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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 ...
The noise factor (a linear term) is more often expressed as the noise figure (in decibels) using the conversion: = The noise figure can also be seen as the decrease in signal-to-noise ratio (SNR) caused by passing a signal through a system if the original signal had a noise temperature of 290 K. This is a common way of expressing the noise ...
Here, k ≈ 1.38 × 10 −23 J/K is the Boltzmann constant and kT 0 is the available noise power density (the noise is thermal noise, Johnson noise). As a numerical example: A receiver has a bandwidth of 100 MHz , a noise figure of 1.5 dB and the physical temperature of the system is 290 K .
Friis's formula is used to calculate the total noise factor of a cascade of stages, each with its own noise factor and power gain (assuming that the impedances are matched at each stage). The total noise factor can then be used to calculate the total noise figure. The total noise factor is given as
The Y-factor method is a common measurement technique for this purpose. [1] By using a noise diode, the output noise of an amplifier is measured using two input noise levels, and by measuring the output noise factor (referred to as Y) the noise figure of the amplifier can be determined without having to measure the amplifier gain.
In RF applications, noise power is defined using the relationship P noise = kTB, where k is the Boltzmann constant, T is the noise temperature, and B is the noise bandwidth. Typically the noise bandwidth is determined by the bandwidth of the intermediate frequency (IF) filter of the radio receiver. Thus, we can define the noise temperature as:
where f 0 is the output frequency, Q l is the loaded quality factor, f m is the offset from the output frequency (Hz), f c is the 1/f corner frequency, F is the noise factor of the amplifier, k is the Boltzmann constant, T is absolute temperature, and P s is the available power at the sustaining amplifier input. [3]
The ratio of (a) total received power, i.e., the signal to (b) the noise-plus-distortion power. This is modeled by the equation above. [2] The ratio of (a) the power of a test signal, i.e. a sine wave, to (b) the residual received power, i.e. noise-plus-distortion power. With this definition, it is possible to have a SINAD level less than one.