The SA44B is not ideal for broadband measurements. We generally only recommend it for narrow-band signal analysis (<250 kHz occupied bandwidth is ideal) due to the limitations of the SA-series software image rejection.
Because we use a software image rejection algorithm, making noise measurements can be tricky. A span of 200 kHz or less should be used. Good settings might be 1 kHz RBW, 30 Hz VBW, power, average. Then you have to factor in that the noise from the image will add to the reading, typically 3 dB.
Hope this helps. The BB60C might be a better choice for noise measurements, as it uses hardware image rejection so it can be used like a normal spectrum analyzer (the BB60C should not be used for noise measurements below about 100 kHz).
Another thing to note is that the preamplifier is off for sweeps that start below 500 kHz or so. So you will not have maximum sensitivity at low frequencies.
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