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Noise Figure Measurement - A Practical Approach
5. What is Noise Figure:
Noise Figure is defined as the ratio of signal to noise ratio at the output to that at the input. In other words,
Where (s/n)I is the signal to noise ratio at the input, and (s/n)o is the signal to noise ratio at the output of the device under test. Note that s/n at the output will always be smaller than the s/n at the input, due to the fact that any circuit will only add to the noise, but never reduces the noise present in the system.
(S/N)in=(signal power) in /(noise power) in
= S in /N in
(S/N)out=(Signal Power)out/(Noise Power)out
=Sout/Nout= [S in *G]/[G*Nin + Ndut]
Ndut is the noise power due to the device.
= [S in /Nin]/[S in *G/[ G*Nin + Ndut]
= [G*Nin + Ndut]/[ GNin]
Effective Noise Temperature:
Let us assume that Te (called Effective Noise Temperature) is the noise temperature that results in Ndut. Then Te is related to the Ndut as follows:
Ndut = KGB*Te
Te= Ndut /[KGB]
Also we know at room temperature, Nin =KT0B
Substituting the values in the above formula,
F=[G*Nin + Ndut]/[GNin] = [T0 + Te]/T0
Or Te = T0(F-1)
F expressed in linear terms is called Noise Factor. In log terms, F is called Noise Figure.
F dB = 10logF
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