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dd2001
Joined: 14 Apr 2002
Posts: 282
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 16 Mar 2003 2:12 Noise Figure? |
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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?
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BigBoss
Joined: 17 Nov 2001
Posts: 928
Helped: 47
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| 16 Mar 2003 2:21 |
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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.
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dd2001
Joined: 14 Apr 2002
Posts: 282
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| 16 Mar 2003 3:12 again? |
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Thanks.
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?
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flatulent
Joined: 19 Jul 2002
Posts: 4856
Helped: 292
Location: Middle Earth
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| 16 Mar 2003 4:13 definition of noise figure |
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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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marcomdd
Joined: 11 Jan 2003
Posts: 158
Helped: 3
Location: Rome
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| 17 Mar 2003 7:50 |
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| Pay attention: for a mixer, even passive, this is non true !
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