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Why noise figure doens't match (Read 5029 times)
tinaxie
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Why noise figure doens't match
Apr 29th, 2004, 11:53am
 
I do Front_End noise simulation, use print noise contribution, I know that RF port noise contribution percent is 50%, that is to say Noise figure equals to 10*log10(1/0.5)=3dB, however ploted noise figure is 5dB, much higher. What's the problem.

When I do LNA noise simulation, ploted noise figure match very well with calculated one according to noise summary.
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Eugene
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Re: Why noise figure doens't match
Reply #1 - Apr 30th, 2004, 7:46am
 
What simulator are you using? Spectre? SpectreRF? Are there any mixers in your front end or are you only looking at the LNA? If there are mixers, are they converting to IF or baseband? And how do you know the port accounts for 50% of the noise?

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tinaxie
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Re: Why noise figure doens't match
Reply #2 - Apr 30th, 2004, 9:34am
 
thanks very much.
I use SpectreRF, The front_end includes LNA and mixer, mixer convert RF signal to high IF. I use print noise contribution to see that RF port contribute 50% of total noise.
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Ken Kundert
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Re: Why noise figure doens't match
Reply #3 - Apr 30th, 2004, 11:29am
 
How much of the noise at the output is contributed by the load? It is ignored in the NF calculation but not when calculating the percentages in the output nosie.

-Ken
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tinaxie
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Re: Why noise figure doens't match
Reply #4 - Apr 30th, 2004, 2:15pm
 
Only 0.44% noise is contributed by loading.
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Ken Kundert
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Re: Why noise figure doens't match
Reply #5 - May 1st, 2004, 8:15am
 
Are you performing the computation using voltages or powers?  Things will not add up right if you use voltages (V/rt(Hz)); you must use powers (V2/Hz).

-Ken
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Eugene
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Re: Why noise figure doens't match
Reply #6 - May 4th, 2004, 9:05am
 
If the question is still unresolved, you might try replacing your circuit models with behavioral models (i.e. VerilogA).  Since the behavioral models simulate only what you include, you can surgically remove noise sources to isolate the offending source. The SpectreRF user's manual describes several behavioral models in detail that might serve as a starting point. That chapter also discusses test circuits, including noise test circuits, that might shed some light on the issue.

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