Output Voltage Noise measurement

[appsee]

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.

 

[abcyin]

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!

 

[appsee]

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)?

 

[abcyin]

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.