Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
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.