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Output Voltage Noise measurement

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Newbie level 3
Nov 2, 2007
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I've been trying to measure the output voltage noise (V/sqrt(Hz)) on a unity gain amplifier with the spectrum analyzer. Now, I've noticed that when I dial down RBW, the noise is also lowered. Can you explain to me:

1. What is RBW and what does it do?
2. How can I find out what my output voltage noise is? Depending on what RBW is set to, its either to smaller or larger than what is spec'd in the datasheet @ 10kHz.

I thought this might have to do with the noise of the analyzer itself but that doesn't seem to be the case because I remove the amplifier, the noise floor drops significantly lower.

As to the 1st question, the noise floor you see at the spectrum analyzer is not the real noise floor, the relationship between them obeys to the following equation:
Real Noise Floor=Noise floor in spectrum analyzer - 10*log(RBW)

hope it helps!

thanks for the reply. i have some follow on questions.

1. there are several ways to represent noise on the spectrum analyzer - dBm, V/sqrt(Hz), etc. Does the -10log(RBW) apply to all of these representations?

2. Can you explain or point me to something on the web that explains why you need to take out 10log(RBW)?

for 1st question, its unit is dBm.
for 2nd question, I have no literature about that, but people around me use this equation, maybe you can prove it from others.

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