Noise Figure And Measurement Techniques
: What Is Noise Figure
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
= S in /N in
(S/N)out=(Signal Power)out/(Noise Power)out
=Sout/Nout= [S in *G]/[G*Nin
Ndut is the noise power due to the device.
= [S in /Nin]/[S in *G/[ G*Nin
= [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