RFICDUDE
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Well, I guess it depends on what is important to the system. A really narrowband or CW signal will have the same power no matter what the resolution bandwidth is, but a widerband signal spectral density will scale with the RBW.
Put another way, if you had a CW signal then the CNR would change with RBW because the noise power measured is a function of the RBW while the CW power measured is independent of RBW. However, if everything is normalized to dBc/Hz then the RBW dependency is removed.
If you changed the RBW to the signal BW then you are effectively measuring SNR if the only interference is the noise floor and the noise outside of the channel is the same across the channel.
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