Andrew Beckett
Senior Fellow
Offline
Life, don't talk to me about Life...
Posts: 1742
Bracknell, UK
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Megh,
When you specify the input port for noise analysis, it does not affect the output noise in any way. It is only used to compute metrics such as input referred noise and noise figure, so that they can compute the gain between that input port or source and the output.
You can even set the input port/source to "none" and the output noise will remain the same.
In your case, because the input is not a single source, you cannot use this method to compute the input referred noise. So instead, you need to compute the output noise as normal (and it doesn't matter what you specify for the input port/source - you could set it to none), and then divide by your computed AC gain.
Note that specifying the AC magnitude on a source is not used in noise analysis - it is only used in AC analysis.
So, compute the AC gain by specifying an appropriate AC magnitude and phase on the I and Q sources, and then measure the signal level at the output - from this you can find the AC gain (sounds as if this is what you're already doing). Then knowing that, you can divide the output noise by this gain, to give you the input referred noise that you're after.
Regards,
Andrew.
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