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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.
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.
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