noise floor measurement

Dear all,

I try to use the spectrum analyzer to measure the noise floor of my environment. There are a vacumm pump beside our laboratory. I want to test the noise floor effect before and after the vacumm pump operation. However, I found different setting in the spectrum analyzer. It gives out different value to me. So, I want to ask yours that which setting is correct. I have the following setting of the specturm analyzer and get the following result:

(1) RBW=1.5MHz, Span=2GHz, -70 dBm @ 4GHz (vacumm pump=on)
RBW=1.5MHz, Span=2GHz, -80 dBm @ 4GHz (vacumm pump=off)

(2) RBW=1Hz, Span=500Hz, -140.598 dBm @ 4GHz (vacumm pump=on)
RBW=1Hz, Span=500Hz, -140.749 dBm @ 4GHz (vacumm pump=on)

Why are so large different? Actually, what is the setting of the spectrum analyzer in order to get a correct noise floor measurement?

Thanks

wccheng

In the case of 1Hz RBW you have the power spectral density or equivalently the power in 1Hz.

In the 1.5MHz case you are integrating the power spectral density over 1.5MHz which is equivalent to adding 10*log(1.5e6) = 60dB gain

Also when making such measurement you should pay attention to set the input attenuation to 0dB. A good way to check your measurement is to compare your measurement with the spectrum analyzer specifications.

Hope this helps

Then which one of the settings is correct for noise floor measurement?

thanks