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Amplifier noise figure - RF Cafe Forums
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| dohagan |
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Post subject: Amplifier noise figure
Posted: Tue Feb 20, 2007 1:43 pm
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| Captain |
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Joined: Sat Mar 19, 2005 3:20 pm
Posts: 6
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Hello,
I've got a variable gain amplifier connected directly to and Analogue-to-Digital
converter (ADC). The input of the amplifier is terminated in 50 ohms and the gain is
set to 20 dB. Measuring the noise at the output of the ADC (i.e. in Matlab) over a 200
kHz bandwidth, I get a value of -92 dB.
So, Pn = -92 dB = k+T+B+Fn+G
where
Fn is the total noise figure of the system, G is the amplifier gain (20 dB) and B is
the bandwidth (200 kHz).
I know the noise figure of the ADC itself to be 25.5
dB, but how do I find the exact value for the amplifier noise figure??
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| nubbage
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Post subject:
Posted: Wed Feb 21, 2007 4:23 am
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| General |
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Joined: Fri Feb 17, 2006 12:07 pm
Posts: 343
Location: London UK |
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Hi
The overall noise factor of a 2 stage system will be dominated by that of the
first stage, particularly, as in your case, where the first stage gain is 20dB.
First
you have to convert dB NF to numerical NF (Fr), and dB gain to numerical gain (G, 100
in your case).
The overall NF is Fn = F1 +(Fr-1)/G
With G = 100, you can see the
NF of the ADC has no importance, so Fn=F1.
The noise power into the ADC is just the
expression you quoted, except Fn is the first stage NF, and the noise power is probably
in dBW, depending on units used for the K value.
Inverting the expression then gives
Fn.
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| sm7ovk
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Post subject:
Posted: Tue Feb 27, 2007 4:41 pm
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Joined: Thu Jan 11, 2007 12:59 pm
Posts: 23
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| OM |
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Post subject:
Posted: Sun Aug 10, 2008 4:40 pm
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| Lieutenant |
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Joined: Sun Aug 10, 2008 3:41 pm
Posts: 3
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To sm7ovk,
I regret to tell you that you are not right. You need to edit this
page.
73!
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| smith08
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Post subject:
Posted: Tue Sep 30, 2008 1:10 am
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Joined: Tue Sep 30, 2008 1:06 am
Posts: 1
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where the signal to noise is measured by an ideal, noiseless receiver. =====================
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Posted 11/12/2012
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