An application example would be with the signal generator and amplifier providing the stimulus test signal to a device-under-test (DUT), as pictured in Figure 1. There are applications where the DUT is sensitive to the noise level at its input. With an amplifier that is always on, turning the test signal from the signal generator on and off provides a test signal power level at the DUT from approximately +35 dBm with the amplifier driven into saturation, to about -26 dBm for the amplifier output noise floor.

Turning the amplifier off at the same time the signal generator’s output test signal is turned off provides a test signal power level at the DUT from approximately +35 dBm with the amplifier driven into saturation, to about -71 dBm for the noise floor at the input to the DUT, or an improvement in dynamic range of about 45 dB.

Amplifier Noise Figure, Noise Factor and Residual Phase Noise

Electronic noise is a random fluctuation in an electrical signal, an unavoidable characteristic of all electronic circuits generated by the random thermal motion of charge carriers, usually electrons, inside an electrical conductor or by a number of other mechanisms and variety of effects. Noise is an error or undesired random disturbance or summation of unwanted or disturbing energy. The noise level is typically measured as an electrical power in Watts or dBm. Noise may also be characterized by its noise spectral density in Watts/Hz or dBm/Hz.

A noise signal is typically considered as a linear addition, in opposition, to a useful information signal. The typical signal quality measure involving noise is signal-to-noise ratio (SNR or S/N). Communication systems strive to increase the ratio of signal level to noise level in order to effectively transmit data. In practice, if the transmitted signal falls below the level of the noise, called the noise floor, in the system, then data can no longer be decoded at the receiver. While there are exceptions, such as spread spectrum where signal processing can recover the signal from below the noise floor, it is still desirable to minimize the noise floor.

Amplifiers are one of the most basic electrical elements in any electronic system. They amplify noise as well as the signal and amplifiers add some amount of their own noise as well. The added noise is characterized by noise figure, noise factor and/or residual phase noise.

Noise factor (F) is the deterioration of the signal-to-noise ratio due to noise from the amplifier as defined by the ratio of the signal-to-noise ratio at the input to the signal-to-noise ratio at the output.

F = (S/N)_IN / (S/N)_OUT= (S_I/N_I) / (S_O/N_O) = S_IN_O / S_ON_I

For an amplifier with gain G and noise N_A, the signal is increased by the gain G so that S_O = GS_I and the noise is increased by both the gain and the amplifier noise so that N_O = GN_I+N_A.

F = S_I (GN_I +N_A) / GS_I N_I = (GN_I +N_A) / G N_I = 1 + (N_A/GN_I)

Note that the result is independent of the signal and approaches unity as N_A approaches zero.

F ≥ 1

In engineering, it is often more convenient to work in decibels and the term noise figure (NF) is the decibel expression of noise factor (F), defined as:

NF = 10 log₁₀ (F)

Note that for an ideal noise factor of unit (F = 1), the noise figure is zero (NF = 0 dB). A few values are shown in Table 2.

Table 2) Examples of conversion of noise factor (F) to noise figure (NF)

Noise Considerations in Broadband Microwave Power Amplifiers

Amplifier Noise Figure, Noise Factor and Residual Phase Noise