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|Amplifier noise figure - RF Cafe Forums|
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Post subject: Amplifier noise figure
Posted: Tue Feb 20, 2007 1:43 pm
Joined: Sat Mar 19, 2005 3:20 pm
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??
Posted: Wed Feb 21, 2007 4:23 am
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
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.
Posted: Tue Feb 27, 2007 4:41 pm
Joined: Thu Jan 11, 2007 12:59 pm
Posted: Sun Aug 10, 2008 4:40 pm
Joined: Sun Aug 10, 2008 3:41 pm
I regret to tell you that you are not right. You need to edit this page.
Posted: Tue Sep 30, 2008 1:10 am
Joined: Tue Sep 30, 2008 1:06 am
where the signal to noise is measured by an ideal, noiseless receiver.
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