Ken Kundert
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When you reduce the resolution bandwidth (RWM) of a spectrum analyzer, the noise floor tends to drop but the signal levels tend to stay the same. To see why, consider a pure tone. It has a particular power and zero bandwidth. Say it has 1mW power and you are measuring it with a 1kHz RBW. The spectrum analyzer will indicate that it is a 1mW signal. Now if you reduce the RBW to 100Hz, it still will report that that you are observing a 1mW signal. The reason being that the signal itself has a bandwidth that is smaller than the smallest bandwidth of the analyzer, and so the analyzer always reports the same amplitude for the signal.
Now consider a white noise signal with a power density of 1μW/Hz. The analyzer will report this as a 1mW signal when using a 1kHz RBW, but only a 100μW signal when using a 100Hz RBW. Thus, when comparing spectrum analyzer results measured with different RBW, you must compensate for the change. And how you compensate depends on the type of signal you are measuring. Generally people normalize their results to a 1Hz RBW. To do so, you would divide the noise power levels by the RBW to get a power density. You would not do anything to signals with bandwidths smaller than the measurement RBW.
-Ken
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