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Basic question about RBW

Question asked by vahidy on Jul 31, 2015
I am trying to specify modulation frequency and frequency deviation for a spread-spectrum technique in a specific application. However, I realized that this could be tightly related to how you measure the EMI in that particular application. Now, I have a basic question about Resolution-Band-Width (RBW) in a spectrum analyzers:

I would like to know how exactly RBW (or an IF filter) affects the measurement. For example I know that for a CW measurement the smaller the RBW, the more accurate measurement you get for the peak power of the CW signal. 

However, assume that you have a two tone signal; why if the RBW is larger than the distance between the two tones, you can not see the distinct frequency separation between the two tones on display. Does the RBW do some kind of a averaging. Or, is it just because the IF filter increases the bandwidth of each tone and therefore they overlap. If the second case is correct, why this only happens for RBW larger than the distance between the two tones? 

The above explanation is based on a picture I found in an Agilent application note for Spectrum Analyzers. As you can see in the picture, as the RBW is increased the distinction between the two tones are faded. One more important question I have is that why in this picture for high RBW, the peak is the same as low RBW. In this picture it seems that the total power measured for this signal is higher for high RBW. How is that possible, does the RBW affect the total power measurement? Shouldn't for high RBW the peak of the signal decrease?