with the former being a unitless ratio and the latter being the logarithm of the noise factor, expressed in units of decibels (dB).

General

The noise figure is the difference in decibel (dB) between the noise output of the actual receiver to the noise output of an "ideal" receiver with the same overall gain and bandwidth when the receivers are connected to matched sources at the standard noise temperature T₀ (usually 290 K). The noise power from a simple load is equal to kTB, where k is the Boltzmann constant, T is the absolute temperature of the load (for example a resistor), and B is the measurement bandwidth.

This makes the noise figure a useful figure of merit for terrestrial systems, where the antenna effective temperature is usually near the standard 290 K. In this case, one receiver with a noise figure, say 2 dB better than another, will have an output signal-to-noise ratio that is about 2 dB better than the other. However, in the case of satellite communications systems, where the receiver antenna is pointed out into cold space, the antenna effective temperature is often colder than 290 K. In these cases a 2 dB improvement in receiver noise figure will result in more than a 2 dB improvement in the output signal-to-noise ratio. For this reason, the related figure of effective noise temperature is therefore often used instead of the noise figure for characterizing satellite-communication receivers and low-noise amplifiers.

In heterodyne systems, output noise power includes spurious contributions from image-frequency transformation, but the portion attributable to thermal noise in the input termination at standard noise temperature includes only that which appears in the output via the principal frequency transformation of the system and excludes that which appears via the image frequency transformation.

Definition

The noise factor F of a system is defined as

[math]\displaystyle{ F = \frac{\mathrm{SNR}_\text{i}}{\mathrm{SNR}_\text{o}} }[/math]

where SNR_i and SNR_o are the input and output signal-to-noise ratios respectively. The SNR quantities are unitless power ratios. The noise figure NF is defined as the noise factor in units of decibels (dB):

[math]\displaystyle{ \mathrm{NF} = 10 \log_{10}(F) = 10 \log_{10}\left(\frac{\mathrm{SNR}_\text{i}}{\mathrm{SNR}_\text{o}}\right) = \mathrm{SNR}_\text{i, dB} - \mathrm{SNR}_\text{o, dB} }[/math]

where SNR_i, dB and SNR_o, dB are in units of (dB). These formulae are only valid when the input termination is at standard noise temperature T₀ = 290 K, although in practice small differences in temperature do not significantly affect the values.

The noise factor of a device is related to its noise temperature T_e:

[math]\displaystyle{ F = 1 + \frac{T_\text{e}}{T_0}. }[/math]

Attenuators have a noise factor F equal to their attenuation ratio L when their physical temperature equals T₀. More generally, for an attenuator at a physical temperature T, the noise temperature is T_e = (L − 1)T, giving a noise factor

[math]\displaystyle{ F = 1 + \frac{(L - 1)T}{T_0}. }[/math]

Noise factor of cascaded devices

If several devices are cascaded, the total noise factor can be found with Friis' formula:

[math]\displaystyle{ F = F_1 + \frac{F_2 - 1}{G_1} + \frac{F_3 - 1}{G_1 G_2} + \frac{F_4 - 1}{G_1 G_2 G_3} + \cdots + \frac{F_n - 1}{G_1 G_2 G_3 \cdots G_{n-1}}, }[/math]

where F_n is the noise factor for the n-th device, and G_n is the power gain (linear, not in dB) of the n-th device. The first amplifier in a chain usually has the most significant effect on the total noise figure because the noise figures of the following stages are reduced by stage gains. Consequently, the first amplifier usually has a low noise figure, and the noise figure requirements of subsequent stages is usually more relaxed.

Noise factor as a function of additional noise

The source outputs a signal of power [math]\displaystyle{ S_i }[/math] and noise of power [math]\displaystyle{ N_i }[/math]. Both signal and noise get amplified. However, in addition to the amplified noise from the source, the amplifier adds additional noise to its output denoted [math]\displaystyle{ N_a }[/math]. Therefore, the SNR at the amplifier's output is lower than at its input.

The noise factor may be expressed as a function of the additional output referred noise power [math]\displaystyle{ N_a }[/math] and the power gain [math]\displaystyle{ G }[/math] of an amplifier.

[math]\displaystyle{ F = 1 + \frac{N_a}{N_i G} }[/math]

Derivation

From the definition of noise factor

[math]\displaystyle{ F = \frac{\mathrm{SNR}_\text{i}}{\mathrm{SNR}_\text{o}}=\frac{\frac{S_i}{N_i}}{\frac{S_o}{N_o}}, }[/math]

and assuming a system which has a noisy single stage amplifier. The signal to noise ratio of this amplifier would include its own output referred noise [math]\displaystyle{ N_a }[/math], the amplified signal [math]\displaystyle{ S_iG }[/math] and the amplified input noise [math]\displaystyle{ N_iG }[/math],

[math]\displaystyle{ \frac{S_o}{N_o}=\frac{S_iG}{N_a+N_iG} }[/math]

Substituting the output SNR to the noise factor definition,

[math]\displaystyle{ F = \frac{\frac{S_i}{N_i}}{\frac{S_iG}{N_a+N_iG}}=\frac{N_a+N_iG}{N_iG} = 1 + \frac{N_a}{N_iG} }[/math]

In cascaded systems [math]\displaystyle{ N_i }[/math] does not refer to the output noise of the previous component. An input termination at the standard noise temperature is still assumed for the individual component. This means that the additional noise power added by each component is independent of the other components.

