Flo,
The image response will be 21.4 MHz above the 20-100 kHz signal. If your configuration is measuring flicker noise or similar, and you have a low frequency amplifier with, say, less than 10 MHz of bandwidth, between your DUT and the SA44B, you would not subtract 3 dB, as the noise at 21.4 MHz would be negligible compared to the noise at 20-100 kHz. You could turn off spur rejection, deal with the spurs, and not worry about VBW settings. Or you could set VBW to 1/10 RBW with spur reject on, and still get a valid reading.
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