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What's the physical meaning of Noise Figure in Lossy network

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Full Member level 4
Apr 14, 2002
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Noise Figure?

NF of Lossy network, ie. filter, passive mixer, is equal to the loss of the network in DB, what is this physical meaning?

That is bad or good?

This is generally unwanted thing. But to escape from that is opposite of the nature of the material.

If you use a passive circuit at the input of a receiver system , NF will impact directly to NF of system .

In fact , it really depends on many factor , bandwith , gain etc ( remember Friis formula ). Sometimes they should be used with amount of attenuation , sometimes they don't create great impact system performance.

Your question is very general , please be specific.



My question is: Why NF = loss of passive filter (in dB)?

that is if my LPF loss is 3dB, then NF of LPS is 3dB! What is meaning of this?

definition of noise figure

The definition of noise figure is the input signal to noise ratio compared to the output signal to noise ratio.

All eletrical circuit components have a noise output power of at least kTB where k is Boltzman's constant, T is absolute temperature and B is the cyclic bandwidth (Hz).

The noise out of a resistive attenuator is kTB with nothing connected to the input. The signal comes in with kTB of noise from its source. After going through the 3 dB attenuator there is 3 dB less signal and the old kTB of noise.

Thus the signal to noise ratlio has been reduced by 3 dB and by the definition of noise figure, the NF is 3 dB.

Pay attention: for a mixer, even passive, this is non true !

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