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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 (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 ...
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.
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 .
If the noise current contribution i n R s >> noise voltage e n, then reducing the source impedance by a factor of 4 reduces the i n contribution by a factor of 4 while the source's thermal noise voltage declines by factor of 2 (ideal transformer with 2:1 turns ratio gives the 4:1 Z ratio); SNR improves by 6 dB. But there's another issue.
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]
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