The standard RBW works well for the BB60C, but not for the USB-SA44B. Because of the software-based image rejection, the default noise bandwidth is 2 x RBW, but then there is the software image reject algorithm. To get NBW close to 2 x RBW, you would need to either disable “spur reject” / image reject, or set video processing to power average and turn VBW down to 1/10 RBW.
To get noise BW close to RBW, you would need to filter out the image frequency. For wide sweeps, this is 21.4 MHz above or below the image frequency. For 200 kHz spans or less, this is 5.8 MHz above or 21.4 MHz above the measured signal. With image/spur reject on, filtering out either image will result in a good measurement.
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