In saConfigSweepCoupling, there is a parameter that gets passed in, bool reject. If you set this to true, and decrease your VBW to maybe 1/10 of your RBW, you should get the expected noise reading and shape.
The reason for this is the USB-SA44B does not have hardware image rejection, and instead relies on a software algorithm. When this is not enabled, an image response shows up 21.4 MHz away from the actual input signal.
For broadband signal applications, the BB60C would be an ideal choice, since it has excellent hardware image rejection. But the SA44B can be configured to work well for signals up to 20 MHz wide, using the image reject flag appropriately.
Generally, if the signal is continuous, the image reject flag should be true. If the signal is pulsed or frequency hopping, you must set the flag to false to catch the signal, and the image response is unavoidable on the SA44B.
Hopefully this helps.
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