Optical noise figure

The above describes noise in electrical systems. The optical noise figure is discussed in. Electric sources generate noise with a power spectral density equal to kT, where k is the Boltzmann constant and T is the absolute temperature. However, there is also noise in optical systems. In these, the sources have no fundamental noise. Instead the energy quantization causes notable shot noise in the detector, corresponding to a noise power spectral density of hf where h is the Planck constant and f is the optical frequency.

In the 1990s, an optical noise figure has been defined. This has been called F_pnf for photon number fluctuations. The powers needed for SNR and noise factor calculation are the electrical powers caused by the current in a photodiode. SNR is the square of mean photocurrent divided by variance of photocurrent. Monochromatic or sufficiently attenuated light has a Poisson distribution of detected photons. If, during a detection interval the expectation value of detected photons is n then the variance is also n and one obtains SNR_pnf,in = n²/n = n. Behind an optical amplifier with power gain G there will be a mean of Gn detectable signal photons. In the limit of large n the variance of photons is Gn(2n_sp(G-1)+1) where n_sp is the spontaneous emission factor. One obtains SNR_pnf,out = G²n²/(Gn(2n_sp(G-1)+1)) = n/(2n_sp(1-1/G)+1/G). Resulting optical noise factor is F_pnf = SNR_pnf,in / SNR_pnf,out = 2n_sp(1-1/G)+1/G.

F_pnf is in conceptual conflict with the electrical noise factor, which is now called F_e:

Photocurrent I is proportional to optical power P. P is proportional to squares of a field amplitude (electric or magnetic). So, the receiver is nonlinear in amplitude. The "Power" needed for SNR_pnf calculation is proportional to the 4th power of the signal amplitude. But for F_e in the electrical domain the power is proportional to the square of the signal amplitude.

If SNR_pnf is a noise factor then its definition must be independent of measurement apparatus and frequency. Consider the signal "Power" in the sense of SNR_pnf definition. Behind an amplifier it is proportional to G²n². We may replace the photodiode by a thermal power meter, and measured photocurrent I by measured temperature change [math]\displaystyle{ \mathrm{\Delta\theta} }[/math]. "Power", being proportional to I² or P², is also proportional to [math]\displaystyle{ (\mathrm{\Delta\theta}) }[/math]². Thermal power meters can be built at all frequencies. Hence it is possible to lower the frequency from optical (say 200 THz) to electrical (say 200 MHz). Still there, "Power" must be proportional to [math]\displaystyle{ (\mathrm{\Delta\theta}) }[/math]² or P². Electrical power P is proportional to the square U² of voltage U. But "Power" is proportional to U⁴.

These implications are in obvious conflict with ~150 years of physics. They are compelling consequence of calling F_pnf a noise factor, or noise figure when expressed in dB.

At any given electrical frequency, noise occurs in phase (I) and in quadrature (Q) with the signal. Both these quadratures are available behind the electrical amplifier. The same holds in an optical amplifier. But the direct detection photoreceiver needed for measurement of SNR_pnf takes mainly the in-phase noise into account whereas quadrature noise can be neglected for high n. Also, the receiver outputs only one baseband signal, corresponding to quadrature. So, one quadrature or degree-of-freedom is lost.

For an optical amplifier with large G it holds F_pnf ≥ 2 whereas for an electrical amplifier it holds F_e ≥ 1.

Moreover, today's long-haul optical fiber communication is dominated by coherent optical I&Q receivers but F_pnf does not describe the SNR degradation observed in these.

Another optical noise figure F_ase for amplified spontaneous emission has been defined. But the noise factor F_ase is not the SNR degradation factor in any optical receiver.

All the above conflicts are resolved by the optical in-phase and quadrature noise factor and figure F_o,IQ. It can be measured using a coherent optical I&Q receiver. In these, power of the output signal is proportional to the square of an optical field amplitude because they are linear in amplitude. They pass both quadratures. For an optical amplifier it holds F_o,IQ = n_sp(1-1/G)+1/G ≥ 1. Quantity n_sp(1-1/G) is the input-referred number of added noise photons per mode.

F_o,IQ and F_pnf can easily be converted into each other. For large G it holds F_o,IQ = F_pnf/2 or, when expressed in dB, F_o,IQ is 3 dB less than F_pnf. The ideal F_o,IQ in dB equals 0 dB. This describes the known fact that the sensitivity of an ideal optical I&Q receiver is not improved by an ideal optical preamplifier.

See also

• Noise
• Noise (electronic)
• Noise figure meter
• Noise level
• Thermal noise
• Signal-to-noise ratio
• Y-factor

References

• Keysight, Fundamentals of RF and Microwave Noise Figure Measurements, Application Note, 57-1, Published September 01, 2019., http://literature.cdn.keysight.com/litweb/pdf/5952-8255E.pdf 

External links

• Noise Figure Calculator 2- to 30-Stage Cascade
• Noise Figure and Y Factor Method Basics and Tutorial
• Mobile phone noise figure