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noise display on Spectrum Analyzer

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Julian18

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Hi, there
Can anybody here tell me is the noise floor displayed on the SA in dBm or dBm/Hz, that is, what 's the meaning when marker sais -70dBm, is that the total noise power? or just the power spectral density around the frequency of interest? If it is power density, why does it not inclue /Hz at the end of reading?

TIA.

Julian
 

madengr

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The spectrum analyzer displays power within the RBW (Resoluation Bandwidth) you have selected. As you narrow the RBW the power will drop. If you use the NOISE MARKER it will normalize the power to 1 Hz and display units of dBm/Hz. If the trace is very noise you decrease the VBW (Video Bandwidth) to stabilize teh trace (average the results). You have to be carefull with noise measuremnts since the spectrum analyzer averages after the log detector. More than likely the spectrum analyzer will make the correction for you in NOISE MARKER mode. If you don't want to use noise marker mode you will have to switch to the linear detector to get a true noise average. Otherwise you must apply the offset and IIRC it's about 2.8 dB however Agilent has an app note on this topic.
 

biff44

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Yep, agreed. dBm/Hz.

Note that the displayed RBW usually does not exactly equal the true noise bandwidth. If the RBW says "1KHz", it might actually be 1.1 KHz when it is accurately measured. This is usually caused by the fact that you can not use brick wall filters.

Why this is confusing is that usually you can change the RBW and the signal power does not seem to change on the spectrum analyzer face. This happens if you are looking at a single tone, where all the power is concentrated at one frequency. It matters very little if you look at that tone in a 1 KHz bandwidth or a 1 MHz bandwidth, because each filter will let in the same tone, which contains 99.999% of the power.

However if you were looking at a broadband noise source, you would see the power level change 30 dB in going from 1 KHz bandwidth to 1 MHz bandwidth.
 
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